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Low-Cost PCO Billing Meter Circuit

This document describes a low-cost circuit that can be used in public call offices (PCOs) to display the billing amount for phone calls. When the phone handset is picked up, an optocoupler energizes a relay that connects the phone line to a phase-locked loop chip tuned to 16 kHz. The chip detects 16 kHz tone pulses sent by the phone exchange to indicate billing pulses. A calculator is used to total the pulses and display the amount due. Manual entries must be made in the calculator for the pulse rate before and after the call. Additional circuitry could automate these entries.

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Shrivlsi Ram
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401 views1 page

Low-Cost PCO Billing Meter Circuit

This document describes a low-cost circuit that can be used in public call offices (PCOs) to display the billing amount for phone calls. When the phone handset is picked up, an optocoupler energizes a relay that connects the phone line to a phase-locked loop chip tuned to 16 kHz. The chip detects 16 kHz tone pulses sent by the phone exchange to indicate billing pulses. A calculator is used to total the pulses and display the amount due. Manual entries must be made in the calculator for the pulse rate before and after the call. Additional circuitry could automate these entries.

Uploaded by

Shrivlsi Ram
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as PDF, TXT or read online on Scribd
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LOW-COST PCO BILLING METER

T
he circuit presented here can be well as upon the impedance of telephone optocoupler IC3. The output of this
used in PCOs for displaying the instrument). Handset is normally lifted optocoupler is used to bridge the =
actual bill. The comparative either for dialing or in response to a ring. button on a calculator (such as Taksun
disadvantages of the presented circuit are In the circuit shown in Fig. 1, when make), which has the effect of pressing
as follows: the handset is off-hook, the optocoupler the = button of the calculator.
1. The calculator used along with this MCT2E (IC1) conducts and forward Considering that pulse rate for a
circuit is required to be switched on biases transistor T1, which, in turn, specific town/time/day happens to be Rs
manually before making a call. forward biases transistor T2 and 1.26 per pulse, then before maturity of
2. Certain manual entries have to be energises relay RL1. In energised the call one enters 1.26 followed by
made in the calculator; for example, for condition of relay, the upper set of relay pressing of + key twice. Now, if a total
a pulse rate of Rs 1.26, number 1.26 is contacts connects the positive supply rail of ten billing pulses have been received

to PLL (phase- from exchange for the duration of the call,


locked loop) IC2 then on completion of the call, the
(LM567) pin 4, calculator display would show 12.60. The
while the lower set telephone operator has to bill the
of relay contacts customer Rs 14.60 (Rs 12.60 towards call
couples the positive charges plus Rs 2.00 towards service
telephone lead to charges).
input pin 3 of For tuning of the PLL circuit around
LM567 via IC2, lift the handset and inject 16kHz
capacitor C1 and tone across the line input points.
resistor R3. Tune IC2 to centre frequency of 16 kHz
The negative with the help of preset VR1. Proper
telephone lead is tuning of the PLL will cause LED
permanently (D6) to glow even with a very low-
to be entered after switching on the capacitively coupled to ground via amplitude 16kHz tone.
calculator followed by pressing of + capacitor C2. As soon as call matures, EFY Lab note. Arrangement used for
button twice. However, possibility exists 16kHz tone pulses would be pumped into simulating a 16kHz pulsed tone is shown
for automating these two functions by the telephone line by the telephone in Fig. 2. Push-to-on switch is used for
using additional circuitry. exchange at suitable intervals. This generation of fixed-duration pulse for
In telephony, on-hook condition is interval depends on the pulse rate of the modulating and switching on a 16kHz
represented by existance of 48V to 52V place called and also the time of the day oscillator.
across the line. Similarly, the off-hook and whether its a working-day or For more details regarding pulse
condition is represented by the line holiday. On receipt of 16kHz pulse, rates, pulse codes, etc, readers are
voltage dropping to a level of 8V to 10V output pin 8 of IC LM567 (which is tuned advised to go through the tariff rates and
(depending upon the length of the local for centre frequency of 16 kHz) goes low pulse code information given in the
lead line (local loop) from telephone for the duration of the pulse. The output beginning pages of telephone directories,
exchange to the subscribers premises as of IC2 is coupled via transistor T3 to such as MTNL, Delhi directory, Vol. I.

188 ELECTRONICS PROJECTS Vol. 21

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