Light Gate With Counter Using 555 And 4033


The circuit described
here counts the number of times that an infrared beam is interrupted. It
could be used to count the number of people entering a room, for
instance, or how often a ball or another object passes through an
opening (handy for playing shuffleboard). The heart of the circuit
consists of – you guessed it – a light gate! Diode D1 is an IR diode
that normally illuminates IR transistor T1. The light falling on T1
causes it to conduct to a certain extent. The resulting voltage on the
collector of T1 should be just low enough to prevent the following
transistor (T2) from conducting. This voltage can be adjusted within
certain limits using P1.

As soon as an object comes between D1 and T1, the light shining on
T1 will be partially or fully blocked, causing the IR transistor to
conduct less current. As a result, the voltage on its collector will
increase, producing a brief rise in the voltage on the base of T2. This
will cause T2 to conduct and generate a negative edge at IC1. This
negative edge will trigger the monostable multivibrator, which will then
hold the output signal on pin 3 ‘high’ for a certain length of time (in
this case, one second). At this point, two things will occur. First, a
buzzer will be energized by the output of IC1 and produce a tone for
approximately one second.

When the buzzer stops, a negative edge will be applied to the clock
input of IC2, causing the counter in IC2 to be incremented by 1. IC2 is
conveniently equipped with an internal binary-to-BCD
decoder, so its outputs only have to be buffered by IC3 and T3 to allow
the state of the counter to be shown on the 7-segment display. Switch
S1 can be used to reset the counter to zero. If a one-second interval
does not suit your wishes, you can modify the values of R3 or C1 to
adjust the time. Increasing the value of R3 lengthens the interval, and
decreasing it naturally shortens the interval.

The same is true of C1. When building the circuit, make sure that T1
is well illuminated by the light from D1, while at the same time
ensuring that T1 ‘sees’ as little ambient light as possible. This can
best be done by fitting T1 in a small tube that is precisely aimed toward
D1. The longer the tube, the less ambient light will reach T1. The
sensitivity of the circuit can be adjusted using P1.


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