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Telephone Call
Meter Using Calculator & COB |
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In 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 circuit 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 department 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. However, 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 available
at terminals A and B, with a phase difference of 90o. The
advantage of using this COB is that it works on a 1.5V DC source.
The clock pulses available from terminal 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 transistor
T1. The collector and emitter of transistor 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/distance, 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
equivalent for pulse rate 4, which is 0.25. Please note that on
maturity of a call in the telephone exchange, the exchange call
meter immediately advances to one call and it 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 position). 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 number 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+’ button. 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 reading, press
‘MRC’ button and slide switch S1 to ‘on’ (start/standby position).
The current local call meter reading will blink once every second.
In prototype circuit, the author used TAKSUN calculator that costs
around Rs 80. The display height was 1 cm. In this calculator, he
substituted the two button-type batteries with two externally
connected 1.5V R6 type batteries to run the calculator for more
than an year.
The power ‘off’ button terminals were made dummy by affixing
cellotape on contacts to avoid erasing of memory, should someone
accidentally press the power ‘off’ button. This calculator has
auto ‘off’ facility. Therefore, some button needs to be pressed
frequently to keep the calculator ‘on’. So, in the idle condition,
the ‘=’ button is activated electronically once every second by
transistor T1, to keep the calculator continuously ‘on’.
Useful hints. Solder the ‘=’ button terminals by drilling small
holes in its vicinity on PCB pattern using thin copper wire and
solder it neatly, such that the ‘=’ button could get activated
electronically as well as manually. Take the copper wire through a
hole to the backside of the PCB, from where it is taken out of the
calculator as terminals G and H.
At calculator’s battery terminals, solder two wires to ‘+’ and ‘–’
terminals. These wires are also taken out from calculator as
terminals E and F. Affix COB on a general-purpose PCB and solder
the remaining components neatly. For giving the unit an elegant
look, purchase a jewellery plastic box with flip-type cover (size
15cm x 15cm). Now fix the board, calculator, and batteries, along
with holder inside the jewellery box. Then mount the box on the
wall and paste the look-up table inside the box cover in such a
way that on opening the box, it is visible on left side of the
box.
Caution. The negative terminals of battery A and battery B are to
be kept isolated from each other for proper operation of this
circuit.
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LookUp
Table |
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Pulse rate
(PR) |
2 |
2.5 |
3 |
4 |
6 |
8 |
12 |
16 |
24 |
32 |
36 |
48 |
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Pulse rate
eqlt. (PRE) |
0.5000 |
0.4000 |
0.333 |
0.250 |
0.166 |
0.125 |
0.083 |
0.062 |
0.041 |
0.031 |
0.027 |
0.020 |
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Note
: Here PRE is shown up to three decimal places. In
practice, one may use up to five or six decimal places. |
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Click
Here For Circuit Diagram
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