This circuit is a simple visible light or IR detector for testing any equipment which outputs light. The circuit can be also used for measuring ambient light levels. The circuit is based on one photodiode which is used in photocell mode so that the photodiode generates a voltage and current signal when it get light.The circuit is best used when you connect it directly to the input of your oscilloscope. Then set the input to the DC inout setting.source

This circuit designed to help a beekeeper count the number of bees going into or out of a hive. The low power circuit uses a slotted opto-sensor to detect the passing bees. The circuit advances an electronic counting module whenever a honeybee passes through the sensor. The device only counts the number of bees going through the sensor.source

This circuit will turn on/off 12V DC fan or CPU fan when temperature above preset temperature.You can set turn on temperature by adjust VR1. This circuit use an NTC (Negative temperature coefficient) as a temperature sensor.The NTC is a thermistor that the zero-power resistance decreased when  increase the temperature. source

This is a light switch or light activated switch circuit.The relay on when LDR uncovered and relay off when LDR covered.Adjust VR1 for light sensitive.LED will turn on at the same time with relay.The circuit use LM311 comparator.This circuit need 12Vdc power supply.source

This circuit use a single-supply system to precisely measure the temperature at a remote location with less than 1°C error over a 0 to 100°C range . The circuit includes a low-cost AD590 temperature sensor, an AD8541 rail-to-rail amplifier, four resistors, a trimming potentiometer and an ADC. source