# Multiplexer not switching any audio input

I have designed this audio switch. 1 out of 4 TRRS inputs is selected.

I am testing it but it does nothing.

I press the switch to switch the next channel and nothing works.

The idea here is:

1. the 555 debounces the key and a pulse is injected on the flipflop when the button is pressed.
2. the flipflop is a 2 bit counter, producing a sequence from 00 to 11 as the button is pressed repeatedly.
3. the two bits from the flip flop selects one of the four channels of the MAX359E switch.
4. The leds display 00, 01, 10, 11, depending on the input selected, obviously... 😃
5. I am not sure what to do with the mic input in relation to the input/output capacitor. This is because some TRRS inputs, like iPhone's, send a voltage on the mic line, to supply the mic. If I put a capacitor it will remove that voltage...

Any ideas why it is not working? I may have connected something wrong on the test board. I have double checked the connections several times but at this point I may be tired and not seeing the problem or the circuit is bad designed...

Any ideas? Thanks

EDIT: I have modified the circuit according to WhatRoughBeast suggestions.

• To debug , check power and gnd on every IC then using a DMM record every input and output to see what it is doing. If your mic's are electret types, this wont work. – Tony Stewart Sunnyskyguy EE75 Mar 8 '18 at 1:33
• Also check input wiring cablechick.com.au/blog/understanding-trrs-and-audio-jacks – Tony Stewart Sunnyskyguy EE75 Mar 8 '18 at 1:36
• When you say "nothing works", exactly what do you mean? What are you connecting the output to? Are you driving a speaker? A scope? A meter? If it were working, what would you expect to happen? – WhatRoughBeast Mar 8 '18 at 14:55
• @WhatRoughBeast - I mean this: I have an iphone. I connect the headphone output of that iPhone (TRRS plug) to entry #1 of my device. I connect my headphone to the output of my device. I push the button of my device to select input #1. I do not hear it on my headphone. – SpaceDog Mar 8 '18 at 15:04
• @SpaceDog - Please see my answer edit. TL:DR - You can't drive headphones. – WhatRoughBeast Mar 8 '18 at 22:46

The most obvious thing I see is that your mux inputs are not ground-referenced. That is, you should have a largish resistor to ground on each analog mux input. If you don't, there is no way to control the "resting" level of the inputs. Try something like 100k.

Second, tie your floating R and S inputs on IC6 to ground. NEVER leave a CMOS input floating - you won't believe the weird things that can happen.

EDIT - In response to a question in the OP comments, you stated that "doesn't work" means that you get no sound from headphones.

Well, there you go. You're not going to, either, with the MAX359. I'm going to assume that you understand impedance/resistance. If you look at the datasheet characteristics (page 2), the very first line is ON resistance, and you should note that typical will be about 1.2 k. A headphone (earbud) will typically have an impedance of about 32 ohms. This means that the signal power at the headphone will be about (32/1200)^2 of normal, or something like 1/1000 of what you expect.

You might try something like this: start by cranking your iphone volume all the way up. Now, make sure that your area is really, really quiet - and I do mean quiet. No background music. Wait a few minutes listening to dead silence. Now put on your head phones and listen, switching between channels and see if you can't hear a faint signal on your desired channel.

The MAX359, and virtually all analog mux ICs, are simply not intended for power switching. They are intended to drive high-impedance loads, such as the input to a subsequent amplifier stage. So, I'd suggest you get an amplifier stage to connect your output to. And a simple op amp won't do very well either, since their outputs are usually not real happy driving loads of less than 500 - 1k or so. Plugging into a headphone amp would be close to ideal.

END EDIT

• please see the circuit now and tell me if this is what you mean? I will test it as soon as I get home. – SpaceDog Mar 8 '18 at 1:00
• @SpaceDog - Yup. – WhatRoughBeast Mar 8 '18 at 1:03
• @SpaceDog - While I appreciate the vote, you might wait until you've tried it to see if I'm right. – WhatRoughBeast Mar 8 '18 at 1:13
• BTW, do you suggest another IC instead of MAX359 for that job? – SpaceDog Mar 9 '18 at 15:02
• @SpaceDog - Nope. What you need to do is provide a stage of amplification after the multiplexer. – WhatRoughBeast Mar 9 '18 at 22:41

I would venture to say that it’s not working because you aren’t generating the -5V rail. The input pin to the negative regulator is floating. You can test it by removing the power jack and adding to 9V batteries as shown below

• what do you mean and what changes do you suggest? – SpaceDog Mar 8 '18 at 15:06
• Your 7905 Vin is only connected to a capacitor from the power jack. No direct connection is shown, therefore the 7905 does not have power connected to it. – DIODEX Mar 8 '18 at 15:18
• you are right! DAMN!!!! You mean this? – SpaceDog Mar 8 '18 at 15:47
• Correct your IC15-Pin2 is not being powered. But also looking a little bit more into your schematic, I don't get it. So you have 2 input jacks that you want to select from to output to a 3 jack. The MAX359 is only one direction, so why would you want to multiplex the speaker (being an output) from the input jacks? I see how you would want to multiplex the MIC as they are inputs into your circuit and multiplex to the output of the speakers in jack J11 – DIODEX Mar 8 '18 at 15:58
• no I have four inputs. Each input is 3 inputs (mic, left, right) from the TRRS jack. – SpaceDog Mar 8 '18 at 16:06

You have miss-wired your ground connections, and should only need one MAX359 IC to do all the work. You should not be switching signal ground, only the left and right signals.

Follow your signal grounds closely and you will see the output is not wired the same as your inputs.

• I am not switching the ground input. That was the mic. This is TRRS (mic, left, right, ground), not TRS. – SpaceDog Mar 8 '18 at 0:40
• Shouldn't TRRS be left, right, ground, mic in that order? The socket on the right looks wrong. – Transistor Mar 8 '18 at 22:21