Specifications
CIRCUIT IDEAS
S.C. DWIVEDI
LASER CONTROLLED ON/OFF SWITCH
Dr K.P. RAO
T
his circuit is built around a 555
timer using very few compo-
nents. Since the circuit is very
simple, even a novice can easily build it
and use it as a controlling device. A laser
pointer, now easily available in the mar-
ket, can be used to operate this device.
This circuit has been tested in op-
erational conditions from a distance of
500 metres and was found to work sat-
isfactorily, though it can be controlled
from still longer distances. Aiming
(aligning) the laser beam exactly on to
the LDR is a practical problem.
The circuit is very useful in switch-
ing on/off a fan at night without get-
ting off the bed. It can also be used for
controlling a variety of other devices
like radio or music system. The limita-
tion is that the circuit is operational
only in dark or dull-lit environments.
By focussing the laser beam on
LDR1 the connected gadget can be acti-
vated through the relay, whereas by fo-
cussing laser beam on LDR2 we can
switch off the gadget. The timer is con-
figured to operate in bistable mode.
The laser pointers are available for
less than Rs 150 in the market. The cost
of the actual circuit is less than Rs 50.
TELEPHONE CONVERSATION
RECORDER
The second part of the circuit con-
trols relay RL1, which is used to switch
on/off the tape recorder. A voltage of 48
volts appears across the telephone lines
in on-hook condition. This voltage drops
to about 9 volts when the handset is
lifted. Diodes D1 through D4 constitute
a bridge rectifier/polarity guard. This
ensures that transistor T1 gets voltage
of proper polarity, irrespective of the
polarity of the telephone lines.
During on-hook condition, the out-
put from the bridge (48V DC) passes
through 12V zener D5 and is applied to
the base of transistor T1 via the volt-
age divider comprising resistors R3 and
R4. This switches on transistor T1 and
its collector is pulled low. This, in turn,
PRADEEP VASUDEVA
T
his circuit enables automatic
switching-on of the tape recorder
when the handset is lifted. The
tape recorder gets switched off when
the handset is replaced. The signals are
suitably attenuated to a level at which
they can be recorded using the ‘MIC-
IN’ socket of the tape recorder.
Points X and Y in the circuit are
connected to the telephone lines. Re-
sistors R1 and R2 act as a voltage di-
vider. The voltage appearing across R2
is fed to the ‘MIC-IN’ socket of the tape
recorder. The values of R1 and R2 may
be changed depending on the input im-
pedance of the tape recorder’s ‘MIC-IN’
terminals. Capacitor C1 is used for
blocking the flow of DC.
S.C. DWIVEDI
52