Flashlight Controlled Remote Control Switch Circuit

It is sometimes very annoying for a person who, while watching a TV
programme or listening to a radio or the like, has to get up to switch
lights or fans or AC off. The same applies when he wants to switch them
on.
Using the simple flashlight remote control circuit
shown in Fig. 1, one can switch almost any domestic electrical equipment
from a distance, simply by flashing a torchlight.  The basic sensor
used in this remote switching mechanism – is a light dependent resistor
(LDR), whose resistance value changes in response to the incident light.
The 555 timer is used in the monostable mode in Fig. 1, which is
triggered when the voltage at its pin 2 falls below l/3 Vcc. This
triggering takes place when light is incident on the LDR, thus lowering
its resistance value. The l0k pot VRI is used  for setting up the
triggering threshold. The output time  period of the 555 is adjustable
up to l2 seconds by varying the 500k pot VR’2, so as to smooth up the
light impulse (similar to switch debouncing); The output of the 555 is
fed to the input ofa JK master/slave flip-flop (7473) which is
configured in the toggle mode (by tying the J and K inputs to Vcc). ln
the toggle mode, the output of the flip-flop changes state with every
pulse at its clock input. The output of the 7473 feeds an amplifier
stage comprising a BC 148 and a SLl00 transistor which are used to drive
a relay that switches the instrument to be controlled. The circuit can
be mounted on a small veroboard and placed in a cabinet with a small
hole for exposing the LDR. Care should be taken to ensure that the LDR
receives only the light from the torch, and that no ambient light falls
on it. Once so installed, it will work reliably, with no false
triggering that sometimes occurs in sound operated remote control units.

The entire digram for the flashight controlled remote circuit is shown below: