A type of transistor in which base current is produced when light strikes the photo sensitive semiconductor base region.Figure shows a circuit in which the relay is de-energized by incident light on the photo-transistor when there is insufficient light, the photo transistor is in cutoff and maximum positive voltage appears at the base of Q2 which turns ON and keeping the energized the darkness operated relay circuit. flight training . witte kist . When there is sufficient light, phototransistor Q1 turns ON, this pulls the base of Q2 low. Thus turning Q2 IFF and de-energized the relay.
These relay circuits can be used in variety of application such as
- Automatic doors activator
- Process counters
- Alarms Systems.
Another simple application is illustrated in following figure.The photo transistor is normally ON, holding the gate of the SCR low. When the light is interrupted, the photo-transistor turns OFF. The high going transition on the collector triggers SCR and sets OFF the alarm mechanism. The momentary contact switch swi provides for resetting the alarm. Smoke detection and intrusion detection are possible uses for these circuits.If you have any question please post here