Mains Pulser


The pulser is intended
to switch the mains voltage on and off at intervals between just under a
second and up to 10 minutes. This is useful, for instance, when a
mains-operated equipment is to be tested for long periods, or for
periodic switching of machinery. Transformer Tr1, the bridge rectifier ,
and regulator IC1 provide a stable 12V supply rail for IC2 and the
relay. The timer is arranged so that the period-determining capacitor
can be charged and discharged independently. Four time ranges can be
selected by selecting capacitors with the aid of jumpers.
Short-circuiting positions 1 and 2 gives the longest time, and
short-circuiting none the shortest.

Circuit diagram:

Mains Pulser Circuit

Mains Pulser Circuit Diagram

In the latter case, the 10µF capacitor at pins 2 and 6 of the timer
IC determines the time with the relevant resistors. The value of this
capacitor may be chosen slightly lower. The two preset potentiometers
enable the on and off periods to be set. The 1k resistor in series with
one of the presets determines the minimum discharge time. The timer IC
switches a relay whose double-pole contacts switch the mains voltage.
The LEDs indicate whether the mains voltage is
switched through (red) or not (green). The 100mA slow fuse protects the
mains transformer and low-voltage circuit. The 4 A medium slow fuse
protects the relay against overload.


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