Introduction: AC LED Amplifier

About: We are a group of people in the Philippines that design circuits.

In this Instructable you will be making an AC LED amplifier.

You can see the circuit working on video.

This circuit does not oscillate on its own. You need to connect the input to an oscillator.


You will need: 1 kohm resistors - 10, 10 kohm resistors - 10, 2 Megohm variable resistor - 5, NPN BJT Transistors - 5, 100 ohm resistor - 10, LEDs/Bright LEDs - 10, 3 V battery, 3 V battery harness, 555 timer oscillator.

Optional: solder, soldering iron, 100 uF capacitor, 100 high power resistor, power supply, photodiode, infrared remote control.

Step 1: Build the Circuit

The circuit shows Zener diodes because the old PSpice simulation software does not have LED components.

This circuit might work with 1 Megohm variable resistors. However, some transistors have a very high current gain and will have a low collector voltage at the maximum variable resistor setting. Thus I have drawn 2 Megohm resistors. Yet you see that the setting is only 0.1 of 2 Megohm which is only 200 kohms. At one end of the potentiometer scale, only one LED is on. At the other end of the potentiometer scale, the second LED is on. You need to set the potentiometer so that both LEDs are off.

I was considering swapping Rb2a and Rb2b to eliminate the need for the Rb2c resistor. However, this would mean that the Q1 collector output is feeding Q2 base without any resistance when the variable resistor is set to zero, thus burning both transistors.

The lower bandpass frequency of the high pass filter is: f = 1 / (2*pi*Rb2c*Cb2) = 1 / (2*pi*10000*(10*10^-6)) = 1.5915 Hz.

The 100-ohm high-power resistor and 100 uF capacitor components are shown in the photo. Those components are used to filter power supply interference and prevent oscillations. They are connected as an RC low pass filter power supply:

I decided that including those components in the circuit could be risky if you are using a current power supply. Because there are only 3 transistors in the circuit it is not likely you will see power supply oscillations.