Transistor LED Bar Graph

Introduction: Transistor LED Bar Graph

About: This project is sponsored by Mental Health Week and Mental Health Society.

This article shows a unique and controversial way of creating an LED bar graph display.

This circuit needs a high amplitude AC signal. You can try connecting a Class D amplifier to its input.

This circuit was designed and published many years ago based on the article shown here:


Components: TO3 heat sink - 1, TO3 BJT NPN power transistor - 1, small LEDs - 10, washers - 5. bolts - 4, nuts - 4, heat transfer paste, solder, matrix board/cardboard/wood, 100 ohm power resistors - 20, 10 ohm power resistor - 1, 10 kohm resistor - 2, 470 uF electrolytic capacitor - 2, 470 nF pillow capacitor - 2, in.

Tools: soldering iron, scissors, wire stripper.

Optional components: encasement/box.

Optional tools: multi-meter.

Step 1: Design the Circuit

I have drawn the circuit in the PSpice student edition simulation software to save time.

The Rd resistor ensures that all LEDs are OFF when the Q1 transistor is disconnected (Rb1 set to the maximum value and no input AC signal is applied).

Step 2: Simulations

Simulations show a low current output for the LEDs. This will not be true in real life if you are using a high-power transistor. Low power transistors have very limited maximum current output.

I varied the Rb1 potentiometer before simulating the circuit.

Step 3: Make the Circuit

You can see that I made this circuit on a piece of cardboard to save money. I could have used a piece of plastic, wood, or even a matrix board.

I used low current, small 5 mA LEDs, and high power resistors. I also attached a high-power transistor and a heat sink.

Step 4: Testing

The testing shows how the variation in the variable resistor setting changing the number of LEDs that are turned ON.

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