Author Topic: Latching relay driver with CMOS - difference between these two circuits?  (Read 3559 times)


Okay, since this thread has come alive again, Here's my 2 cents worth, after countless hours of trial and error testing on the CMOS switching scenario. Here are my results, one circuit for Bypass @ Startup, and one circuit for FX on @ Startup, power on.
Both Circuits have a dual Temporary mute function that sends to GND the signal @ switch trigger, to minimize any switching audible clicks, thwacks, thumps, etc.. These circuits are tested and work well, hope this helps for any who are not yet ready for the MCU Switching Bypass scenario.

Thanks for posting!  If I'm not mistaken, that's a tweaked and more feature-filled version of Circuit B from the original post, right?

The second inverter (IC1B) and mosfets are responsible for the muting during switching, correct?  I.e., if you removed those from the circuit, you'd be left with a viable switching scheme that may suffer pops/thumps/thwacks during switching.

I'm having trouble understanding how that first inverter (IC1A) doesn't always see a "1" and thus always output a "0"?  The switch is a momentary switch, right?  I can see how the inverter would output a "1" momentarily when the switch is closed, but once the switch re-opens, won't the inverter once again go back to constant "1" input and "0" output?


Hi Matt,
Yes it is a kind of reiteration on the circuit in the first post. Most of my understanding and concept of this type switching came from "Mictester "over on the Freestomboxes forum, there's a great thread there that details a lot on this type switching and various ways to achieve the end result , see the link -

Only there was never a way to reduce the inadvertent switching  clicks, thwacks and the like. So I utilized the concept of the temporary mute type arrangement where one temp mute circuit activates on switch in and the other on switch out, which is same ideally as how its done with an MCU microprocessor and also in various other audio equipment to keep out the unwanted switch clicks. If you want to test the efficiency of the temp mute, just add in a resistor from the Mosfet drain to an LED Cathode, with LED Anode to a positive voltage, this will give an indication on the time span of the temp mute and show it working adequately, it is pretty effective in reducing the switch noise issues.

And yes it is IC1B pin 4 of the 40106 that activates one side of the temp mute circuit either in or out, depending upon whether you are using the Bypass @ startup or the FX on at Startup circuit, they both differ in opposite ways where the mute function works,
The second side of the temp mute circuit comes off the Relay Positive input, where the charge into the 220uF Electrolytic allows a short pulse that activates the mosfet open to allow temporary GND to the Guitar Output Signal.

As for the way the circuit works it has an active "High Reset" circuit in components D4 and C4, which force the first stage schmitt trigger to always start up in High positive output, which is held in place, the second stage at pin 4 starts up in a low state GND, 0V. Then when the momentary switch is pressed the first stage goes low and the second stage goes high, always being held at the voltage accordingly.

Put it on a breadboard and see how it works, its a reasonable alternative to the Microprocessor pathway to True Bypass Relay Switching with a mute function. And there aren't a whole lot more components, especially if you end up shrinking them all down via SMD components, the circuit almost fits on a postage stamp.

« Last Edit: March 11, 2023, 01:20:57 AM by bluelagoon »


Thank you for the explanation, I appreciate it!  I definitely plan to play with this circuit.

I came here via that mictester thread you linked, as I've gone down a bit of a rabbit hole researching relay-based switching schemes.  Regarding those momentary muting mosfets, in that thread you mentioned, around here, concern was brought up about implicitly having diodes in the signal path, and the potential to clip - even the bypassed signal - if the voltage swing is big enough.  Do you have any thoughts on that?

Thanks again!


With the dual MOSFETs and MOSFET relays the two substrate diodes are connected in series with opposite polarity so clipping would only occur at the breakdown voltage of the substrate diode. In most cases that would be over 60 volts. A lot of the higher voltage MOSFET relays are designed to work with up to 400 volts across them safely.