2 LED Flasher with Calculator

Here is a typical 2 LED Flasher (astable multivibrator) which is actually a cross-coupled two-stage amplifier, where the output of one stage feeds the input of the other. Each transistor turns ON in saturation, and we can take the square wave from either collector. This circuit will light a pair of standard red LEDs and therefore the series resistor is 330 Ω in value.

As you can see, this is a balanced circuit and therefore it uses identical values of R and C at either ends. The values of R and C determine the frequency of flashing operation given by the simple formula below.

This extremely simple circuit is usually something that schoolchildren make on a breadboard. An advantage of this circuit is that it uses the 2N3904 transistors that are cheaply and readily available, which means that everyone in a class can have a go at making it. Once this circuit is working try experimenting with different resistor and capacitor values to see how it changes the timing. With simple switching circuits such as this, you can use a wide range of cheap general-purpose transistors.

Calculator

Experiment

If we use a commonly available 10-µF capacitor for C, and vary the value of the resistors (R) using standard values from the E24 Series, then we can obtain these frequencies.