Electronic – LED’s flash with music


I am wanting to put have a few LED's flash with the intensity of my music. I don't need a color organ, but just want the LED's to flash in sync with the music. I am amplifying the signal with a LM368 chip and playing music from a 4ohm 3W speaker and it's sound quality is great. The problem is my LED's don't turn on at all..enter image description here

I am using the configuration that many people claim works well but its usually done with a TIP31 transistor and not a 2N3904. I tried inserting an opAmp with a gain of 10 before the base of the transistor with no success.

Does anyone see a problem with my circuit or know of a reason the LED's arn't turning on? Any advice would be appreciated


Best Answer

Odds are that Q1 is smoked. You forgot to add a current limiting base resistor to limit the current.

You should probably add a reverse diode on the base (after the resistor) to protect the transistor. The diode is recommended because you are feeding the base with an alternating voltage that swings above and below zero volts. When it swings negative the base-emitter junction is reverse biased. It will probably survive given that you are operating on a low voltage but it's good practice anyway.

Test the transistor with your multimeter diode test function. You should get 0.7 V b-e and b-c with + lead on base. You should get high reading when leads reversed.


simulate this circuit – Schematic created using CircuitLab

Figure 1. Modified circuit.

When you get Q1 going again the next problem will be that you'll probably smoke the LEDs. You have no LED current limiting resistor in your schematic. You might get away with it if your supply voltage is low.

Edit: I couldn't read the supply voltage. I now see it's only 5 V. That won't be enough for four LEDs. As others have suggested, try them in parallel pairs.