Mute BMW head unit

Thread Starter

conandrum

Joined Mar 10, 2024
20
Hi all, this is my first post here:

Mute button:
My BMW head unit has a mute/volume button.
The encoder, when pushed-in mutes the head unit.
Today, I accessed the encoder and soldered 2 wires on the 2 pins which join when the encoder is pushed-in.
See photos/video.

Triggering:
I have a triggering device, which has a mute wire.
I asked support: "Can you at least tell me what the mute wire does when your device is alarmed?"
Answer: "normally ist sends ground." (they are German)
(I have not confirmed this, but I am going by what they said)

Plan:
So when the trigger wire goes to ground, the 2 wires must be joined (momentarily at least).
(usually we push down momentarily to mute - don't know if pushing down continuously is ok but I will test tomorrow).

How do I do this? Some sort of relay?
Need some help here as I am not familiar with all the devices out there, but if you tell me what I need to buy, I can put it together np.
 

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Ya’akov

Joined Jan 27, 2019
9,248
Welcome to AAC.

Two things:

1. Isolation—you should use an optoisolator to galvanically (electrically) isolate the output from the mystery device from the pins on the encoder. This is pretty important. Sacrificing an optoisolator, should that occur is better than sacrificing a BWM part—nearly any BMW part.

A relay will function as an isolation device, but it is overkill because the signal on those two pins is going to be very low current. It also won’t protect from a high enough voltage to make it across the contacts. The relay alone would still work and probably be almost as safe that the opto, but why risk it? You seem willing to do things right, so I am suggesting it.

The relay I would use would be a small reed relay designed for logic level operation. These relays are very reliable, and use very little power. A relay like this one, available on Amazon, would be ideal. You can get them cheaper from real distributors but with time and shipping the Amazon route might be best.
1710156847142.jpeg
NTE Electronics R56-7D.5-6
This is the size of an 8-pin DIP IC

2. A one shot—you should put a monostable multivibrator also called a “one shot” between the alarm wires and the relay so it only closes the mute wires momentarily. Even if holding them closed doesn’t seem to cause problems, down the road odd behavior and apparently unrelated troubles could stem from it. Better to simulate “normal” operation when hijacking a function like this—if you are going to do it at all.

The one shot could easily be made from a 555 IC (cheap, ubiquitous) and the information on how is almost literally everywhere, like here. And you can get lots of help doing it, also here.
 

Thread Starter

conandrum

Joined Mar 10, 2024
20
Wow guys thanks for the input.
First off, I just measured the voltages on the 2 pins of the encoder (connected to the 2 wires). One is 4.9V (reference I guess) and the other is 0V.
Secondly, I tested the muting behaviour and it is most reliable when I touch the wires for a split second. Touching them longer than that does not always result in a mute toggle. Therefore, a "one shot/monostable multivibrator" is an excellent idea Ya’akov.
Thirdly, I do not know the mute wire's voltage (on the mystery device) when triggered/untriggered, so I will come back with that info.

If possible, I would prefer going to a shop (or ebay/amazon) and asking for readily available parts which I can solder together, instead of building 555 circuits.
And yes I have 12V available, which turns into 14.4-15V when alternator is running.

BTW. A guy at electronics.stackexchange made this diagram for me, but as I said, I do not have the inclination of putting a bunch of electrical components together. Something simpler and readily available on amazon maybe?
 

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Thread Starter

conandrum

Joined Mar 10, 2024
20
"one shot/monostable multivibrator" on Ebay"
3PCS New DM9602N DIP-16 Monostable Multivibrator Dual Retrig One Shot
Fairchild DM74LS123M Dual Retrigglerable One-Shot Multivibrator, SOIC-16, Qty.10
DM74LS123N Fairchild Dual Retriggerable OneShot Monostable Multivibrator 74LS123
5PCS CD14538BE CD14538 CMOS Dual Precision Monostabl - New IC

I have no idea which to buy and how to connect them.
I would appreciate if you could tell me which one if appropriate and how to connect it to the relays you recommended earlier?
Very much appreciated!
 

Ya’akov

Joined Jan 27, 2019
9,248
A brief search suggests that this is your best bet from amazon. Not the smallest thing—though not enormous either, but you can bury it once it is working. The “P1” mode is what you want—it does a bunch of different things. Drok is a decent, longstanding brand.

There is a video that demonstrates connection and adjustment. If you do get it, don’t bury too deep, in case it needs adjusting.
 

Ya’akov

Joined Jan 27, 2019
9,248
That looks perfect.
However, can it get triggered by a Ground instead of a +ve voltage as in the video?
You can
That looks perfect.
However, can it get triggered by a Ground instead of a +ve voltage as in the video?
This PDF has a lot of information in it. You will see that the “low level trigger” option uses a very tiny current. If you make sure the signal ground on the timer is common with the ground used by the triggering device, connecting the low level trigger output from the relay to the triggering device‘s trigger line should work for you.
 

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Thread Starter

conandrum

Joined Mar 10, 2024
20
I noticed that the images in the pdf look different to the one on Amazon ie. more connections on the left side.
I modified the image on Amazon to show where the encoder would connect ie. right side, bottom 2 poles.
When the relay is triggered the 4.9V pin will join with the GND pin. Right?

Program: I am guessing P1 is fine.
OP: I should set operate time to a split second like 0.1s.
CL: not required for P1
LOP: not required for P1

I will make a final diagram showing the mystery device and connections.

Low level trigger:
If you make sure the signal ground on the timer is common with the ground used by the triggering device, connecting the low level trigger output from the relay to the triggering device‘s trigger line should work for you.
Are you saying that if I use the car's USB to power the DROK, then the low level trigger might not work?
 

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Thread Starter

conandrum

Joined Mar 10, 2024
20
@Ya’akov
Hey... I have been thinking about the device you recommended and its modes of operation.
Until a few moments ago I thought that the first and simplest mode would suffice.
ie. P1: Relay will turn ON for time OP after getting a trigger signal and then turn relay OFF.
The input sighal is invalid if it gets a trigger signal again during delay time OP.

However after some thought, when the 'mute wire' will drop to GND it could be for a number of seconds.
Therefore, if I choose P1, this means that :
1. 'mute wire' drops to GND for 10 seconds
2. trigger signal sensed
3. Relay will turn ON for time 0.1s
4. Relay will turn OFF
5. will go back to #2 and repeat? (If so, then at #3 it will UNMUTE the radio!! Right?)
or for the trigger to be sensed a second time, it has to go High and then go GND again? (in this case it will work properly)
 
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Ya’akov

Joined Jan 27, 2019
9,248
The way I understood it, it will need the trigger to change state before it can be retriggered. But testing will show, and it if doesn't work it can be worked out.
 
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