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The integrated circuit is a 4516
up/down counter. Diode 2 determines the count direction.
The value of the resistors R’ depends on the type of diode
used. (You will probably need resistors in the range 100k
to 1M.) When entering the room, the beam illuminating
diode 1 must be cut first. This causes the counter to
count UP one. If (when a person leaves the room) the beam
illuminating diode 2 is cut and while this beam is
still cut the beam illuminating diode 1 is cut, the
counter will count down one. The output can be used
(with a relay and driver) to switch on a light when a
person first enters the room and then to switch the light
off when the last person leaves the room (maximum number
of people, 15).
Pins 1,3,4,5,12, and 13 of the 4516 must be connected
to battery negative (see vero diagram
below). The "switch-on reset"
network (R and C) can have almost any values; try 100k and
1.5nF. |
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Use I.R. receiver diodes with black
insulating tape (or something similar) around them.
Alternatively, put the diodes in a small plastic box, as
shown below. |
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Vero diagram |
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You can add a buzzer
which sounds each time someone enters or leaves the room. |
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© David Hoult 2009
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