Electrical – How to light a bulb from an audio signal

audioledlighttransistors

So this is my first post, hello everyone. 😀

Well, I am trying to make my computer/synthesizer light up a bulb – Reacting to the voltage. Right now this is working with an LED: (I using a 2n3904 and some random LED)

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But when I am using an incandescent light bulb (12v, 30mA) it does not work. I have removed the 1k resistor from the supply, and tried different lower value resistors for the transistor base. I even added another transistor to make more gain, but with no luck.

Maybe I need more energy to light it up? A capacitor? More gain? Or am I totally forgetting obvious (Diode vs. bulb)?

Please enligthen me wise people. 😀

Best Answer

The basic problem is that the light bulb requires a lot more current to light up than the LED does. You said yourself that the bulb requires 30 mA. From the 12 V and 1 kΩ resistor in series with the LED, it's clear that the LED is drawing less than 10 mA. You can probably see a visible indication with just 1 mA or so. That's a lot less than a incandescent bulb requires to light visibly. There is at least a order of magnitude difference between lighting the LED and lighting the "bulb".

To get higher output current with the same input signal, you need more current gain. One easy way to do this is to use a second transistor:

Q1 works as your transistor does now. But instead of turning on the LED or bulb directly, it turns on Q2. With Q1 fully on, Q2 gets about 1 mA of base current. It needs a minimum gain of 30 to support 30 mA of output current. Many small signal transistors can do this, including the one shown.

This circuit will works to drive the LED, although you can now use a lower resistor in series with the LED to get higher current and therefore higher brightness. Let's say you've got a common green LED that drops 2.1 V and can take 20 mA. Just about any cheap green LED you can find can do this. A 510 Ω resistor in series with the LED will limit to the current to just under the 20 mA the LED can handle. This will produce more light than the "light bulb" at 30 mA, assuming you mean a incandescent bulb.

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