Latching relay driver with CMOS - difference between these two circuits?

[matt_garman]

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?

[bluelagoon]

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 -
https://www.freestompboxes.org/viewtopic.php?t=31684&sid=cfa1ec3ce2be598ad191301a0ed79cbe

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.

[matt_garman]

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!

[FSFX]

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.

[bluelagoon]

Hi Matt, As for the concerns using just a single mosfet as the temp mute switch, the remedy to that is to use either 2 mosfets together or an optocoupler.
This eliminates any concern for grounded diodes causing clipping on the signal path. See the following image and web link for further explanation.
Also refer to FSFX response in the post above.
Cheers.

https://www.diystompboxes.com/smfforum/index.php?topic=130300.msg1263736#msg1263736

[bluelagoon]

Hey FSFX, I was just wondering what you meant in what you said in that earlier post concerning the diodes and any potential clipping. Were you referring to the 2 mosfets that I had used for temporary muting function as shown in my posted circuit this thread?
You say that for the 2 substrate diodes in series with opposite polarity would only clip at the breakdown voltage of the substrate diodes, being in most cases over 60 volts.
Does that mean since these are being run in lower voltage type circuitry, that then they would never reach a clipping stage due to never being pushed to a 60 volt breakdown voltage?  Thanks, could do with some clarification on your meaning there.
Cheers

[matt_garman]

Based on @bluelagoon's incredibly helpful post, I created some PCBs of this momentary-switch controlled relay bypass circuit.  I created two versions, a "lite" version that omits the muting circuitry, and another that includes the muting function.

The only change I made to the design was to add another PNP between the logic gate and the relay-driving circuit.  The intent is to reduce any potential loading of the logic gate.

I used my "lite" PCB for a build I did, you can see it in my Snozzberry build report.  I've been using it daily for a couple weeks now; it works great!

Here is the schematic of the "lite" circuit:



The nice thing about this circuit is it's fairly simple, and uses readily available commodity parts.  But it is a lot of parts.  I also worry about the long-term effects of using the capacitor for the current-pulse: the duration and voltage aren't precisely controlled.  If the pulse voltage is higher, or the pulse longer, than the relay design specifies, will that reduce its longevity?

That said, if anyone is interested, I'm happy to share the Gerber files for my PCBs and/or the KiCad files.  I also have plenty of spare PCBs, I'm happy to give those away, PM me if interested.

Also:

Hey FSFX, I was just wondering what you meant in what you said in that earlier post concerning the diodes and any potential clipping. Were you referring to the 2 mosfets that I had used for temporary muting function as shown in my posted circuit this thread?
You say that for the 2 substrate diodes in series with opposite polarity would only clip at the breakdown voltage of the substrate diodes, being in most cases over 60 volts.
Does that mean since these are being run in lower voltage type circuitry, that then they would never reach a clipping stage due to never being pushed to a 60 volt breakdown voltage?  Thanks, could do with some clarification on your meaning there.
Cheers

I think I can answer this by looking at your previous post, with the optocoupler and dual mosfets.  The way I look at it, with the "trigger" (i.e. mosfet gates) off, the circuit essentially becomes this:

Code Select Expand


---->|-----|<----

I.e., two series diodes, with reverse polarity.  Generally, current won't flow through such an arrangement, unless the breakdown voltage of the diodes is exceeded.  At least with solid-state pedals, we should never get anywhere near 60v, which makes such a diode arrangement effectively open.  Based on your image, looks like the optocoupler works on exactly the same principle, except the trigger is light-controlled (as opposed to voltage controlled).

[bluelagoon]

Hey thanks Matt for you explanation on the diodes array, had thought that was what FSFX was alluding to.
And nice work on your redraw adaptation on that switching circuit, looks good, glad its working well for you.

Stumbled upon this recently, Its another CMOS type switcher that replicates the old mechanical type radio buttons, very cleverly designed. You can also replicate the same with Microcontrollers, but this using cmos, is another option without the programming needed'

https://www.instructables.com/Electronically-Interlocking-Radio-Buttons/

https://content.instructables.com/pdfs/EHL/MC3L/K2I415YA/Electronically-Interlocking-Radio-Buttons.pdf

Cheers.