Timer

The basic circuit operates at a frequency determined by R1, R2 and C1 and has a pulse width range of 0 to 100 percent.
The following diagram shows a basic circuit with an open collector output that would require a pull up resistor at its output. The parts values are the nominal values of [...]

This little timer kit can be easily added to any existing project to allow for automatic shutoff. Tons of applications; heating, cooling, lighting, entertainment systems, fans, pumps, sprinklers, cryogenic stasis tubes? well maybe not that last one! Just set the jumper for short or long timers, use the push button to select your desired timer [...]

These circuits show methods of changing the operating frequency of astable LM555 timers electronically. Any source that can drive the base of transistor Q1 can control these circuits.
The advantage of using this type of frequency control is that the duty cycle of the timer is not affected when the frequency is changed. [...]