Electrical – Music reactive LEDs


I want to make a bigger project, but I have run in to a problem that I do not understand. I want to make an LED flash to music, for use in a bigger project. I have looked at about 20 different tutorials and googled my fingers off, because I cannot make it work.

I am using a 2N3904 transistor like him. I have tried with several jack sticks. I am simply lost, please help.

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Best Answer

There are two basic problems with your circuit.

  1. The transistor needs at least 0.6V to turn on. Any audio signal with a peak voltage below this won't light the LED at all.

You can solve this problem by using a voltage divider which raises the DC Base voltage up close to 0.6V.

  1. The audio source will probably have a capacitor on its output to remove any DC bias that may be present. Unfortunately, when the signal peaks do get large enough to turn on the transistor on, its Base will draw current and charge up this capacitor, creating a negative bias voltage that turns the transistor off until the capacitor discharges again (which will take a very long time because there is no discharge path).

This effect can be reduced by providing a path to ground for the capacitor to discharge into, and adding a resistor in series with the Base to reduce the peak current.

The following circuit has been found to work reasonably well:-


simulate this circuit – Schematic created using CircuitLab

R3 and R4 create a voltage divider with a ratio of 5.7:1, dropping 3.3V down to 0.58V. This is enough to just barely turn the transistor on, so the LED will glow weakly with no signal. Any signal will now cause the transistor to turn on harder and make the LED glow brighter.

R2 reduces Base current during positive peaks of the signal waveform, reducing the amount of charge stored in the audio source's coupling capacitor. R4 provides a discharge path for this charge during the rest of the waveform. As R4 is much lower than R2, the capacitor will only build up a small bias voltage that doesn't affect the LED brightness much.

Note that even with these improvements this circuit will never produce maximum possible brightness, because it only turns the LED on during positive peaks of the audio waveform.