Simple Latching Circuit
This circuit is good for alarm circuits, for example, a laser trip wire (once it has tripped, then it will stay on regardless if the laser line is restored). We shine a laser or bright light onto the LDR (Light dependent resistor) to break the circuit. The voltage divider from omhs law tells us that if we have two equal resistors in series we get half the supply voltage in the middle.
Vout = Vin * R2/R1 + R2
We can use this to turn on or off the transistor. At ambient light the LDR has a small resistance, but when the surface is illuminated enough, the resistance will shoot up and thus clamping the voltage to a level not enough to turn the transistor on via the base pin.
This circuit requires you to have the light source hitting the LDR before you fire it up. If you don't, it will latch instantly because of the low resistance of the LDR due to the ambient light.
To reset the Thyristor (also know as a Silicon Controlled Rectifier) you must interrupt the current flowing through the anode and cathode, it would make sense to add a momentary switch to do this.
Simple circuit created by myself.
Vout = Vin * R2/R1 + R2
We can use this to turn on or off the transistor. At ambient light the LDR has a small resistance, but when the surface is illuminated enough, the resistance will shoot up and thus clamping the voltage to a level not enough to turn the transistor on via the base pin.
This circuit requires you to have the light source hitting the LDR before you fire it up. If you don't, it will latch instantly because of the low resistance of the LDR due to the ambient light.
To reset the Thyristor (also know as a Silicon Controlled Rectifier) you must interrupt the current flowing through the anode and cathode, it would make sense to add a momentary switch to do this.
Simple circuit created by myself.