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 P8

555
Monostable

 

In Brief…..

In this circuit, the output of the 555 Integrated Circuit (I.C.) will momentarily go to "on", once a brief trigger input has been applied to its input on pin 2. It will then stay in the "on" state for a pre-determined amount of time (t), before returning to its "off" state. It will remain "off" until it is triggered once again. As it is stable in one state only, it is known as a monostable circuit.

The 555 Integrated Circuit is also described in P7, where it is used in an astable circuit. In this application it can make its output repeatedly turn "on and off", at a chosen rate.

The 555 I.C. will source, or sink, current up to 200mA at its output. The I.C. is built on a small element called Silicon. Its circuit consists of 22 transistors, 2 diodes and 16 resistors and its package is smaller than one square centimetre. The I.C. is commonly used in electronic devices and only costs a few more pence than a single transistor.

Circuit Diagram


How the circuit works....

The astable 555 circuit uses R and C to determine how long the output of the monostable will remain in the high state (+ supply volts) before it returns back to the low state (0 volts). The sequence begins when the voltage (trigger) input is taken down to 0 volts, as shown below.


The change in I/P voltage sends the output of the 555 timer to the high state (power supply + volts). At the same time the capacitor C starts to charge up via resistor R. When the voltage on the capacitor reaches 2/3 of the + supply rail, the output returns to 0 volts. If the reset input on pin 4 is taken to 0 volts, at any time, the ouput of the 555 timer will immediately return to the low state. The other components in the circuit have been added for stability.

Calculating the time (t) the output stays high....

t (in secs) = 1.1 R x C

(Note R is measured in Ohms and C in Farads)