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Here
is an interesting circuit for a magnetic proximity switch which
can be used in various applications.
The magnetic proximity switch circuit, in principle, consists of a
reed switch at its heart. When a magnet is brought in the vicinity
of the sensor (reed switch), it operates and controls the rest of
the switching circuit. In place of the reed switch, one may, as
well, use a general-purpose electromagnetic reed relay (by making
use of the reed switch contacts) as the sensor, if required. These
tiny reed relays are easily available as they are widely used in
telecom products. The reed switch or relay to be used with this
circuit should be the ‘normally open’ type.
When a magnet is brought/placed in the vicinity of the sensor
element for a moment, the contacts of the reed switch close to
trigger timer IC1 wired in monostable mode. As a consequence its
output at pin 3 goes high for a short duration and supplies clock
to the clock input (pin 3) of IC2 (CD4013—dual
D-type flip-flop). LED D2 is used as a response indicator.
This CMOS IC2 consists of two independent flip-flops though here
only one is used. Note that the flip-flop is wired in toggle mode
with data input (pin 5) connected to the Q (pin 2) output. On
receipt of clock pulse, the Q output changes from low to high
state and due to this the relay driver transistor T1 gets
forward-biased. As a result the relay RL1 is energised.
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