Speaker Power Detector Part 4: Construction and Calibration

At the risk of sounding elitist, if you haven’t been comfortable following and understanding the description so far, if you aren’t comfortable figuring out the enclosure and speaker connections yourself, and if you aren’t comfortable assembling the circuit by referencing the diagrams and photograph above, you maybe shouldn’t be building this project. If you connect something wrong in the speaker/potentiometer/capacitor section, you could permanently destroy your receiver or amplifier.

You need to be able to wire it correctly and confidently, or find a friend to build it for you. I can’t guarantee that you’ll be assemble this correctly, nor that it will work right on your stereo even if you do. If you continue, you do so at your own risk. There’s no way I can foresee every possible problem in advance, and I really don’t want anything bad to happen to your stereo.

That said, there are reasonable precautions to take along the way to maximize the likelihood of finding any errors that do creep in, and to minimise the risk.

Check the Board for Short-Circuits

Assemble at least the right section of the board, from the trim pots all the way through the LED, and the power supply. If you have an ohmmeter, before powering up, measure the resistance with the red probe on the V+ connection and the black probe on the V- connection. If it’s less than 1kΩ, double-check your wiring and component placement — there’s probably something shorted or connected wrong.

Turn on power and make sure the LM339 isn’t heating up. If you’re using a wall wart (wall transformer) that doesn’t have a switch, I like to plug it into a power strip with a switch. Then I can keep one hand on the power strip’s switch and one thumb on the IC, to turn off the power right away if anything gets hot.

Check the LED Polarity

With no connections to the speaker inputs, turn on the project power and use a spare piece of wire to connect the ground row (at the bottom) to either one of the rightmost pins of the IC (where I have the purple wire in my picture). The LED should light. If it doesn’t, it’s probably connected backwards — shut off the power, reverse the connections, and try again.

If it still doesn’t light, you have a problem with your power supply, LED, or R5 connections, or a short circuit somewhere. Don’t continue until you’ve found and fixed that.

Set the Bias Voltage

With no connections to the speaker inputs, and with the power on, turn the two trim pots all the way CCW; hopefully the LED goes off. Turn one at a time CW; the LED should come on.

If the LED doesn’t ever go off, turn the trim pots all the way CW, or try each combination of one fully CW and one fully CCW. If one or both trim pots have to be CW to turn off the LED instead of CCW, pull it (them) out, turn it (them) around 180°, and stick it (them) back in and test again.

If the LED doesn’t come on at all, check your wiring.

Turn both trim pots all the way CCW, so the LED goes off. Turn the top one CW until the LED comes on, then back CCW until the LED goes off and then just a little bit more. Repeat with the lower trim pot.

You now have the bias voltages set a little below the reference voltage.

Check the Speaker Wiring for Short-Circuits

If you haven’t already, power off and build out the rest of the circuit, including connecting your receiver/amplifier and the speakers to the jacks (with the amplifier still off). If you have a smaller or cheaper stereo, like a boom box or computer speakers, you might want to use that instead of your main amp for initial testing. Now:

  • Turn your amplifier volume all the way down (still with the amp off).
  • Pick which set of speakers you’re going to test with. They should be a set you can hear from where you’ll be testing.
  • Press the amp’s buttons to enable whichever set of speakers you’re using to test and disable any other sets.
  • Leave the project powered off for now.
  • Turn on your amp.
  • Slowly turn up the volume until you’d expect to be able to hear your speakers. If you can’t hear sound from both speakers, immediately turn off the amp and check/fix your speaker input wiring — there’s an open and/or shorted connection somewhere. Do not continue until this is fixed and you can hear both speakers.

You’ve now confirmed that the speaker connections work and aren’t shorted.

Calibrate the Input Potentiometers

Turn both speaker input potentiometers (or your single stereo pot) all the way CCW. Turn on circuit power — the LED should stay off. Turn on amplifier power and turn it up to the lowest volume at which you want the circuit to warn you that your speakers are on. Double-check that you hear sound from both left and right speakers — kill the amp and check/fix your speaker wiring if you don’t.

If the LED is on or flickering brightly, turn the speaker pot(s) all the way CW. If that makes the LED go off, your speaker pot(s) is(are) in backwards — power everything down, reverse the speaker pot connections, and check again. Don’t proceed until the LED is off with the speaker pot(s) at the CCW end of travel.

If you used one stereo speaker pot, slowly turn it CW until the LED starts flickering, and a little further until you’re happy with the LED brightness. Turn off the amplifier or push the button to disconnect those speakers, and the LED should go off. You’re done!

If you used separate speaker pots, turn one CW until the LED comes on to a brightness you like. Then disconnect that speaker wire from the amplifier, and the LED should go out. Turn the other speaker pot CW until the LED again comes on to a brightness you like. Turn off the amp or push the button to disconnect the speakers, and the LED should go off. You’re done!

Fine Tuning and Troubleshooting

If you ever decide that the circuit isn’t sensitive enough to audio at a low volume, adjust the speaker potentiometer(s) CW until the LED is on when the amp is playing at a volume level you want to detect, and still goes off when the amp is off.

If the LED stays on even when the amp is off, turn the speaker potentiometer(s) CCW until the LED goes off.

If you can’t adjust the speaker pot(s) to get the LED to come on at a reasonable volume, turn the trim pot(s) slightly CW. This increases the bias voltage to be closer to the reference voltage, so it requires less help from the speaker to trigger the comparator. You may want to do this one channel at a time, by disconnecting one speaker wire at a time at the amplifier end.

If the LED flickers or stays on even when the amp is off, and even with the speaker pot(s) all the way CCW, turn the trim pot(s) slightly CCW. This decreases the bias voltage to be further below the reference voltage, so it requires a stronger signal from the speaker (and speaker pot) to trigger the comparator. As above, you may want to do this one channel at a time.

If the LED is too bright, change R5 for a bigger resistor. If it’s too dim, swap in a smaller resistor.


That’s it! You have a working speaker power detector.

The exact nature of the enclosure and packaging is up to you. Because you’ll have to run speaker wires to and from the box, you may want to hide it behind the stereo instead of putting it out in the open. You can solder long wires to your LED and run them back to the box — just be sure to insulate (preferably heatshrink) the connections so they don’t short out.


If you build this project and it’s useful to you, or if you have suggestions for improving it, I’d love to hear from you. My blog software doesn’t allow comments on “pages” (like this); so please go to my related post and add a comment.


Part 0: Project Overview
Part 1: Schematic and Operation
Part 2: Parts and Enclosure
Part 3: Layout and Prototype
Part 4: Construction and Calibration