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Low-Cost
PCO Billing Meter
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The
circuit presented here can be used in PCOs for displaying
the actual bill. The overall cost of this circuit is
less than Rs 200 while a commercial equipment serving
similar purpose may cost more than Rs 10,000 in the
market. The comparative disadvantages of the presented
circuit are as follows:
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| 1.)The
calculator used along with this circuit is required to
be switched ‘on’ manually before making a call.
2.)Certain
manual entries have to be made in the calculator; for
example, for a pulse rate of Rs 1.26, number 1.26 is
to be entered after switching ‘on’ the calculator followed
by pressing of ‘+’ button twice. However, possibility
exists for automating these two functions by using additional
circuitry
In telephony,
on-hook condition is represented by existance of 48V
to 52V across the line. Similarly, the off-hook condition
is represented by the line voltage dropping to a level
of 8V to 10V (depending upon the length of the local
lead line from telephone exchange to the subscriber’s
premises as well as upon the impedance of telephone
instrument). Handset is normally lifted either for dialing
or in response to a ring
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| In
the circuit shown in Fig. 1, when the handset is off-hook,
the optocoupler MCT2E (IC1) conducts and forward biases
transistor T1, which, in turn, forward biases transistor
T2 and energises relay RL1. In energised condition of
relay, the upper set of relay contacts connects the positive
supply rail to PLL (phase-locked loop) IC2 (LM567) pin
4, while the lower set of relay contacts couples the positive
telephone lead to input pin 3 of LM567 via capacitor C1
and resistor R3. |
| The
negative telephone lead is permanently capacitively coupled
via capacitor C2. As soon as call matures, 16kHz tone
pulses would be pumped into the telephone line by the
telephone exchange at suitable intervals. This interval
depends on the pulse rate of the place called and also
the time of the day and whether it’s a working-day or
holiday. On receipt of 16kHz pulse, output pin 8 of IC
LM567 (which is tuned for centre frequency of 16 kHz)
goes ‘low’ for the duration of the pulse. The output of
IC2 is coupled via transistor T3 to optocoupler IC3. The
output of this optocoupler is used to bridge the ‘=’ button
on a calculator (such as Taksun make), which has the effect
of pressing the ‘=’ button of the calculator. |
| Considering
that pulse rate for a specific town/time/day happens to
be Rs 1.26 per pulse, then before maturity of the call
one enters 1.26 followed by pressing of ‘+’ key twice.
Now, if a total of ten billing pulses have been received
from exchange for the duration of the call, then on completion
of the call, the calculator display would show 12.60.
The telephone operator has to bill the customer Rs 14.60
(Rs 12.60 towards call charges plus Rs 2.00 towards service
charges). |
| For
tuning of the PLL circuit around IC2, lift the handset
and inject 16kHz tone across the line input points. Tune
IC2 to centre frequency of 16 kHz with the help of preset
VR1. Proper tuning of the PLL will cause LED1 to glow
even with a very low-amplitude 16kHz tone. |
| EFY
Lab note.
Arrangement
used for simulating a 16kHz pulsed tone is shown in Fig.
2. Push-to-on switch is used for generation of fixed-duration
pulse for modulating and switching on a 16kHz oscillator. |
| For
more details regarding pulse rates, pulse codes, etc,
readers are advised to go through the tariff rates and
pulse code information given in the beginning pages of
telephone directories, such as MTNL, Delhi directory,
Vol. I. One may also dial 183 for getting more details.
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