Sometimes, we need to know whether someone is
still inside a building before it is locked. And if so, how many persons are
still inside? There can be many other similar situations for which the circuit
presented here can come handy. But it needs separate in and out gates
to work and can count up to nine persons (inside) only. Though, it
can be extended to count more.
Circuit and working
Fig. 1 shows circuit of the difference counter,
built around four ICs, for use between entrance and exit doors. Switch S1,
which is an SPDT switch, together with gates N1 and N2 of CD4011 (IC4)
generates a pulse for incrementing counter CD40192 (IC1). CD40192 is a BCD
pre-settable up/down (bidirectional) counter. The counter moves up or down with
the positive clock edges applied to its corresponding pin 5 (clock up) or pin 4
(clock down).
When you press S1, its pole comes in contact with
its terminal 3, which is connected to pin 5 of IC4. When you release S1, its
pole returns to its original position at terminal 1. Thus, for every action of
pressing and releasing switch S1, there is an increment in display DIS1 till 9.
Pressing switch S3 momentarily resets the counter to 0.
Fig. 1: Circuit diagram of the difference counter
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Fig. 2: Actual-size PCB layout for the counter
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Fig. 3: Component layout for the PCB
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Switch S2 is also an SPDT switch that works
exactly like switch S1, but it is used for decrementing the count in DIS1.
Switch S2, together with gates N3 and N4 of IC4 generates a pulse for
decrementing the counter in IC1.
The outputs of IC1 are decoded by CD4511 (IC2),
which has internal 4-bit latch, decoder and output drivers. The internal latch
is always enabled because pin 5 (LE) is connected to ground permanently. The
outputs of IC2 drive common-cathode 7-segment display LTS543 (DIS1). Since pin
configurations may be different for the other displays, always check their
datasheets before use. Resistors R5 through R11 are usually in the range of 120
to 470 ohms, depending on the type of DIS1. Here, we have used 330-ohm resistors.
Power supply of the circuit is built around linear regulator 7805 (IC3). The
input voltage is applied to connector CON1 and can be AC or DC in the range
9-15V.
Construction and testing
An actual-size, single-side PCB for the
difference counter is shown in Fig. 2 and its component layout in Fig. 3.
[Mount the components on the PCB
to save time and avoid assembly errors. Carefully assemble the components and
double check for any overlooked error. Check/verify the voltages at various
points as per the table.
You may
use any type of SPDT switches (S1 and S2) to check the increment and decrement
in DIS1 display. But for practical application, S1 and S2 should be SPDT foot
switches, which could be fitted in level with the floor at the
entrance and exit doors, respectively]
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