Specifications
CIRCUIT IDEAS
CIRCUIT IDEAS
S.C. DWIVEDI
DHURJATI SINHA
LOW-COST PCO BILLING METER
T
he 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 com-
mercial equipment serving similar pur-
pose may cost more than Rs 10,000 in the
market. The comparative disadvantages
of the presented circuit are as follows:
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 ex-
ists 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 volt-
age 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.
In the circuit shown in Fig. 1, when
the handset is off-hook, the optocoupler
MCT2E (IC1) conducts and forward bi-
ases transistor T1, which, in turn, for-
ward biases transistor T2 and energises
relay RL1. In energised condition of re-
lay, the upper set of relay contacts con-
nects 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 nega-
tive telephone
lead is perma-
nently capaci-
tively coupled
via capacitor
C2. As soon as
call matures,
16kHz tone
pulses would
be pumped
into the tele-
phone line by
the telephone exchange at suitable inter-
vals. 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, out-
put 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 ‘=’ but-
ton on a calculator (such as Taksun
make), which has the effect of pressing
the ‘=’ button of the calculator.
Considering that pulse rate for a spe-
cific 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 dis-
play would show 12.60. The telephone op-
erator 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 os-
cillator.
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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