This application is for an analogue synth: it uses a 555 based clock and a 4017 counter to cycle through a set of voltage levels which are set by pots. The unusual aspect is that it displays the step of the sequence on a Nixie tube. A separate circuit is used to generate the
180V for the Nixie, and the digits are switched by MJE340 transistors. The HV supply is on a separate board from the sequencer itself.The circuit diagram is here:
(I "designed" this myself, and am by no means an expert - so please be gentle.)
The problem I have is (not surprisingly) voltage spikes on from the HV supply appearing on the output of the sequencer.
This set of scope traces shows, at the top, the HV supply (20V/cm,
2mSec/div) and on the bottom the sequencer output with the clock disabled (0.1V/cm, 2mSec/div).I seem to have about 150mV of spikes on the output :-(
Things I have tried: decoupling the HV board +9V supply rail from the sequencer board by a 100 Ohm resistor, and various smoothing capacitors across the 180V supply ... with very little success.
I'd welcome some suggestions on how to reduce the interference. I did wonder about using some optocouplers I have lying around, but maybe that's a bit far-fetched?!
Thanks, Julian