Specifications

CIRCUIT IDEAS
T
achometer is nothing but a simple
electronic digital transducer. It
finds many applications in our
day-to-day life.
Normally, a tachometer is used for
measuring the speed of a rotating shaft,
gear, or a pulley. A tape or a contrasting
stripe is pasted on the rotating part of
the machinery. The reflected light from
the contrasting stripe falls on the junc-
tion
of the sensor module to alternately acti-
vate and deactivate it, depending upon
the size and width of the tape pasted
on the pul-
ley or ro-
tating part
of the ma-
chine.
Hence,
one gets
the output
in the
form of
sharp pulses for every revolu-
tion.
Apart from counting the
revolutions of a moving object,
the circuit can also be used
for counting the objects on a
conveyer belt.
The basic digital tachom-
eter circuit consists of two
stages. The first stage is a
simple monostable, wired
around IC NE555. The second
stage consists of a digital counter based
on 4-digit counter IC 74C926. When light
from any source falls on junction of the
infrared module, its output goes low. This
output is connected to pin 2 of NE555
(configured as a monostable) to trigger it.
The output pulses from pin 3 are con-
nected to clock pin 12 of 74C926. Hence,
on receipt of every pulse, the count of IC
74C926 increments by one. For checking
the revolutions in a predetermined time
period, a stopwatch may be used. Before
counting starts, depress reset switch S1
and release it as soon as counting is to
start. At the instant when counting is to
end, one should immediately withdraw ei-
ther the sensor module or switch ‘off’ the
light source to see the final reading (revo-
lutions) on the display. Accuracy will be
better if the counting period is compara-
tively larger.
The 74C926 is basically a 4-digit
counter module, which can count from
0000 to its maximum possible value of
9999. It can be operated with V
CC
of 3 to
15 volts. Here, regulated 5-volt supply is
ADITYA U. RANE
ELECTRONIC DIGITAL
TACHOMETER
S.C. DWIVEDI
83