Chapter 27, Problem 7Q

To determine

Usage of photoelectric circuit in burglar alarm, a smoke detector and in photographic light meter.

Explanation of Solution

(a)A burglar alarm:

The alarm consist of a photoelectric cell connected in a closed electrical circuit and an infrared source of light. The light from the infra red source is continuously incident on the photoelectric cell, maintaining a constant current in the circuit. The device is kept conceated near the place to be protected.

If a burglar intercepts the invisible infrared rays, the photoelectric current stops. This automatically operates a relay,which sends an electric current in another circuit connected bell. The bell starts ringing and the alarm is raised.

(b)A smoke Detector:

A light beam could be focused on a photocell. When the light beam is interrupted by a smoke as it become denser and denser (thicker)then the current in the (thicker)circuit would decrease or stop. An alarm could be set to go off when the current decreased below a certain level.

Many smoke detectors use the photoelectric effect to detect tiny-particles of smoke that interrupt the flow of light and so alter the electric current.

(c)Photographic light meter:

The amount of current in the circuit depends on the intensity of light, as long as the frequency of light is above the threshold frequency (minimum frequency of incident light which cause photoelectric emission). The ammeter in the circuit could be calibrated to reflect the light intensity.