Home Page.
A photoresistor or Light Dependent Resistor operates in ambient light. It can be used in some circumstances to detect the presence of a train. Operation is from 3 volt battery or a 3 to 12 volt DC supply and the output is a LED for display only.
With light the photoresistor has a low resistance which biases the transistor off and the LED is extingquished. When a train is over the sensor its resistance goes high which raises the voltage on the transistor base. The Emitter follows this and the voltage drives the LED., 1.6 to 1.8 volts is required to light the LED.
Sensitivity is adjusted by the 20k potentiometer. Starting at the centerpoint of the potentiometer and with the sensor covered by a train, adjust until the LED is at the correct brilliance. Expose the sensor and the LED should go out. If it doesn't then reduce the brilliance. The higher the voltage the brighter the LED but be careful not to allow too much voltage on the LED as it will burn out.
The circuit can be built on vero board.
A photoresistor or Light Dependent Resistor operates in ambient light. It can be used in some circumstances to detect the presence of a train. Operation is from 3 volt battery or a 3 to 12 volt DC supply and the output is a LED for display only.
With light the photoresistor has a low resistance which biases the transistor off and the LED is extingquished. When a train is over the sensor its resistance goes high which raises the voltage on the transistor base. The Emitter follows this and the voltage drives the LED., 1.6 to 1.8 volts is required to light the LED.
Sensitivity is adjusted by the 20k potentiometer. Starting at the centerpoint of the potentiometer and with the sensor covered by a train, adjust until the LED is at the correct brilliance. Expose the sensor and the LED should go out. If it doesn't then reduce the brilliance. The higher the voltage the brighter the LED but be careful not to allow too much voltage on the LED as it will burn out.
The circuit can be built on vero board.
LINK TO IMAGE - LDRdetector.gif
Circuit Schematic
