Voltage Controlled Switch using 555 Timer


Description:

In this circuit the 555 timer is used in a novel way, as a voltage
controlled switch.The old and omnipresent NE555 can be very good at
something it was not meant for: driving relays or other loads up to 200
mA. The picture shows an example circuit: if the input level rises over
2/3 of the supply voltage – it will turn on the relay, and the relay
will stay on until the level at the input drops below one third of the
supply voltage.

If the relay and D1 were connected between pin 3 and ground, the
relay would be activated when the input voltage drops below one third,
and deactivated when the input voltage goes over two thirds of the
supply voltage. It is also a nice advantage that the input requires only
about 1 uA, which is something bipolar transistors can’t compete with.
(This high impedance input must not be left open.) A large hysteresis
makes the circuit immune to noise. The output (pin 3) can only be either
high or low (voltage-wise), and it changes its state almost
instantenously, regardless of the input signal shape.

Circuit

Circuit diagram

The voltage drop across the NE555’s output stage (at 35-100 mA) is
0.3-2.0 V, depending on the way the relay is connected and the exact
current it draws. D1 is absolutely vital to the safety of the integrated
circuit.


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