Specifications

CIRCUIT IDEAS
so the displays (DIS.1 and DIS.2) remain
off. However, at the end of positive pulse
at pin 1 of IC1 (and base of transistor T1),
the frequency counter is latched. During
the following 4-second low level period,
transistor T1 conducts and displays DIS.1
and DIS.2 show the current count. At be-
ginning of the next positive timing pulse
the frequency counter resets (as the reset
pin 15 of IC1 and IC2 receives a differenti-
ated positive going pulse via capacitor C1)
and starts counting afresh.
Thus, this digital water level meter
shows water level reading for four-second
duration and then goes off for a variable
period of 0.5 second to 3 seconds. Thereaf-
ter the cycle repeats. This type of display
technique is very useful, because if there
is ripple in water, we shall otherwise ob-
serve rapid fluctuations in level readings,
and shall not get a correct idea of the
actual level.
Timer IC4 is used
as a free-running
astable multivibrator
which generates con-
tinuous 30Hz clock
pulses with 52 per
cent duty cycle. The
output of IC4 is avail-
able at its pin 3,
which is connected to
pin 2 of IC1.
The other timer,
IC3, is used as tim-
ing pulse generator
wherein we have in-
dependent control
over high and low du-
ration (duty cycle) of
the timing pulses available at its output
pin 3. The different duration of high and
low periods of the timing pulses are
achieved because the charging and dis-
charging paths for the timing capacitor
C2 differ. The charging path of capacitor
C2 consists of resistor R19, diode D1, and
potmeters VR1 for OHT (or VR2 for sump
tank, depending upon the position of slide
switch S1) and VR3. However, the dis-
charge path of capacitor C2 is via diode
D2 and variable resistor VR4.
By adjusting preset VR4, the low-du-
ration pulse period (4 seconds) can be set.
The low-pulse duration should invariably
be greater than 3 seconds.
Potmeter VR1 (or VR2), a linear
wirewound pot, is fitted in such a way in
the overhead tank (or sump tank) that
when water level is minimum, the value
of VR1 = 2 kilo-ohm. When water level in
the tank is maximum, the in-circuit value
of potmeter VR1 increases to 340-kilo-ohm
(approximately). This is achieved by one-
third movement of pot shaft. The change
in resistance of VR1 results in the change
in charging period of capacitor C2. Thus,
depending upon the level of water in the
tank, the in-circuit value of VR1
(or VR2)
resistance changes the charging time of
capacitor C2 and so also the duration of
positive pulse period of IC3 from 0.5 sec-
ond to 3 seconds.
Adjustment of presets for achieving
the desired accuracy of count can be ac-
complished, without using any frequency
counter, by using the following procedure
(which was adopted by the author during
calibration of his prototype):
1. First adjust the values: VR1 (or
VR2
) = 2 kilo-ohm, VR3 = 64 kilo-ohm,
VR4 = 90 kilo-ohm, VR5 = 23.5 kilo-ohm.
2. Now switch on the circuit. The 7-
segment DIS.1 and DIS.2 blink. If neces-
sary, adjust VR3 such that the display
goes ‘on’ for 4-second period.
3. If display readout is 15, increase
value of pot VR1 from 2-kilo-ohm to 340-
kilo-ohm. Now, the display should show
90. If there is a difference in the displayed
count, slightly adjust presets VR5 and VR3
in such a way that when VR1 is 2-kilo-ohm
the display readout is 15, and when VR1 is
340-kilo-ohm the display readout is 90.
4. If 15- to 90-count display is achieved
with less than one-third movement of pot
shaft, increase the value of VR5 slightly.
If 15- to 90-count display is not achieved
with one-third movement of pot shaft, de-
crease value of VR5.
If you need this digital water level
meter to monitor the levels of water in
the overhead as well as the sump tanks,
it can be done by moving DPDT slide
switch to down (DN) position. The indica-
tion of the position selected is provided
by different colour LEDs (refer Figs 1 and
2) or by using a single bi-colour LED. To
monitor water level in more than two
tanks, one may use a similar arrange-
ment in conjunction with a rotary switch.
For timing capacitors C2 and C4 use
tantalum capacitors for better stability.
UNIVERSAL HIGH-RESISTANCE
VOLTMETER
YOGESH KATARIA
TABLE I
Position 1 of Function Switch
E
dc
input Meter Current
5.00V 44 µA
4.00V 34 µA
3.00V 24 µA
2.00V 14 µA
1.00V 4 µA
T
he full-scale deflection of the uni-
versal high-input-resistance volt-
meter circuit shown in the figure
depends on the function switch position
as follows:
(a) 5V DC on position 1
(b) 5V AC rms in position 2
(c) 5V peak AC in position 3
(d) 5V AC peak-to-peak in position 4
The circuit is basically a voltage-to-
Fig. 2
Fig. 3
S.C. DWIVEDI
37