Prototype PCBs

Well, I have to say, im rather impressed!

I have been using the online EasyEDA to design the PCB layouts for the various Clansman projects im working on. Now, as much as I dislike the way EDA software uses device package symbols that bear little relation to how I was taught to lay out a schematic, you cant deny that the advantage of being able to let the software design the circuit boards takes a lot of the hard work of prototype design out. But what has impressed me is something that I can be forgiven for being skeptical of - that EasyEDA is 'tied in' with a Far East supplier of PCBs and components.

Having designed the SMD PCB for the PRC344 battery warning circuit, I decided I would take advantage of the offer of 10 PCBs for just $2 (about £1.60), to see how they came out.

I did make a bit of a mistake by not spotting which shipping service was selected, and got an invoice for well over £20! But having corrected that, I decided that the cheaper shipping, which brought the total up to £6, was still worth the risk. The first thing that raised an eyebrow was the near flawless tracking - the entire process was tracked! But even more surprising was being told the boards were etched and ready to ship just 2 days later!

They took about a week to arrive, which really isnt bad at all from China! And, as it turned out, most of the delay was by Royal Mail! And I have to say, they do look pretty good

Due to their very small size, the silkscreen print is rather limited, and given a choice I personally would have made some of the track a little wider ( I could have done that, but had no reference from which to decide how much thicker to go from the default).

Next step of course is to make one up and test it. Unfortunately, I am still awaiting delivery of several components, not least the LM358 op-amp that is at the heart of the circuit!

I gather that the low cost offer for 10x PCBs holds for boards up to 100mm x 100mm! My boards are about 15mm x 25mm! If I could just find a way to layout SMT parts on both sides of the PCB, I could create some amazingly compact circuits!

But, there are difficulties! To make these boards fit in the only safe available space in the PRC344, I had to choose 0603 sized parts. These are damn small! And to attach them to the board im using solder paste out of a tub! This is a new idea to me - in the past whenever ive had to rework SMT, ive had access to solder paste in dispensing syringes and a controlled pump, plus hot air reflow equipment. Here, im having to apply the paste with the tip of a probe, and then melt it with the tip of my soldering iron. Add on the fact that I dont have access to the high power magnification I used to use either, and it all makes for a rather tricky job!

But, that said, ive made a start. Im making three up at the moment, one is for my '344, the other two for further test and evaluation.

Fitting the miniMELF zener

The Zener diode is a miniMELF package. These are tricky buggers, as they are little cylinders and apt to roll off! As the only parts that I currently have stocked are the zener, opto and capacitor, these are the only parts ive been able to install just now. The capacitor is just about visible in the photo below!

zener, 1uF capacitor and optocoupler installed

 Each was then tested for continuity to ensure there were no shorts underneath! Once completed and tested, then connecting wires will be measured and attached, and the whole board housed in a length of heatshrink tubing. As nice as it is to be able to see the electronics, putting it into heatshrink will ensure it cannot cause any problems inside the radio!

As this design uses fixed resistors to save complexity and cost, its likely that a little compromise will be needed as to the exact threshold voltage at which the warning is activated. I'll aim to make this as close to 20v as I can, but it depends on where the standard resistor values come out. If I cant make it close enough with standard values, then one option is to parallel standard values by mounting them on top of each other! All that can be determined though using standard through hole parts on the breadboard once the DIL LM358s arrive! Of course, if I decide to offer some of these as kits, then the buyer can experiment as they see fit with the values of RA and RB, likewise, if they are not happy with the duty cycle of the alarm, they can play with other values as they wish.