Specifications

CIRCUIT IDEAS
CIRCUIT IDEAS
M
ost of the power-supply failure
indicator circuits need a sepa-
rate power-supply for them-
selves. But the alarm circuit
presented here needs no ad-
ditional supply source. It em-
ploys an electrolytic capacitor
to store adequate charge, to
feed power to the alarm cir-
cuit which sounds an alarm
for a reasonable duration
when the mains supply fails.
During the presence of
mains power supply, the rec-
tified mains voltage is stepped
down to a required low level.
A zener is used to limit the
filtered voltage to 15-volt
level. Mains presence is indicated by an
LED. The low-level DC is used for charg-
ing capacitor C3 and reverse biasing
switching transistor T1. Thus, transistor
T1 remains cut-off as long as the mains
supply is present. As soon as the mains
power fails, the charge stored in the ca-
pacitor acts as a power-supply source for
S.C. DWIVEDI
M.K. CHANDRA MOULEESWARAN
POWER-SUPPLY FAILURE ALARM
transistor T1. Since, in the absence of
mains supply, the base of transistor is
pulled ‘low’ via resistor R8, it conducts
and sounds the buzzer (alarm) to give a
warning of the power-failure.
With the value of C3 as shown, a good-
quality buzzer would sound for about a
minute. By increasing or decreasing the
value of capacitor C3, this time can be
altered to serve one’s need.
Assembly is quite easy. The values of
the components are not critical. If the
alarm circuit is powered from any exter-
nal DC power-supply source, the mains-
supply section up to points ‘P’ and ‘M’
can be omitted from the circuit. Follow-
ing points may be noted:
1. At a higher DC voltage level, tran-
sistor T1 (BC558) may pass some collec-
tor-to-emitter leakage current, causing a
continuous murmuring sound from the
buzzer. In that case, replace it with some
low-gain transistor.
2. Piezo buzzer must be a continuous
tone version, with built-in oscillator.
To save space, one may use five small-
sized 1000µF capacitors (in parallel) in
place of bulky high-value capacitor C3.
RUPANJANA
T
he heart of this circuit is a COB
which is used in quartz clocks.
SCR1 is used for ‘start’ and ‘stop’
operations. LED1 used in the circuit
serves two purposes. It provides the path
to satisfy the minimum holding current
requirement (about 6 mA for a low-power
SCR) for the SCR, to maintain it in ‘on’
state. By placing the LED in the vicinity
of LCD, one can read the display even
during darkness. The positive going out-
put pulses from the two points of the COB
ANANDAN M.A.
(from Ajanta timepiece used by EFY) are
combined to obtain one pulse per second
output across resistor R4.
(EFY Lab note: Please note that
COBs used in different clocks may give
different outputs—frequency as well as
polarity—which may necessitate reversal
of diodes, use of additional transistor in-
verter stage, and modification of key op-
eration sequence of calculator.)
The voltage developed across resistor
R4 provides forward bias for transistor T1.
Transistor T1 conducts and switches ‘on’
the optocoupler, whose output (across col-
STOPWATCH USING
COB AND CALCULATOR
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