Home > Designing basic digital circuits/circuit wizard > Astable circuit using a 555 timer

Astable circuit using a 555 timer

This is a simple astable circuit using a 555 timer.

Astable print screen

An astable circuit will produce what is known as a square wave.

This digital waveform has sharp movements between low (0v) and high (+Vs).  The durations of the low and the high states may be different hence the reason this is an astable circuit.  It is astable as it is not stable in any state and the output (led) is continually changing between low and high.

The time period (T) of the square wave is the time for one complete cycle, but it is usually better to consider frequency (F) which is the number of cycles per second.

T = 0.7 x ( R1 + 2R2) x C1


F = 1.4 divided by (R1 + 2R2) x C1
The time period can be split into two parts: T = Tm + Ts

T   = time period in seconds (s)
F   = frequency in hertz (Hz)
R1 = resistance in ohms (ohm)
R2 = resistance in ohms (ohm)
C1 = capacitance in farads (F)

Mark time (output high): Tm = 0.7 × (R1 + R2) × C1
Space time (output low): Ts  = 0.7 × R2 × C1

Many circuits require Tm and Ts to be almost equal, this is achieved if R2 is much larger than R1.

For a standard astable circuit Tm cannot be less than Ts, but this is not too restricting because the output can both sink and source current. For example an LED can be made to flash briefly with long gaps by connecting it with a resistor between +Vs and the output. This way the LED is on during Ts, so brief flashes are achieved with R1 larger than R2, making Ts short and Tm long.

I had a little trouble with this circuit, it wasn’t the actual reading of the schematic that was the problem it was more myself trying to remember how the tracks ran on the breadboard.  The problem turned out to be my led, I had inserted it in such a way both terminals were on the same track vertically and so the current was going nowhere.  After getting the circuit working I introduced a potentiometer and was able to control the speed in which the led was flashing.

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