How to kill the clicks while bypassing effects using a relay?

[tss]

I've installed a simple bypass circuit inside an overdrive pedal I had built. The circuit is pretty much the one that in shown here:


The problem I am having is that I can always hear a click while switching (on and off). The order of execution of my code is:

1. 5V through 1.5k resistor to the LED of the TLP222G
2. Wait 20ms
3. Switch relay state
4. 0V to the TLP222G (close)

I have tried running just the relay and just the opto-mosfet and they both make a noise while switching. The opto-mosfet is actually louder I think. The grounded terminal of the TLP222G goes to the output ground tab of the output jack.

Any ieads what could be causing this? Any help is appreciated.

[GibsonGM]

I'd look up 'switch popping' in the search function and try some of the cures out there. Common problem.   Two things come to mind...
1)  do you get any DC at all in your signal path at that relay?  If so, that's a problem that must be corrected (bad/missing cap...)!   The other way pops can get into audio paths are by induction and capacitive coupling...

2) you can add a cap or R/C network around the stomp switch to 'ease' the transition when you switch.  Ideas for this are all over the place by using search.

Clicks/pops are caused by a change in DC levels when you suddenly connect two things together...differences in potential.  They are fixed with pulldown resistors and caps to 'pad' the rush of current when you switch.   You may also make sure your grounds are good and done correctly.

[R.G.]

Is your logic ground connected to your audio ground?

If so ... how?  What currents flow on which wires?

[tss]

Regarding caps, there is a cap on the output of the effect circuit, that should block the DC (it's also pulled down). I think that even if I did have DC there, when I am opening the mosfets any current will flow though them to ground since they have about 3 ohms resistance when on so for practical reasons I think they are almost as good as a short. I keep the mosfets open up until after the switching is done.

RG - I actually connect the digital and analog grounds at a single point and from there they go to the output jack ground.

[tss]

Just hooked up a scope to my signal to see what I can see there and there's no DC, however there is about 130mV rms of 50Hz that are not coming from the effect since runs on 9V battery - so from the amp.

[R.G.]

ooops.

[tss]

This is not great... what can I do about it, could this be causing the problems with the switching? (50Hz is pretty much DC in many perspectives).