Miroslav Adzic - Serbia & Montenegro
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
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.