Specifications

CIRCUIT IDEAS
S.C. DWIVEDI
I
n this circuit, a simple calculator, in
conjunction with a COB (chip-on-
board) from an analogue quartz
clock, is used to make a telephone call
meter. The calculator enables conversion
of STD/ISD calls to local call equivalents
and always displays current local call-
meter reading.
The circuit is simple and presents an
elegant look, with feather-touch operation.
It consumes very low current and is fully
battery operated. The batteries used last
more than a year.
Another advantage of using this cir-
cuit is that it is compatible with any type
of pulse rate format, i.e. pulse rate in
whole number, or whole number with
decimal value. Recently, the telephone de-
partment announced changes in pulse rate
format, which included pulse rate in whole
number plus decimal value. In such a
case, this circuit proves very handy.
To convert STD/ISD calls to local calls,
this circuit needs accurate 1Hz clock
pulses, generated by clock COB. This COB
is found inside analogue quartz wall clocks
or time-piece mechanisms. It consists of
IC, chip capacitors, and crystal that one
can retrieve from scrap quartz clock
mechanisms. These can be purchased
from watch-repairing shops for less than
Rs 20.
Normally, the COB inside clock
mechanism will be in good condition. How-
ever, before using the COB, please check
its serviceability by applying 1.5V DC
across terminals C and D, as shown in
the figure. Then check DC voltage across
terminals A and B; these terminals in a
clock are connected to a coil. If the COB
is in good condition, the multimeter needle
would deflect forward and backward once
every second. In fact, 0.5Hz clock is avail-
able at terminals A and B, with a phase
difference of 90
o
. The advantage of using
this COB is that it works on a 1.5V DC
source.
The clock pulses available from ter-
TELEPHONE CALL METER USING
CALCULATOR AND COB
will be further incremented according to
pulse rate. So one call should always be
included before counting the calls.
For making call in pulse rate 4, slide
switch S1 to ‘off’ (pulse set position) and
press calculator buttons in the following
order: 1, ‘+’, 0.25, ‘=’. Here, 1 is initial
count, and 0.25 is PRE. Now calculator
displays 1.025. This call meter is now
ready to count. Now make the call, and
as soon as the call matures, immediately
slide switch S1 to ‘on’ (start/standby posi-
tion). The COB starts generating clock
pulses of 1 Hz. Transistor T1 conducts
once every second, and thus ‘=’ button in
calculator is activated electronically once
every second. The calculator display
starts from 1.25, advancing every second
as follows:
1.25, 1.5, 1.75, 2.00, 2.25, 2.50, and
so on.
After finishing the call, immediately
slide switch S1 to ‘off’ position (pulse set
position) and note down the local call
meter reading from the calculator display.
If decimal value is more than or equal to
0.9, add another call to the whole num-
ber value. If decimal value is less than
0.9, neglect decimal value and note down
only whole numbers.
To store this local call meter reading
into calculator memory, press ‘M+’ but-
ton. Now local call meter reading is stored
in memory and is added to the previous
local call meter reading. For continuous
display of current local call meter read-
ing, press ‘MRC’ button and slide switch
S1 to ‘on’ (start/standby position). The cur-
LOOKUP TABLE
Pulse rate (PR) 2 2.5 3 4 6 8 12 16 24 32 36 48
Pulse rate
eqlt. (PRE) 0.500 0.400 0.333 0.250 0.166 0.125 0.083 0.062 0.041 0.031 0.027 0.020
Note: Here PRE is shown up to three decimal places. In practice, one may use up to five
or six decimal places.
minal A and B are combined using a
bridge, comprising diodes D1 to D4, to
obtain 1Hz clock pulses. These clock
pulses are applied to the base of transis-
tor T1. The collector and emitter of tran-
sistor T1 are connected across calculator’s
‘=’ terminals.
The number of pulses forming an
equivalent call may be determined from
the latest telephone directory. However,
the pulse rate (PR) found in the directory
cannot be used directly in this circuit. For
compatibility with this circuit, the pulse
rate applicable for a particular place/dis-
tance, based on time of the day/holidays,
is converted to pulse rate equivalent
(PRE) using the formula PRE = 1/PR.
You may prepare a look-up table for
various pulse rates and their equivalents
(see Table). Suppose you are going to
make an STD call in pulse rate 4. Note
down from the table the pulse rate equiva-
lent for pulse rate 4, which is 0.25. Please
note that on maturity of a call in the tele-
phone exchange, the exchange call meter
immediately advances to one call and it
K. UDHAYA KUMARAN, VU3GTH
67