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# monostable circuit

A monostable circuit is an electronic device called a multivibrator that has two distinct states, one of them stable (having a steady voltage) and the other one unstable (having an unsteady or variable voltage). Normally, the circuit is in the stable condition. The circuit can be removed from the stable condition temporarily by an input pulse called a triggering signal. After a certain period of time, which can be adjusted or set by choosing specific component values, the circuit returns to the stable state until another triggering signal is applied.

A monostable circuit comprises an IC (integrated circuit), usually a device called a 555 timer, along with an external resistance and an external capacitance. Normally, the voltage output equals zero or near zero. This state is called low. When a triggering pulse is applied to the input, the output voltage rises to the supply or battery voltage (the high state, which usually ranges from +3 to +15 volts) for a length of time that depends on the values of the resistance R and the capacitance C.

If we specify R in ohms and C in microfarads, then the pulse duration t, in microseconds, can be calculated as

t = 1.1 RC

This formula also applies for values of R in ohms, C in farads, and t in seconds.

After the delay time t has elapsed, the monostable circuit returns to the low state.

Monostable circuits are used as timers, pulse generators, waveform generators, and sweep generators for CRTs (cathode ray tubes) such as the ones found in some television (TV) receivers, older computer displays, oscilloscopes, and spectrum analyzers.

Tim Surtell describes the construction and operation of a simple monostable circuit using a 555 timer IC.

Computer Support Group offers a timeout calculator for monostable circuits based on the 555 timer IC.

This was last updated in April 2012
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