This design allows emulating a photo-thyristor, ie a SCR photo using a common phototransistor as a sensor. The circuit can be used in remote control systems, alarms and others by triggering a higher power SCR.
What we have is a very sensitive regenerative configuration that fires very fast when the sensor receives light. The circuit is used as a common component and the power can be made with voltages from 9 to 15 V.
In figure 1 we have the diagram of the photo thyristor.
In figure 2 it is mounted on a small printed circuit board.
Equivalent phototransistors can be used, as well as any commonly used PNP for Q1.
Material list
Q1 – BC558 –PNP transistor
Q2 – TIL81 or equivalente – fototransistor
RL – see text
R1 - 33k - resistor
R2 – 1k – resistor
C1 – 10 nF – capacitor
Several: printed circuit board,, wires. Solder, etc.
