1445326735900-0

Starting electronics
voltage (about 3 V). Figure 5.3(a) shows an equivalent circuit
to that of Figure 5.2 for the times during which the electronic
switch of the 555 is off.
You should be able to work out that the capacitor C1 of the
circuit is connected through resistors R1 and R2 to the positive
power supply rail. The time constant τ 1
of this part of the
circuit is therefore given by:
When the voltage across the switch rises to about 6 V, however,
the switch turns on (as shown in Figure 5.3(b), forming
a short circuit across the capacitor and resistor, R2. The
capacitor now discharges with a time constant given by:
Of course, when the discharging voltage across the switch
falls to about 3 V, the switch turns off again, and the capacitor
charges up once more. The process repeats indefinitely,
with the switch turning on and off at a rate determined by the
two time constants. Because of this up and down effect such
oscillators are often known as relaxation oscillators.
As you might expect the circuit integrated within the 555 is
not just that simple and there are many other parts to it (one
part, for example, converts the charging and discharging exponential
voltages into only two definite voltages — 9 V and 0 V
— so that the 555’s output signal is a square wave, as shown
in Figure 5.3(c). But the basic idea of the astable multi-vibrator
formed by a 555 is just as we’ve described here.
106