XR2206 Function Generator kit, and Chinese project boxes

A number of items from the Far East have been trickling in over the holiday period, including 2N3906 PNP transistors, veroboard, audio transformers and project boxes. One such packet arrived today with five project boxes, snap-on lid types, for about £3. The quality is good, not perfect, but certainly adequate, and quite impressive for the cost! Two of these will become small Radiation detectors using the SI-19BG and the SBM-10 G-M tubes.

Another packet with two larger white boxes has also arrived. These are for the BOI-33 tube based detectors. The photo above shows the first version board fitted into the box. I need now to sort out a battery supply (waiting on coin cell holders - this will run on a 3V lithium cell), plus power switch, LED and neon indicator holes, and a sound hole for the loudspeaker, plus a row of holes along the side to allow better beta penetration to the tube.

Also for about £3 shipped, I received this little function generator kit. Actually a little smaller than I expected, the board was of reasonable quality, although the edges are a little rough, and some of the holes are a little too large.

Putting it together was very easy, although the instructions that came with it were pretty rubbish! The parts list at least was accurate.

Hard to see in the photo, but the clear acrylic case is etched with the names of the controls. The board is not bolted into the case, as the five(!) M3 nuts and bolts were too short to do so! However, the case itself is a reasonably tight fit and so there is little movement.  I've yet to test it though.

SBM-10 G-M tube

Yesterday I received an SBM-10 G-M tube from Russia. This tiny tube is just 6mm diameter and about 20mm long. I've tested it and it works well.

But, like the tiny SI-19BG, it has no easy way to connect to it. I'm wondering whether the socket for the SI-19BG can be made to work?

I've also realised that the voltage doubler I was working on yesterday wasn't complete! There should have been another diode, or series string. I need to test that out later.

Thinking the other way up!

All the Geiger circuits im working on, are intended for portable use, and so need the lowest voltage and lowest current supplies, to make them compact...

...but not the 1600V supply for the MST-17! This tube is far too delicate for portable use, and will only ever be used on the bench.. so why the heck am I trying to make it work from such low voltages? I know I can get 1500V from a 9V battery, even with the simple blocking oscillator circuit, so almost certainly I can get well above this, and hence enough to feed the regulator chain, with 12V!

As my bench PSU goes up to 30V, I can choose any voltage up to this, I'm not limited to a small battery for this tube!

Incidentally, the CCFL transformer I've been playing with has a primary of about 30 turns (found by unwinding one), so I can also rewind for a better turns ratio if I wish.

Voltage Doubler Oddity

Just prior to tidying up and the workshop 'closing' for christmas, I've been playing with the CCFL transformers taken from a defunct LCD monitor, a simple blocking oscillator, and a Cockcroft voltage doubler.
As this transformer is rated for 3kV AC, I have been very conservative and used several 1N4007 1kV diodes, plus the 3kV 4n7 capacitors from the same board as the transformer.

I already knew from previous tests that when fed with 9V, the blocking oscillator, which is extremely simple, draws a lot of current, around 150mA, and the 2N3904 transistor gets hot, so had been working with 3V where it only took around 17mA.

To this simple blocking oscillator/flyback topography, I added the voltage doubler, and was disappointed to get only 550V from it. Thinking that I had no need for the spread voltage handling of three diodes per half-section of doubler, I changed the circuit to the more common form with just two diodes - and got under 400V!

Now, this has me confused! Why should I get a higher output voltage when I have more diodes, and hence more forward voltage drop, in series?

So, with this thought troubling me, I decided to try it on 9V. As expected, the transistor did get hot, though not as hot as previously. At a current draw of 37mA (interestingly having the ammeter in circuit lowered the output voltage as well) I was now reading a little above 1600V - 1800V without the ammeter.

Clearly this circuit is not at all efficient - in fact loading the mid-point of the doubler with a neon bulb and 4M7 resistor drops the output first to 40V, then slowly lower until there is no output, and the current drawn jumps to over 60mA! This can probably be improved by adding a feedback path to regulate the output. I have seen a Geiger circuit using this type of transformer, operating from 1.2V, admittedly supplying a standard 400V tube, and using a zener feedback system.

Update - The thought occurs to me that, with just the two diodes forming the Cockcroft circuit, the reason for seeing just 550V may have been due to the combined reverse breakdown of the two diodes! These are rated 1kV, but the pair operating across the AC supply from the transformer could well have been operating in a controlled breakdown on each half-cycle, resulting in the observed 550V. With three in series for each side of the doubler, the combined breakdown voltage of each chain of three would be 3kV - much more than the applied voltages, and hence the more sensible 1500V output.

The Owon SDS1102 100MHz Digital Storage Oscilloscope - Pt.2 - In Use

Before even starting acceptance testing, the first thing I did with this 'scope was to change the fuse in the kettle-lead plug for a 3A. Even this is big for the actual rating of the 'scope, but a damn sight safer than leaving a 13A fuse in there!

So with a safer fuse rating fitted, I powered up the 'scope, and set up my Geiger circuit as a test source.

Having folded out the two feet from underneath the scope, which weights about 1kg, and attached the scope probes, I first carried out the compensation adjustment to the probes, before connecting them to test points on the Geiger circuit.

In use, the 'scope is reasonably intuitive. Most controls are where you'd expect them and work as you'd expect them to work. The buttons have a reasonably positive action to them.

Blocking oscillator and flyback

Please forgive the poor colour rendering on the photos! I've used the same set of test points and measurements that I did with the DSO138 kit. So the first photo shows the trace of the blocking oscillator and the flyback pulse, the second is the flyback pulse expanded to show the inductor ringing.

Inductor ringing

The photo below shows the blocking oscillator with the FFT mode enabled, the top trace being the time domain, and the lower the FFT frequency domain.

FFT frequency domain function

Another option is to open a measurement window, which gives statistics of the signal being monitored. The menus, as with most menu driven devices, take a little getting used to. Most are accessed from a dedicated button on the main panel, then the options selected by the buttons beside the display.

Channel measurements option

And finally, both channels enabled, showing the G-M tube cathode connection, and the blocking oscillator. Here the side menu is shown as well for the Trigger options - the 'scope being set to trigger on rising edge 'normal' mode. The trigger signal in this case being the radiation event pulse shown in the middle of the screen.

Blocking oscillator lower trace, cathode pulses upper trace

All in all, so far, I'm quite happy with it. One thing missing though which would have made it very portable, is a DC supply connector! Despite its compact, lightweight portability - it still needs mains power.

Si-I-19BG G-M tube replaced and working!

Readers may recall that I bought an SI-19BG miniature Geiger-Müller tube from ebay seller 2015_Gurie in the Ukraine, and the delivered tube would not work. Some discussion with the seller resulted in an agreement to supply a replacement, on the understanding that if I couldn't make that one work (i.e. it was likely my fault not the tubes) I would return both.
Well, today at 15:50 - conveniently as I was acceptance testing the Owon oscilloscope, using the Geiger detector circuit as a signal source - the replacement tube was delivered.

After visually inspecting it and being happy it was undamaged physically, I connected it in place of the BOI-33 tube - and nothing happened! At this point I was a little miffed, but, I had a shiny new oscilloscope in front of me monitoring the cathode port - and yes, there were very faint, low level pulses! In fact, looking at the indicator LED of the Geiger circuit in the dark, I could see it occasionally flash dimly.

Ah-ha! So, this tube is doing something! So, what is amiss that prevents a good count pulse? The first thing to eliminate is the additional capacitance and resistance of the croc-clip leads used to connect the tube. With the first, faulty tube, I had an anode 4M7 resistor connected right to the tubes short anode lead, but this time, I was using the boards anode resistor, and a 30cm long croc-clip lead. So, I moved the anode lead of the tube to the anode clip on the board, and just used one croc-clip to secure it - and yes! Good, strong pulses and indications!

Of course, being such a tiny tube, its sensitivity is nothing like that of the BOI-33. Its background count is around 2cpm, compared with the BOI-33's 20cpm, but, it is a fraction of the size! It is also an end-window device, so is most sensitive from that end and little from around the body. This is fine though for a sub-miniature detector intended for sniffing out sources at charity shops and flea-markets! I think this tube will go into a circuit using 555 timers as HV generator and monostable indicator, but that will need to wait until I receive the project boxes from China.

NanoVNA and a curious kit

I spent much of yesterday Christmas shopping, which I hate (can't stand being in large crowds), but in the evening started to get to grips with the NanoVNA. But before that, I put together the perspex case kit that had arrived for the DSO138 oscilloscope.

Some instructions would have been nice!

This kit contained three more panels than I had expected, and absolutely NO instructions! So it took quite a bit of experimenting to discover the correct way to put it all together. From the photo above, the top right piece is the base - add to that the four long bolts and two nuts (to lock the bolts in position), the bottom left panel then goes over the main PCB but BEFORE the display! So, the display had to be removed for that stage. There were also four tiny nuts and bolt to secure the display to this panel. Once that is done, the two pieces with the large cut-out go on, these stack up and form the channels into which the button and switch levers fit. I found that these could move in use and make the switches stiff, so Ive added more nuts to secure them in position 8mm above the main board. Then the side panels go on, ensuring that the cut-outs align. This is easy for the BNC and power connector as the holes are different sizes, but the USB port cut-out is slightly off-set and needs some playing to find the right way around for the panel. The buttons and switch levers are now added, making sure the switch levers fit correctly over the switches. Finally, the top panel (top left in photo) goes on. A bit of jiggling is required here to align the eight studs in the side panels, and also the five button tops. The whole is then secured  with four dome nuts.

Actually looks quite good

 The completed case and kit actually looks quite good! I did find that the test point was now rather hard to access using the croc-clip probe, so used needle-nosed pliers to adjust this to make it easier. Other builders might want to take that into account during soldering and make a bigger loop!

An agreement was also found with the seller, concerning the faulty display, resulting an an acceptable partial refund.

So onto the NanoVNA. For such a tiny and low cost device, this thing is incredibly capable! However, when you first turn it on - it is rather daunting! It boots up in a full display mode - all four measurement channels, 50kHz to 900MHz range, making for a very busy and confusing display. Once you master the basics of driving the menu, which can be done via the touch-screen or the rocker control switch (which I actually found less reliable) and have turned off the traces you don't need, it becomes more manageable and easier to understand.

VSWR and Smith chart plot of a 2m rubber ducky antenna

So far the most I have done with it is to look at a 2m band rubber ducky antenna, but this is enough for finding your way around the menus, learning the modes and features etc. One thing I didnt expect, its charger port is a USB-C rather than the more usual micro-USB. Luckily it comes with a cable - as this is the only USB-C device I have!

I paid a little under £30 for this, shipped from the Far East of course. Compare that with the cost of an MFJ-259B, which doesnt display graphs, has to be manually adjusted for each frequency, is massively bigger, only goes to about 200MHz if that - and costs around 5x as much even second hand!

The Owon SDS1102 100MHz Digital Storage Oscilloscope - Pt.1 - Unboxing

Despite a bit of a worry over the seller and the risk that it was coming from China and not within the UK, the 'scope arrive safe this morning. Maybe I had been a little hasty over the weekend!

So, first impression is of the packing. The outer packing was a thick polythene shipping bag. No additional padding so only half-marks there! I suppose it was a bit much to expect double boxing!

Inside the bag the scope was enclosed in a sturdy cardboard box with plastic carry handle.

As supplied in box

Opening the box revealed the 'scope wrapped in a thick packing bag and positioned centrally by two moulded packing pieces. Down one side was the poly zippybag containing the instruction booklet and a CD/DVD, down the other side the bag containing the probes. In a space at one end of the box was the mains cable and a USB lead.

Split bag containing probes

I did notice that the edge seam of the probe bag had split, but this isnt of much concern. Inside it was two 1x/10x probes, with removable spring clips, a trimming tool for adjusting the compensation capacitors, spare springs, and different ID colour rings. The probes themselves, which are about 1 1/2m long, already had yellow and blue ID rings on them. 

13A fuse for a 15W device! Change that!

If you buy one of these, or in fact any mains powered item these days - check the rating of the fuse in the plug! This 'scope has a specified power requirement of just 15W - yet the plug has a 13A fuse! Swap that for a more sensible fuse as soon as you can! At the very least a 3A fuse - but even this is much bigger than needed!

All the controls were in good order, straight etc, no scratches or marks anywhere on the unit. The unit itself is only about 3inch deep, and has fold out feet, and a fold out carry handle. It weighs very little.

On switching on, the unit displays a splash screen while booting up. This takes about 10-15sec.

Initial start up splash-screen

As its late, and Im very tired from work, the furthest I am going to test it tonight is to quickly connect one of the scope probe leads to the 5V square wave internal test signal port. The two BNC connectors for the input channels have protective plastic caps.

Internal test signal trace display

As can bee seen, the probe needs its compensation adjusting! But that is only to be expected. Over the next few posts I will look at using this 'scope, and some of its features. Of interest is the 'MATH' button, which brings up a menu of things it can do with the traces - including Fast Fourier Transforms.

More Toys

Today, I took delivery of my new Owon SDS1102 100MHz Oscilloscope, and the nanoVNA. These will get tested over the weekend. The nanoVNA cost just £27 and looks to be an impressively capable little device - albeit with, for me, a very steep learning curve! The 'scope on the other hand has cost considerably more, and will be the subject of detailed future posts.

Something Ive not said much about is the Seafix 2000 RDF unit. This is considerably chunkier, and heavier, than the Euromarine Radiofix unit. I got this from an ebay seller and it came with the original headphones, which work quite well. As with the other unit, I need to find time to get it up onto the top of the local hill where it is radio quiet, and try it out properly.

Seafix 2000 marine RDF 'gun'

A few images of traces on the little DSO138 'scope kit now. I discovered an extra set of details that can be displayed by holding down the OK button when the timebase is highlighted. The trace below shows the HV generator on the Geiger circuits blocking oscillator, as measured at the switching transistors collector.

Blocking oscillator pulses

What is interesting, is what happens when the timebase is decreased and the trace adjusted to show just one of the blocking oscillators pulses -

Inductor ringing during oscillator cycle

It is possible to see the 'ringing' of the inductance as the induced currents oscillate. Having not worked with blocking oscillators and flyback converters before, I found this to be quite fascinating, and it really shows the value of a modern oscilloscope - even a £10 200kHz one!

Geiger circuit working

Having completed the oscilloscope kit, and with my car away at the garage for its MoT, Ive had time to finish the PCB for the Geiger counter.

This is now complete and working. Some slight alterations will be needed when I receive the case for it to go into, as the LED and loudspeaker will need positioning on the case, and proper battery connections, with a power switch, will be needed. I have also still to decide on how best to give a 'pulse out' connection for external equipment.

The case will need holes for the LED and the loudspeaker, and also for the HV ON neon indicator. It will also need a slot cutting along the tube side to allow the most radiation to enter. The boxes are on their way from China, so I guess I will get those when I get them!

Ive also now made the first use of the DSO138 oscilloscope in anger! Using it to look at the oscillator waveform and the cathode pulses from the Geiger-Müller tube.

Monitoring the flyback converter oscillator

I can see from this little gadget that the oscillator is pulsing low every 40ms, but with some seemingly random positive spikes. This equates to 25Hz, which seems rather low (I'd expected several kHz), but its not a worry, so long as its generating the HV supply!

A radiation event pulse

Having finally mastered setting the trigger threshold value (right hand triangle on the display), I can now get the 'scope to trigger on each pulse at the G-M tube cathode. With the cathode to ground resistor I have, each pulse is about 1.5V and decays away over a period of around 200uS - this equates well to the expected 190uS dead time of the tube.

So this little £10 oscilloscope is actually quite a handy little device! Its just a shame about the horrid bright white column of duff pixels!

Update - Some Geiger counter circuit measurements...

Pulse height - approx 1.6V; Pulse duration - approx 200uS; Standby current draw - 1.24mA; High count-rate (Radium dial at 5mm from tube) current draw - approx 3.1mA; Background count - 18-20cpm.

I'd like to get the standby current down below 1mA if possible. Thats something to look at in future.

Building the DSO138 Oscilloscope - Pt.3

Following on from the main board build, the next thing was to perform some voltage checks. With a 9V PP3 battery powering the board, these checks were completed satisfactorily and the various supply jumpers bridged.

At this point I installed the BNC, after giving my 45W iron time to warm up. This is one of the few issues I'm going to take with this build - there is no way the recommended 20W iron would ever solder this! Even with my 45W iron, it was all but impossible to solder the body tags of the connector. I would recommend anyone else building this to file the plating off of these studs before soldering!

Main board complete

So with all the main board now done, I installed the TFT display panel, and powered it up

1st Boot splash screen

2nd Boot splash screen

 The unit goes through two different displays while initializing. It then goes to the oscilloscope display.

Initial 'scope display, before calibration

Internal Test signal display, again before calibration

 In use, it is a little quirky at first - working out how to drive the menus. Options on-screen are highlighted using the SEL button, and then changed with the + and - buttons. The OK button in normal use selects RUNNING or HOLD mode. Input signal levels and coupling are selected by the three switches on the left.

Defective display column

All in all, its a fun and quite straightforward kit to build, and fairly easy to operate. There are header pins for UART and DATA, plus the mini-USB socket, which the instructions say little about, but which are probably for flashing updated firmware, and perhaps PC display/control.

The second issue I found, and the one which for me has marred the experience, is that one column of pixels on the TFT is permanently ON. This fault is probably a failed driver transistor in the TFT controller, and so likely not repairable. I've flagged this up with the seller.  Ideally I'd like a replacement, working display, but I will see what they come back with!

Ive not tried it with any signal other than the internal test square-wave yet, but I might use it to probe the oscillator in the HV generator circuit for the Geiger counter.

Building the DSO138 Oscilloscope - Pt.2

The nicely laid out glossy, full colour guide made much of the build very easy, but first, I had to get myself comfortable! So, with the iron and my cup of tea hot, I laid out the necessary tools and the construction guide,

Ready to start

Resistors fitted

One thing to note is that many parts connect to the extensive ground-plane - a little extra heat and a little longer with the iron tip on the joint is required,

Diodes, Inductors, crystal and USB socket

Ceramic capacitors and push-buttons added

LED, regulators and transistors

Electrolytics, power inductor, trimmers

Switches and connectors

At this point, the only part left to install was the BNC socket. Taking it very steady, and with a spot of lunch and lots of cups of tea in the meantime, getting to this stage took about 3h. I was going very carefully and methodically though, and following the guide to the exact letter.

Building the DSO138 200kHz Oscilloscope Kit - Pt.1

One of the things I think is wrong with how electronics is taught these days, especially in apprenticeships, is that the student is no longer required to start by making their own tools! In the old days of proper industry apprenticeship, a new apprentice would start in the toolshop, by being shown how to make a file. That file would then be used to make screwdrivers, and so onwards... resulting in proper engineers who could turn their hand to anything.

Likewise, in home correspondence courses of the post-war era, budding electrical engineers would start by building their own oscilloscope!

Of course, these days with modern ICs, its harder to understand exactly whats going on, but the idea of starting learning by building your own test equipment, starting I'd suggest with a simple analogue multimeter, is a sound concept.

Of course, so much is available as cheap and useful kits from the Far East these days. Which brings me to the DSO138 - a kit 200kHz single channel digital storage oscilloscope.

Bought on ebay for just over £10 (plus another £3 ish for the box), mine was supplied with a very nicely done PCB, with all the SMT parts already installed, leaving just the through hole parts for me to add. The paperwork consists of three quite glossy colour A4 sheets - circuit diagram, layout diagram, operating and fault-finding instructions, and a photographic step-by-step construction guide, in 'tick-box' style. Far better than I had expected.

Having a maximum bandwidth of only 200kHz means its really just an audio frequency tool, but at just a few inches in size, it could prove handy for simple tests and demonstrations. The resistors supplied are very small, and 5-band marked, so the advise on the instructions to "confirm with a meter" is to be followed!

Over the next few posts I will document my building of this kit, stage by stage.

Finally! What a Muppet! Working HV.

Well it's taken almost all evening (not quite, I've been to my youngest lads school performance, plus cooked tea for me and my eldest) but I finally discovered why I couldn't get the HV circuit to work...

Working HV circuit

After testing, and replacing anyway, all the transistors, testing the diodes, trying various open/short circuit connections, trying different bias resistor values, and even testing the timing capacitor(!), it was while trying the different bias values I spotted my mistake! The 100k resistor from the flyback diodes anode, was connected wrong - it was connected to the oscillator transistors emitter! Literally, on the wrong end of the emitter 1k resistor. It should have connected at the 'supply' end. You can easily see in the photo above of the board, where I have had to change it for a fresh one with long enough leads to reach the right connection point.

I'm very pleased to have got it working, as this board has been hand milled using my Dremel, so a lot of physical effort has gone into its making. I was starting to think I would have to abandon it.

With the neon lamp in the regulator chain glowing, and the ammeter now showing 1.2mA rather than 35mA (itself an improvement on the 1.8mA I was getting on the breadboard, probably from changing the BC327 for a BC212L with more gain), I'm now going to leave it until tomorrow to add the indication circuitry.

This circuit now has a random mish-mash of transistors in it, rather than the nice complementary pair it was designed with - a BC212L as oscillator, MPSA42 as switch, and BC337 as feedback. The feedback transistor may yet be changed again, the current design uses collector control feedback. When my supply of high-gain MPSA18's arrives, I may change this for base control feedback, to see if I can lower the current drawn even more.

Regardless, its late now, and I'm quite tired. Since towards the end of fault finding I noticed myself rather too casually holding the board by the G-M tube anode clip (in other words - the place that should have had 400V on it!) it would probably have been a little too risky to work on it anymore tonight!

Geiger PCB not going well

Well, I thought I had it sorted, but on powering the HV circuit on the new PCB it wouldn't work. I've checked the layout and the circuitry, and the voltages, and I can't see what is wrong with it, but it won't oscillate.

My suspicion is that I've killed one of the transistors, probably the PNP BC327. For some reason, I have trouble getting my head around the emitter NOT normally going to GND!

This is quite annoying as I had hoped to have it working this evening!

I'm still awaiting delivery of my new 'scope, but at least the seller has actually shipped it now! Amazingly though, the little kit oscilloscope from China has arrived!

Chinese 200kHz TFT Oscilloscope Kit

 The SMT components are all pre-mounted, only the through hole parts need soldering, so it should be a quick and easy build. I'm a little miffed that the display is not exactly squared on the PCB, but its not a deal-breaker!


Update - Well, both of the main transistors test fine! So at the moment I'm really stumped!

Tricky things these helicopter seats

Under orders from the management (Julie) I've had some time off electronics to finally fit the fold away seats in my Merlin helicopter.

I had been putting it off as they are very fiddly to install- being only 3/4 inch tall!

This is, of course, an Airfix 1:28 scale model! It is what my boys got me for my birthday nearly 2 years ago. I promised to do a really good job of it, so it is taking a long time. All the interior is now complete though, so it's back to larger parts which will be quicker. 

Another item of vintage RDF arrived today- a seafix 2000. Good working order, I've yet to take a peek inside to see what electronic architecture it uses. Instead, I've made a start on the first permanent PCB geiger counter. This will be done on a hand milled PCB, using my dremel tool.

Tube clips and anode resistor fitted

Thoughts on the SI-19BG tube

My seller has agreed to supply me another tube, on the presumption that the first has failed. This may not be the case, and if I discover otherwise I will return the spare, of course if I cant make either work I will have to return both.

A few thoughts on possible issues, other than complete failure -

1. Some tubes dislike certain current levels. A higher anode resistor may be required to get this tube to work;
2. Perhaps 400V is not enough, or maybe too much? Although if too much I would have expected it to avalanche. Characterizing it on a variable HV supply might find a working operating voltage. Available data shows 360 to 440V, and I have heard mention of successful operation at 440V.
3. Capacitance. Although too much shunt capacitance can kill tubes, most have an inherent capacitance of about 5pF - this tube being so tiny is 1pF. Perhaps a little extra capacitance will give it a charge 'kick' to help it to ionize.

Ive now received the 100Ω precision presets to allow me to properly control the 555 timer based HV supply. I will modify the Cockcroft doubler on this with an extra stage, to allow me to reach higher voltages. This I need to do anyway, as I also have a MST-17 tube coming, that requires 1600V!

SI-19BG tube disappointment

Earlier today I connected the tiny SI-19BG tube to the test circuit. Unfortunately, not one single count was registered. 

I obtained this tube from eBay seller "2015_gurie", and am now in contact  with the seller concerning a replacement tube.

It might be that I've just been unlucky and received a failed unit, or it might be an error in operation due to the lack of good information on these tubes.

I will transfer it to my variable HV supply and probe the tube with the scope, to see if there is any activity at all.

I've just today ordered a new, modern scope as well!

Ooh! Remembered I have a check source!

Before going to bed, I tried out the RC pulse stretcher on the breadboard.  With a 100nF capacitor, and various resistors from 2k2 up to 4k7, I can say it had no detrimental effect on the circuit.  Whether it actually stretches the pulses will need the oscilloscope to verify.

Complete Geiger mock-up

 Then I suddenly remembered - Bob M1BBV once gave me a WW2 fob watch! A bit of a hunt later (also finding the replacement strap for my Fitbit) I had it in hand and powered up the Geiger circuit...

...oh my! This thing is HOT! At no closer than 2 inches the circuit was going crazy! With the dial glass against the tube, the rate was almost more than the LED could cope with!

So, that's a nice bit of Radium I have.

RC constant practical use

I've been pondering ways to 'clean up' the pulses generated by the Geiger tubes, and make them longer and more uniform. To do this I've been revisiting RC constant and their practical applications.

For the Geiger tubes, and RC circuit can form a pulse stretcher. The 1st transistor in my interface switches on the 2nd transistor, which switches the indicator LED and clicker. By adding an RC circuit between the two transistors,  I can stretch the pulses out by approximately the RC time constant. Since the tubes dead-time is 190uS, choosing an RC constant for 200uS makes a good pulse size.

The capacitor I added to cure the flickering problem was 22nF, and is probably acting as an RC pulse stretcher anyway with the 1st transistors collector resistor. 

This got me thinking of another,  really useful RC circuit.  Many years ago I used to service pendant radio alarms made by Tunstall telecom. These used a momentary push button to trigger them, and a clever RC circuit to hold the power on only long enough to complete the transmission cycle. This clearly has many interesting uses. The circuit shown is, I think, how it works.

Momentary power switch circuit, I think

Annoyed - wrong TIG rod!

The long awaited package from eBay seller "mitools" arrived today - and it's the WRONG item!

It should of course be a 2% thoriated rod, colour coded red, and 1.2mm thick. What they have sent is a 2% CERIATED 2.1mm rod, colour coded grey. This is useless as a check source for the blindingly obvious reason of it not being radioactive!

So it's going back for exchange  and perhaps sometime soon I might finally have a check source!

I've ordered some MPSA18 transistors as well to try the base control method of regulating the HV circuit.  These have very high gain. 

The circuit diagram of my current Geiger counter is shown below. I also need to add an external pulse output to drive a counter or meter.

Simple Geiger 'clicker'

Flicker problem solved

Adding a small bypass capacitor across the supply, in this case a 22nF polyester that just happened to be loose on the bench, made no difference to the flickering LED problem (which I discovered also manifest as a low rapid clicking from the loudspeaker!). So, I thought to try it instead as bypass across the G-M tube cathode resistor. 

This solved the flickering problem, but created another - no more pulses! 

Clearly this was absorbing the pulses without triggering the indicator circuit.  So I moved it instead to bypass the base of the 2nd transistor - bingo!

Good, solid flashes and clicks with each pulse, but no more dim flickering!

22nF might be too high a value, so at a later time I will try and find the lowest value that gives good results. 

Improved Clicker - and an Oddity...

The original single transistor driving the indicator LED and sounder was poor. I have improved this by using the first transistor to drive a second, which then switches the LED and sounder. I have also replaced the small sounder with a 1.5" cone 8Ω loudspeaker. The result is a much louder click and a nice bright flash from the LED, to indicate a detected particle/quanta.

I did attempt to use a trick I've seen on others builds of similar architecture, that is, using an extra diode and capacitor to obtain an extra supply from the HV circuits, higher than the supply but low enough for safe use for the indicator circuits. This gave me around 6V, but the current went through the roof (35mA) and it was clear that the drive oscillator was not cutting back as it should. For the moment I have abandoned this approach. The 3V circuit I'm using seems quite adequate.

But there is an oddity... the LED is always very dimly lit, and on close observation, can be seen to be flickering. Now, if I touch ANY part of the low voltage side of the system - the LED goes out, except when pulsed by the transistor switch. And I do mean anywhere! I can put a finger on the +Ve supply connection, or the ground, or the battery itself! However, if I touch the potential divider input to the 1st indicator transistor (i.e. the G-M tube cathode connection), the LED lights with a very bright fast flicker!

It seems quite obvious that the cause of this is leakage of the HV generators oscillator. I suspect that some bypass capacitors will be required to tame this.

In this configuration on the breadboard, the circuit draws just 1.63mA from the 3V supply. This is with the regulator transistor controlling the oscillator transistors collector. I might be able to get it lower, and improve the regulation, by controlling the base, but that's an experiment for later, as is lengthening the output pulses to a fixed pulse length for an external counter interface.

1st working Geiger counter mock-up

This is the first breadboard mock-up  Geiger counter working. Background count is about 20cpm using a BOI-33 G-M tube. Anode voltage 400V, supply 3V.

BOI-33 G-M tube mock-up

The indicator section of the circuit is a simple transistor switch. Its nowhere near good enough for a finished project - the LED flash is too dim and the sounder 'click' is far too quiet. There is also too much leakage through it meaning the LED is very dimly lit all the time. This section then needs more gain, and perhaps a pulse extender to allow a much more robust indication.

The current drawn from 2xAA cells in the absence of a pulse, is about 1.8mA. Its a bit higher than I would like, but quite acceptable. Ideally, I would get the quiescent current down to under 1mA, which might be achievable with some component value changes. Ive also tested this circuit with my huge STS-6 G-M tube, and it works great. Background count is appreciably higher of course, due to the much larger sensitive area of the tube.

Clearly though there are things with the breadboard version that will need changing for a complete, enclosed project.  One thing to change is the high voltage capacitors - the ones used on the mock-up are about 4x the physical size of the ones i've recently bought, yet have identical ratings. The neon lamp as part of the regulator does take up more space than zener diodes, but I will keep that - the glow is a perfect indicator that the circuit is live!

More electronics from behind the curtain

I'm old enough to remember the fall of the Berlin wall (though not as many would have it the abandonment of Hadrians!) and it's because of that moment in history that today I could take delivery of a Soviet era SI-19BG Geiger tube.

This tube really is tiny! I would never have thought that a G-M tube could come through the post in a normal envelope! And that's wrapped up in bubble wrap!

I've done some tests on the breadboarded HV circuit now I have the 1Gohm resistor. The 3V circuit has trouble cutting back the current if final output regulation is used, but seems to happily give around 400V with the regulator chain connected at the first multiplier, dropping the current from 4.5mA to about 1.5mA.

I'm sure I can get this lower, with a bit of circuit tweaking. Right now though, the daft 11M impedance of my DMM means that the voltage reading on the LCD is not quite accurate,  so I hope to work out a way you correct this using parallel resistance to get a true 100:1 divider chain. I might check the impedance of some of my older less useful DMMs - I might be able to make a dedicated HV probe meter.

Edit - Well Doh! It isn't a parallel resistance I need! Its more series resistance of course! 1089MΩ in fact.  With the 1GΩ resistor this means I can make up the extra using standard low cost, low voltage resistors, plus a preset to allow precise calibration.

STS-6 tube arrives

The huge STS-6 tube has arrived safely from Volgograd, Russia- yet I'm still waiting for the delivery from RS Components!

I had hoped to have had at least one Geiger circuit working by now, but the late delivery of the 1GΩ resistor, plus some short notice IT work, have delayed me.

PIN diode radiation detectors?

The Geiger-Müller gas discharge tube is of course only one of several ways to detect ionizing radiation. It is perhaps generally the most convenient - its sensitive area is greater than optical systems; its much lower cost than photomultiplier tubes and scintillation crystals; and its a heck of a lot more portable than a cloud chamber!

But I'd quite like to try all the methods that are within my reach! One of these, the cloud chamber, I am working on. I have suitable high current power supplies and heatsinks for a small chamber cooled using Peltier effect devices - I'm just awaiting delivery of those devices! The cloud chamber is of course the best for visually demonstrating radiation, as the paths of the ejected particles are visible to the eye.

Another method is by detecting the impact of a particle or energy quanta on a semiconductor junction. Most junctions, for instance a TO-92 transistor, are tiny, but there are a number of PIN photodiodes that have rather large junction areas, in the region of 5-7mm² which, while pretty small still, represent a much greater target area. One such, the BPW34, has been used in several simple detector circuits, and can be obtained for very low cost. As the likelihood of a particle event is still quite low, wiring them in parallel to increase the effective surface area is a trivial matter. Ive ordered five for less than £1.50. My intention is to build all five into a detector, but perhaps allow switching to select the actual detection area.

Of course the big problem with using PIN photodiodes for radiation detection is keeping them from detecting the radiation they were designed for - light! And extremely light-tight enclosure is needed, plus any indicator LED has to be very well isolated optically!