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Q & A: Lamps reacting to sound

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Most recent answer: 10/22/2007
Q:
Some lamps today are able to react to sound and change intensity and colour acording to the pitch of the sounds they are sensing. I would like to know exactly how they work esspecially the circuitry envolved detecting the sound and translating it in the the changing light.
- Luke Wyatt
England
A:
Lamps that react to sound have become popular novelty items lately, and with the ever-decreasing cost of electronics, they have become inexpensive to make. All of the parts you need can be found at a local electronics specialty store (like Radio Shack, for instance).

Here's a fairly simple way to get a set of lights to react to volume. Attach a microphone (dynamic, so you don't need a power supply) to a capacitor with a diode in the circuit. Hook a resistor across the capacitor and choose values for the resistor and capacitor such that 1/RC is about 0.5 seconds. Attach an analog-to-digital converter IC's inputs across the capacitor, and use the digital outputs to drive light-emitting diodes.



------------Diode--------------------------
| | |
microphone Cap Resistor To ADC inputs
| | |
-------------------------------------------

This is just a simple digital volume meter. You will have to check the average voltage output of this thing in selecting the overall R and C values (smaller C will make for more voltage, easier for your ADC to measure).

You can do the same thing with pitch by building several circuits with bandpass filters and one of these digital volume things on each one.

Tom

(published on 10/22/2007)

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