Specifications

CIRCUIT IDEAS
successive overloads. Under this condition,
the system has to be manually reset. (Suc-
cessive overload condition indicates that
the inverter output is short-circuited or a
heavy current is being drawn by the con-
nected load.)
The circuit uses an ammeter (0-30A)
as a transducer to detect overload condi-
tion. Such an ammeter is generally
present in almost all inverters. This am-
meter is connected between the negative
supply of the battery and the inverter, as
shown in Fig. 2. The voltage developed
across this ammeter, due to the flow of
current, is very small. It is amplified by
IC2, which is wired as a differential am-
plifier having a gain of 100. IC3 (NE555)
is connected as a Schmitt
‘trigger’, whose output goes
low when the voltage at its
pin 2 exceeds 3.3V. IC4
(again an NE555 timer) is
configured as a monostable
multivibrator with a
pulsewidth of 6 seconds.
IC5 (CD4017) is a CMOS
counter which counts the three overload
conditions, after which the system has to
be reset manually, by pressing push-to-
on switch S1.
The circuit can be powered from the
inverter battery. In standby condition, it
consumes 8-10 mA of current and around
70 mA with relay (RL1), buzzer (PZ1),
and LED1 energised. Please note the fol-
lowing points carefully:
• Points A and B at the input of IC2
should be connected to the corresponding
points (A and B respectively) across the
ammeter.
• Points C and D on the relay termi-
nals have to be connected in series with
the already existing ‘on’/‘off’ switch leads
of inverter as shown in Fig. 1. This means
that one of the two leads terminated on
the existing switch has to be cut and the
cut ends have to be connected to the pole
and N/O contacts respectively of relay
RL1.
• The ammeter should be connected
in series with the negative terminal of the
battery and inverter, as shown in Fig. 2.
Move the wiper of preset VR1 to the
extreme position which is grounded.
Switch ‘on’ the inverter. For a 300W in-
verter, connect about 250-260W of load.
Now adjust VR1 slowly, until the inverter
just trips or shuts down. Repeat the step
if necessary. Use good-quality preset with
dust cover (e.g. multi-turn trimpot) for
reliable operation.
The circuit can be easily and success-
fully installed with minimum modifica-
tions to the existing inverter. All the com-
ponents used are cheap and readily avail-
able. The whole circuit can be assembled
on a general-purpose PCB. The cost of
the whole circuit including relay, buzzer,
and PCB does not exceed Rs 100.
S.C. DWIVEDI
TELEPHONE LINE BASED AUDIO
MUTING AND LIGHT-ON CIRCUIT
V
ery often when enjoying music or
watching TV at high audio level,
we may not be able to hear a tele-
phone ring and thus miss an
important incoming phone call.
To overcome this situation, the
circuit presented here can be
used. The circuit would auto-
matically light a bulb on ar-
rival of a telephone ring and
simultaneously mute the mu-
sic system/TV audio for the du-
ration the telephone handset
is off-hook. Lighting of the
bulb would not only indicate
an incoming call but also help
in locating the telephone dur-
ing darkness.
On arrival of a ring, or
when the handset is off-hook,
the inbuilt transistor of IC1
(opto-coupler) conducts and capacitor C1
gets charged and, in turn, transistor T1
gets forward biased. As a result, transis-
tor T1 conducts, causing energisation of
relays RL1, RL2, and RL3. Diode D1 con-
nected in anti-parallel to inbuilt diode of
IC1, in shunt with resistor R1, provides
an easy path for AC current and helps in
limiting the voltage across inbuilt diode
to a safe value during the ringing. (The
RMS value of ring voltage lies between
70 and 90 volts RMS.) Capacitor C1 main-
tains necessary voltage for continuously
forward biasing transistor T1 so that the
relays are not energised during the nega-
DHURJATI SINHA
Fig. 2
117