How it works

The heart of the circuit is a '555' integrated circuit. This is arranged to work in 'Astable' mode. that is to say that, when power is applied to the circuit, the output pin (3) switches from line voltage to zero voltage and back again continuously and with a frequency that is set by resistors R1, R2 and capacitor C2.In general, the lower the values, the higher the frequency and vice versa. The values shown give a frequency of between one and two flashes per second.

When the output (pin 3) is at line voltage, transistor Q1 will be switched off. Q2 will also be switched off and the indicator light (connected between line voltage  and connection 2) will not be lit. When the output (pin 3) is at ground (zero) voltage, current will flow through R4 switching transistor Q1 hard on. Current will then pass through R5 to Q2 switching that hard on as well. This means that current will flow through the indicator bulb via Q2, to ground  and the lamp will light.  To put it another way, the lamp will flash on and off in time with the output of the IC.

Another feature of the '555' IC is that pin no 4 is a reset connection. When this pin is pulled low, the output (pin 3) will be at ground voltage and will stay there all the time the reset is held low.

In this circuit, pin 4 is pulled high by R3. A current sensor is used to pull the pin low when current is flowing through the circuit load.  When this happens, the output (pin 3) on the '555' goes low and the indicator lamp is switched on.

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