Touchy circuits!

Having received the LM358s, I set about checking the circuit for the PRC344 battery warning would behave as required on the breadboard.

Working now with just a single 8-pin DIL device meant that much of the circuit was scrunched up on the breadboard, making it tricky to see if everything was connected properly, so it was no surprise when it didnt bloody work!

After some time fiddling and testing, I discovered that the battery level detector side of the circuit did work just fine, but the astable multivibrator side didnt!

This now was where it got strange! The astable would work, but only if I had the tip of the multimeter probe touching pin5! Or, even weirder, if I myself touched pin5 or the leg of the resistor connected to it!

I checked everything! I tried different positions on the breadboard, different feedback resistors, battery supply, capacitors bypassing the pin to ground, capacitors bypassing supply to ground, even THREE different ICs! The damn thing just would NOT work unless I touched it!

By now, I was starting to think the worst! Ive got around £20 of PCBs and components tied up in this! What the heck can be wrong? Are the ICs junk?

No. It turns out, with thanks to MrBungle on the UK Vintage Radio Repair and Restoration forum who pointed this out to me, that pin5 in this case requires a 'virtual ground', made up of the two feedback resistors I had, plus another to the supply rail! I nipped out and added another 100k resistor, and lo and behold the bugger is oscillating again!

This means of course that my shiny PCBs are already out of spec and need modding! But, an additional resistor is hardly a big mod and wont make much difference to the space requirements. One aspect of this though is that the circuit now operates correctly, which it seems it wasnt doing with the 741 op-amp, and gives a 50/50 3sec duty cycle. Making it now do the 10sec off/2sec on that I previously had, requires even more modification, with the addition of at least another resistor and a diode, most likely two diodes. I can of course do this on the existing run of PCBs but its going to start getting a bit messy! If the board does prove to have some demand for it, then a mkII version will have to be made with the extra parts.

For now, im just glad that the damn circuit does in fact work! And yes, the big problem of the optocoupler being always slightly on has been solved by using the LM358!