A popular element of night caches in the game "Geocaching" are reactive lights. That are small electronic devices with a light emitting diode and a sensor for the brightness. In the dark one can activate it using a torch light. Then they are blinking a preset sequence. Apart from simple blinking it can be morse code, too. Depending on the sensitivity and selected type of brightness sensor a reactive light can be triggered over a distance of several hundret meters reliably. Beside photoconductive cells, photodiodes and phototransistors also light emitting diodes can be used as sensors. That makes it possible to construct a complete reactive lights with just a couple of elements. Normal batteries are used for power supply. Using ingenious mechanism for saving power makes it possible to run a reactive light several years with just two AA cells.
There are two different kinds of reactive lights. The first one uses the microcontroller ATtiny13V of the company Atmel. The controller is programmed using the language "Bascom". It provides a high level of flexibility, but knowledge of electronics and programming microcontroller is needed. To avoid this, the second type can be used. It bases on CMOS gates that are connected by external elements. That results in a higher number of elements as in the first variant. Because of the lack of ability to program the circuit, no variation of the blinking sequence is possible.