(i) (a) In automatic door openers: As a approaches the door, light between the emmiter and the receiver is cut off. The current stops flowing. It activates the switch and the door is opened automatically.
(b) In intrusion alarm(burglar alarm): When a family is not at home and they lock the door, they switch on the photocell. When a thief enters the house, the switch is activated and the bell starts ringing .Photo cell thus used sometimes is also called electric eye. This is also used in bank security system.
(c) In turning street lights on and off during dusk and dawn:- This is done by installing photo cells on the light poles.
(ii) In a factory, the items on a conveyer belt pass between a beam of light and photocell. As each time passes the beam, it interscepts the light. The current in the photo cell stops and counter is turned on.With this method,the exact number of items living the factory can be counted.
(iii) (a) Solar cells can be used to provide small amounts of electricity in remote location where other sources are not available.
(b) In solar lantern in place of torches.
(c) In solar geysers for heating water.
There use is not so common because:
(1) Individual solar cells produce a low voltage of about 0.6 volt.
(2) They are quite expensive.
