Page 1134 - Basic Electrical Engineering

17.7.6 An IC 555 Timer Astable Oscillator Circuit

A practical circuit of an IC 555 timer astable oscillator circuit has been
shown in Fig. 17.13 drawn in a slightly different way. The students may try

out this circuit for its working. We may choose R = R = 10 kΩ, C = 10 µF,
1
C = 0.01µF, supply voltage to the IC, i.e., V = +9 V, a red LED, a series
1
CC
resistance of 470 Ω. The LED red light has been used to indicate clearly that

the circuit is working. A LED will require about 5 mA to 20 mA of current to
emit light.

The current flowing through the LED circuit will be equal to the output
voltage divided by the LED series resistance of 470 Ω. Assuming a voltage

drop across the LED of 1.7 V, the current flowing through t will be

which is sufficient for the LED to light up. The timer will drive

a ‘high’ or ‘low’ output depending on the charging cycle of the series resistor
and the capacitor. High output will be nearly V and low output will be
CC
nearly zero voltage output. The LED will be on on/off mode due to the
periodic output waveform, which will alternate between high and low states.

The trigger pin 2 and the threshold pin 6 are attached to the two
comparators (op-amps) and are connected together to the external capacitor.

When power is first supplied, i.e., V is switched on, the capacitor is
CC
uncharged and the lower comparator sets the output voltage to high. This

allows the capacitor to charge through R and R . When the charge of the
1

capacitor reaches V , the upper comparator is triggered, causing the
CC

output to go low. When the voltage across the capacitor is V , the lower
CC

comparator is triggered, setting the output to high.