Specifications

CIRCUIT IDEAS
used for the entire circuit. The circuit
shown in Fig. 2 employs 7805 regulator.
This 4-digit counter can be readily inter-
faced to many circuits such as clock-fre-
quency meter, digital voltmeter, tachom-
eter (as explained here), stop watch, etc.
A reset switch is connected between pin
13 and V
CC
.
The chip 74C926 pulls its carry out-
put (pin 4) ‘high’ when the counter reaches
6,000. This output can be suitably used
in clock circuits for resetting; for example,
if the clock input is 100 Hz per second,
the carry output will be available every
minute. However, here we are not using
the carry output.
There are different versions of 4-digit
counter modules for different applications.
For example, in 74C927, the second most
significant digit gets divided by 6, rather
than 10. Similarly, in 74C928, the most
significant digit
gets divided by 2,
rather than 10,
and its carry out-
put goes ‘high’ at
the count of
2000, and ‘low’
only when the re-
set switch is
pulled ‘high’.
Figs 3 and 4
show the applica-
tions of the elec-
tronic digital ta-
chometer. Basic
principle in both
these applica-
tions is the same.
The important factor is that maximum
reflected light from the contrasting stripe
should fall on the IR detector, i.e. θ1
should be equal to
θ2. The other im-
portant thing is
that the contrast-
ing stripe may be
a mirror with a
small piece of tape
pasted on it, as
shown in Fig. 5.
Fig. 6 shows another application of
the same circuit for counting the objects
moving over a conveyer belt. The only
difference between applications shown
in Figs 3, 4, and 6 is that in the first
two applications, one requires a contrast-
ing stripe, whereas in case of Fig. 6, one
requires a light source and a sensor mod-
ule which are kept on the opposite sides
of the conveyer belt.
When there is no object between the
source (light) and the sensor module, one
gets a continuous pulse at the output pin
3 of monostable IC NE555. But as soon
as the object on moving conveyer belt ob-
structs the light path, the output of NE555
goes ‘low’. Since output pin 3 of NE555 is
connected to the clock input pin 12 of
74C926, the number of objects get counted
continuously—up to 9999, using a single
74C926. A simple IR light-source circuit
is shown in Fig. 7.
The total cost of fabrication of the
complete circuit is approximately Rs 250.
PRADEEP G.
H
ere is a light-operated, remote-
controlled solidstate switch to
operate a lamp. During darkness,
the resistance of LDR shoots up to meg-
ohm range. Thus, the triac does not get
gate drive and hence it does not conduct.
When LDR is illuminated by means
of a torch-light beam, the resistance of
LDR suddenly decreases (below 10-kilo-
ohm). This causes the triac to conduct
and switch ‘on’ the lamp. Light received
LIGHT-OPERATED LIGHT
SWITCH
from the lamp (not from the torch) keeps
LDR’s resistance low. So, the lamp re-
mains continuously ‘on’. Once the lamp is
‘on’, it can be switched ‘off’ again by in-
terrupting the light falling on LDR, by
either waving hand in front of it or by
interrupting
power supply to
the circuit for a
moment.
RFC emplo-
yed here can be
made by winding
about 15 turns of
18 SWG wire
over an insulated
ferrite rod.
S.C. DWIVEDI
84