timer circuit

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Electronic device for creating a time delay. The main component of most electronic timing devices is a capacitor in series with a resistor. Together they create a simple time delay. The size of the time delay can be found by using a simple equation:

t = C × R × 1.1

where C is the capacitor value in farads, R is the resistor value in ohms, and t is the time in seconds.

Dedicated integrated circuits (ICs) can be used in timing devices. The 555 IC is perhaps the most popular. There are three basic types of timer: a monostable, an astable, and a bistable. A monostable counts a fixed length of time. It is said to have one stable state. This can be monostable ON, where a device stays on for a fixed time and then turns off, or a monostable OFF which stays off for a fixed length of time and then comes on and stays on. Astable timers have no stable state. They are sometimes called square-wave timers because they switch on and off repeatedly, and the waveform seen on an oscilloscope has a square shape to it. This repeated on and off switching gives out an accurate pulse or beat that can be used as a clock pulse. The pulse rate is altered by adjusting capacitor and resistor values. A bistable is a timer that has two stable states because it has two outputs and two inputs. Switching the inputs causes alternate outputs to switch on and then off. This is why they are sometimes called ‘flip-flops’. Timers are not only used in timing devices such as clocks, but can also be found in electronic alarms and counting devices.

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