Page 1010 - Basic Electrical Engineering

dark conditions, the lights should automatically get switched on. A 12 V
battery supply is connected to the relay coil which has an LDR resistance
connected in series as shown in Fig. 14.47. During night darkness the LDR

resistance is very high, and hence a very small amount of current will flow
through the relay coil. During night, or during darkness, and even during

cloudy days, the lights L , L , L should be on as they get 230 V supply
3
2
1
through the normally closed contact of the relay. Current through the relay
coil is very low because the LDR resistance during darkness is very high. The
armature (the movable part of the relay) of the relay will stay at the position

shown. When during day time light falls on the LDR which may be placed in
a convenient position, its resistance will decrease, and hence current through

the relay will increase. At a particular value of current, the armature will be
attracted and pulled towards the fixed part of the relay against the spring

pressure. Then the relay contact, ‘a’ will open. The lights will not get any
supply, and hence will be off. When darkness comes, the LDR resistance will
increase, current through the relay will decrease, the attractive force of the

relay coil will decrease, the armature will return to its original position due to
the spring action. The relay contact, ‘a’ will close, and hence the lights will

get full supply and will be switched on.

Figure 14.47 Lights controlled by relay-operated LDR circuit