Electronic – Driving LEDs from audio signal

audioled

I am making this very simple hobby circuit and I need help.

This utilizes the sound (variations) from the device through the jack to provide light (intensity) variations in the LEDs using TIP31.

Hence, the sound is not heard (pretty obvious, I know).

I want to know the additions that can be made to this circuit to enable the music (sound) from the device to be heard AND provide enough to drive the transistor and LEDs.

Best Answer

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That's a pretty bad circuit. First it needs a current limiting resistor in series with the LEDs. Something like 150 Ω will probably do.

Next, the transistor needs at least 0.7 V to conduct, which you may not have if you use a line output. If you do get the LEDs on, then add a 1 kΩ resistor in the red wire connection. (They seem to have never heard of resistors.) That should be sufficient to allow you to hook up both light box and amplifier to the audio output.

And swap that TIP31 for a BC337-40. The TIP31 has almost zero current gain (HFE).

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C goes to the left LED's -, B goes to the resistor you added to the red wire, and `E' to the orange wire.

And like m.Alin says, you may need to protect the transistor against negative voltages (though from a line input that won't be a real problem): place a diode between the battery's - connection and the red pin on the transistor, with the black band to the transistor.

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