Help me identify the missing component... Sound to light circuit

[edugimeno]

hI there!
Im trying to help a friend repairing a large, real-sized short-circuit robot.
This is remotely controlled and has a 2 way voice send/receive. Also while the hidden operator is talking, the red bulb inside the "mouth" should flash according to the sound
Some other technician used to service it and he was fired leaving all the repairs unfinished and no documentation at all.

What I found is a sound receiver, a speaker, and a wire directly to the mought bulb (a normal motorbike 5W 12V bulb).
So I bought a simple 2X30W amplifier to feed the loudspeaker and this works, but we need to find out what's missing for the light in the mouth.

Movinf stuff around in old drawers and boxes we found this circuit. It has an LM324N and TIP122. To me this looks like the missing part, as the LM324 could be used to amplify the signal after a var resistor, compare the signal to a predefined level with another var resistor, and with this drive the output compare to a darlington transistor to feed 12V to the mought bulb. This all makes sense.

I tried it and it doesn't work. First I noticed, is the previous guy desoldered a missing component right between the 2 lines of scre plugs. A capacitor? A diode? I can't make out what it could be!
Also what parts should I replace? IC and Q? Im a bit outdated in analog electronics....

Any help will be appreacited!!
Thanks

[lotas]

There is not enough electrolytic capacitor (negative at the bottom, positive at the top).

[edugimeno]

There is not enough electrolytic capacitor (negative at the bottom, positive at the top).

Yes, t thought that too, a capacitor would be needed. I just came from the shop to buy an LM324N, TIP120 (hopefully it works as equivalent as they didn't have TIP122) and a 10uF 25V cap. I will try with that
Also the diode coming from right side to that capacitor I understand is for reverse polarity protection, but the other diode at the bulb output? What's the point?

Thanks

[redwire]

I am able to draw a schematic. I could not see all part values but you can see how the circuit works. It has a couple mistakes but still would work.
The diode at the bulb output protects against spikes from the lamp+wiring inductance. The missing cap C4 is at least 100uF 16-25V.

[edugimeno]

Wow!! You are a genius!! I assembled the new LM324 and the capacitor (althugh I had already bought a 10uF, it uses a car battery so Vcc should be pretty clean) and tried it. Using the output from my phone and just using a voltmeter at the darlington output I could see the level jump to 12v so I just need to try it tomorrow at the robot and see if it likes the RF receiver voltage levels, and if not see if the VR's can help to fit it.
I was expecting the second VR to be a "trigger set level" to be compared with (like "IF MUSIC > 4.5V THEN LIGHT ON"), but your schematic shows it as a Low Pass Filter in combination with C3 right? I guess in order to make the effect follow fast or slow music variations, correct?

Thanks a lot for your time!

[redwire]

Yes the second trimpot just sets how long the lamp stays on, once it is triggered. The threshold is fixed (trigger level) by the zener which I'd guess is 6V or 9V. It is a "peak detector" with variable decay time.
What I don't like is the first op-amp stage is referenced to ground and there is no split-supply... so it sort of ignores the -ve peaks coming in. I guess it's good enough though.

[edugimeno]

Awesome!! Thanks for all the detailed explanation!
I went today to the shop and installed the board in the robot and it works perfectly, the mouth bulb lights with every voice burst.
I didn't need to swap the Q, it seems only the LM324 was damaged, and addind the capacitor was a good time to do it too.

I can call this closed, thanks again!

[redwire]

Post a picture of the robot
If you want to make more channels, look at MSGEQ7 IC that is also good for sound-to-light as it has 7 EQ channels (but multiplexed). I have to make a colour organ with it.