Speaker Power Detector Part 2: Parts and Enclosure

Parts List

I used parts from my junk bin, but I chose all parts that are available at Radio Shack.

Qty Item
1 breadboard or circuit board
? speaker jacks (see below)
1 5-30V DC power supply
1 jack for DC power supply
1 LM339 quad comparator (or substitute any low-voltage dual or quad comparator with open-collector outputs
4 .01-.1uF 100V capacitors (ceramic disc or whatever — just find some with a high voltage rating to protect them from your amplifier)
2 100kΩ trim potentiometers
20-100kΩ potentiometers

20-100kΩ stereo potentiometer (nice if you want to mount the sensitivity control where you can adjust it without opening the case)
2 10kΩ (brown-black-orange) resistors
1 resistor for LED (see table below)
1 any color LED, preferably with a frosted lens for wide-angle viewing
hookup and jumper wire

Enclosure and Power

If you want to build one, I’d suggest building it on a solderless breadboard first, then (if possible) transferring it to a perfboard that has the same copper patterns as the connections under the breadboard.

You’ll need an enclosure large enough to mount speaker jacks on, and you’ll need one large or two small speaker jack panels — four red/black pair total, to have left in, left out, right in, and right out.

Connect the corresponding wires straight through on each in/out pair (left red in to left red out, left black in to left black out, etc.), and then connect from there to the appropriate place on the board. When the circuit is done and working, use short pieces of speaker wire to connect your amp to the power detector, and the usual wires to run from there to your speakers.

Alternatively, you could have a single set of speaker wire jacks on the project box, and connect two sets of speaker wires (speaker and power detector) to the speaker outputs you want to monitor. Given the way most amplifier speaker connections are made, this may be harder to do well, but may be more convenient (not having to run two sets of wires to the project) if you can manage it.

You should be able to run the project from any DC power supply in the 5-30V range, and the power supply doesn’t need to be regulated. (Pick the lowest appropriate voltage you can find easily.) You’ll need to change out R5 if you use a higher-voltage power supply, to keep the LED from burning up, probably along these lines:

5V 220 – 330Ω red-red-brown or orange-orange-brown
9V 470 – 560Ω yellow-violet-brown or green-blue-brown
12V 680 – 820Ω green-blue-brown or grey-red-brown
15V 820Ω – 1kΩ grey-red-brown or brown-black-red
18V 1 – 1.2kΩ brown-black-red or brown-red-red

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

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