Specifications

CIRCUIT IDEAS
board, in a single line, so that they may
be pushed through the cutouts in the front
panel of the enclosure from inside.
The water level at 30cm intervals is
monitored by corresponding sensors, caus-
ing the input to the concerned inverters
(normally pulled high via resistors R1
through R10) to go low, as soon as water
reaches the respective sensors.
On initial switching on of the power
supply, when the tank is empty, all the
electrodes are open. As a result, all the
inverter inputs are high (via the pull-up
resistors R1 to R10) and their outputs are
all low. Thus, all the LEDs are off. As
soon as the water starts filling the tank,
the rising water level grounds the first
sensor. The logic 1 output of first inverter
gate N1 causes conduction of transistor
T2 to extend ground to one side of resis-
tors R14 through R23 via emitter-collec-
tor path of transistor T2. The LED D1 is
thus lit up.
Similarly, other LEDs turn on suc-
cessively as the water level rises. As soon
as the water in OHT reaches the top level,
the output of gate N10 goes to logic 1 and
causes flashing-type LED D11 to start
flashing. At the same time, transistor T1
conducts and cuts off alternately, in syn-
chronism with LED D11s flash rate, to
ground the base of transistor T2 during
conduction of transistor T1. As a result,
transistor T2 also starts cutting off dur-
ing conduction of transistor T1, to make
the LED meter (comprising LEDs D1
through D10) flash and thus warn that
the water has reached the top level. When
the water level goes down, the reverse
happens and each LED is turned off suc-
cessively.
The novel feature of this circuit is that
whenever the water level is below the first
sensor, all the LEDs are off and the qui-
escent current is very low. Thus, a power
on’/‘off switch is not so essential. Even
when the LED-meter is fully on, the cur-
rent drawn from the power supply is not
more than 120 mA. A heat-sink may, how-
ever, be used for transistor T2, if the tank
is expected to remain full most of the time.
A power supply unit providing unregu-
lated 6V DC to 15V DC at 300mA current
is adequate.
Caution. A point to be noted is that
water tends to stick to the narrow space
at the sensor-spacer junction and can
cause a false reading on the LED-meter.
This can be avoided if the spacers are
made wider than 10 mm.
H
ere is a simple yet very useful
circuit which can be used to
eavesdrop on a telephone con-
versation. The circuit can also be used as
a wireless telephone amplifier.
One important feature of this circuit
is that the circuit derives its power di-
rectly from the active telephone lines, and
thus avoids use of any external battery
or other power supplies. This not
only saves a lot of space but also
money. It consumes very low cur-
rent from telephone lines without
disturbing its performance. The
circuit is very tiny and can be
built using a single-IC type
veroboard that can be easily fitted
inside a telephone connection box
of 3.75 cm x 5 cm.
The circuit consists of two sec-
tions, namely, automatic switching
section and FM transmitter section.
Automatic switching section
comprises resistors R1 to R3, preset
VR1, transistors T1 and T2, zener D2,
and diode D1. Resistor R1, along with pre-
set VR1, works as a voltage divider. When
voltage across the telephone lines is 48V
DC, the voltage available at wiper of pre-
set VR1 ranges from 0 to 32V (adjustable).
The switching voltage of the circuit de-
pends on zener breakdown voltage (here
24V) and switching voltage of the transis-
tor T1 (0.7V). Thus, if we adjust preset
VR1 to get over 24.7 volts, it will cause
the zener to breakdown and transistor T1
to conduct. As a result collector of transis-
tor T1 will get pulled towards negative
supply, to cut off transistor T2. At this
stage, if you lift the handset of the tele-
phone, the line voltage drops to about 11V
and transistor T1 is cut off. As a result,
transistor T2 gets forward biased through
resistor R2, to provide a DC path for tran-
sistor T3 used in the following FM trans-
mitter section.
The low-power FM transmitter sec-
tion comprises oscillator transistor T3, coil
L1, and a few other components. Transis-
tor T3 works as a common-emitter RF
oscillator, with transistor T2 serving as
an electronic on’/‘off switch. The audio
signal available across the telephone lines
automatically modulates oscillator fre-
quency via transistor T2 along with its
series biasing resistor R3. The modulated
RF signal is fed to the antenna. The tele-
phone conversation can be heard on an
FM receiver remotely when it is tuned to
FM transmitter frequency.
Lab Note: During testing of the cir-
cuit it was observed that the telephone
used was giving an engaged tone
when dialed by any subscriber. Addi-
tion of resistor R5 and capacitor C6 was
found necessary for rectification of the
fault.
S.C. DWIVEDI
PHONE BROADCASTER
ANJAN NANDI
66