Introduction: Transistor Blinking LEDs

About: We are a group of people that design cheap electronic hobby circuits.

This article shows you how to make a quick and simple blinking LEDs circuit. The circuit shown is basically a square wave oscillator made from two-transistors.

You can click on this link to find out more information on this topic here:


Parts: LEDs - 4, resistors (shown in the circuit), general-purpose transistors - 4, battery harness, battery, 100 uF electrolytic or bipolar capacitors - 2, cardboard, piece of plastic or matrix board, 1 mm metal wire or insulated wire.

Tools: scissors, pliers.

Optional tools: soldering iron, wire stripper, multimeter.

Step 1: Design the Circuit

You can click on the link in the introduction of this article to find out how to calculate the frequency of this oscillator.

I have drawn the circuit in the old PSpice simulation software. There was no component for LED. Thus I used three diodes to model the LED component.

Calculate the maximum LED current:

IledMax = (Vs - Vled - VceSaturation) / Rc1

= (9 V - 2 V - 0.2 V) / 680 ohms = 0.01 A = 10 mA

This is the approximate current for a typical LED.

Calculate the maximum transistor base current:

IbMax = (Vs - Vbe) / Rb1 = (9 V - 0.7 V) / 18000 ohms = 0.00046111111 A = 461.1111 uA

Calculate the transistor minimum current gain to ensure transistor saturation:

BetaMin= IcMax / IbMax= IledMax / IbMax = 10 mA / 461.1111 uA = 21.686746988

Transistors can have a current gain as high as 500. A typical transistor current gain is 100. Current gain is influenced by:

- production tolerances,

- temperature,

- aging,

- biasing current.

Simulations show that the circuit will have an oscillation period of 2.15 * 2 = 4.3 seconds or Frequency = 1 / Period = 232.55813953 mHz (milliHz).

You can reduce the voltage to a minimum of 3 V because the LEDs only need 2 V to turn on and transistors only need minimum voltage of 1 V.

Rc1 = (Vs - Vled - VceSaturation) / IcMax

IcMax = 10 mA

Vled = 2 V

Vs = 3 V

Rc1 = 100 ohms

IbMax = IcMax / BetaMin = 10 mA / 20 = 0.01 / 20

Rb1 = (Vs - Vbe) / IbMax

Vs = 3 V

Vbe = 0.7 V

Reducing the Rb1 resistor will increase the frequency. You can increase the capacitor values to reduce the frequency. More information on this topic is in the link specified in the introduction.

Step 2: Make the Circuit

I did not use a soldering iron. I connected one of the connections underneath the piece of plastic. This is why you do not see it in the photo.

I specified four transistors in the parts because you can connect two transistors in parallel to improve the reliability of the circuit.

I suggested that you purchase more LED because you might burn a few.