Instead of using a counter-controlled, pushbutton-activated range switch, you can use an optical encoder. Inexpensive encoders are available, and they occupy minimal space on the front panel of an instrument. Moreover, an encoder gives you the opportunity to select the optimum operating speed. However, at positions near the transition points in counter position, mechanical shocks can provoke false switching. The circuit in figure above overcomes the false-switching problem. source

A programmable reference diode, such as the Motorola TL431A, can supply constant-current bias for a silicon pressure-sensor bridge. This circuit is simpler than using an op amp and separate reference diode or than using a current diode, which requires temperature compensation.source

This circuit has a very low standby current requirement yet has very high sensitivity toward the light flashes from a xenon lamp. When connected to a flip/flop it can serve as an on on/off controller.source

The circuit uses a very inexpensive C-MOS IC that is connected to a small photodiode. Using a unique inductive feedback network, the circuit provides high sensitivity under high ambient light conditions. It is a great circuit when you want to extend the range of an optical remote control transmitter.source

This is an infrared gate with two sensors planned to use in the wall in the way behind a door. It can be applied in a toilet to keep track of that someone is inside exceeding a certain amount of time. After that time elapsed, the circuit triggers the digital output wich can turn on a ventillator. The time period the output is turned on can be separately controlled by a second timer.source