VU Meter(?) Puzzle

[MoJay]

Hello everyone,

I am apprentice as an electronics technician and have my final exam in a few weeks. For this, I have to assemble a PCB (amplifier, the layout is fixed) beforehand. In the exam itself I have to solder an extension to the existing circuit on perfboard (among other assignments, of course).
Based on the parts left over in the BOM for the whole project, you can usually guess what the extension will be. 80% of the time it has been some kind of simple op-amp circuit, but not here. I can't quite figure out what one could make with the leftover components. So any help from you people is greatly appreciated.

The parts are:
1x 2-pole screw terminal
3x MOSFET, n channel, IRFZ44N
2x Diode, 1N5817 (schottky)
1x Diode, 1N4001
3x LED, red, 2mA
1x Capacitor, electrolytic, 220u
1x Resistor, 10R
(Load resistance on the amplifier is a 3R9, 11W chunker. I do not know if it stays connected in parallel with the extension.)

Usually all the components are used, but some could be omitted.

My main circuit is a class-D amplifier. The only signal coming from the main PCB is the amp output (signal and ground), since there are no other screw terminals on the main design. The signal will be a 1kHz sine, triangle or square wave with around 9V peak-to-peak.
I guess you could grab the 5V supply from a test point, but why wouldn’t they then put a screw terminal for that?

I have played around with the components and think it might be some kind of VU meter. However, I can’t think of a working circuit (see the attachment for my sad attempt). Perhaps you know a solution to this puzzle.

Thanks again!

[Chris Mr]

It's interesting that there are 3 LEDs and only a single 10R current limiting resistor.  10R is pretty low for limiting current to an LED.

In your circuit the input gets rectified to supply the gates, and the top gate is grounded so would never switch on.  If it were a VU meter then the rectified voltage would more likely be the supply for the LEDs.  But with only 10R limiting the current, I suspect not.

I wonder if it might be some sort of over current detector, to see if the amplifier is short circuited?

The MOSFET turn on threshold is 2-4v, and 10R with 2-4v is 200-400mA load current, so probably not.. just a thought.

[Terry Bites]

see https://tubecad.com/2017/07/blog0386.htm for some ideas