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THE 555 AS A MONOSTABLE


The 555 can be wired as a monostable. A monostable has one stable state and that is the OFF state. The unstable state is called the ON or HIGH state.
When it is triggered by an input pulse, the monostable switches to its temporary or ON state. It remains in that state for a period of time determined by an RC network and returns to its stable state. In other words, the monostable circuit generates a single pulse of a fixed time duration each time it receives and input trigger pulse.

The monostable circuit can also be called a ONE-SHOT due to the single-pulse it creates. This type of circuit can be used for activating an external device for a specific length of time. They can also be used to generate delays.

Another use for this type of circuit is to take the brief pulse of a push-button and activate a device. This is called a PULSE-EXTENDER.
It can also be used to clean-up the noisy output of a push-button and this is called SWITCH DEBOUNCING.
The diagram below shows a push-button connected to a 555. When the button is pressed, the relay operates for 5 seconds. The button must be released before the time-interval has expired otherwise the time is extended. This is the only limitation of this circuit.

The next circuit is an improved design. The switch can be pressed for any length of time and the circuit will only produce a 5 second output. The circuit is prevented from re-triggering by the addition of a 470k and 100n capacitor. When the switch is pressed, the uncharged capacitor takes pin 2 low and triggers the circuit. If the button is kept pressed the 100n charges and takes pin 2 high. The potential across the voltage divider formed by the 47k and 470k resistors is insufficient to re-trigger the monostable. The circuit "times-out" and the output goes low. When the button is released, the 100n discharges through the 470k and is ready for the next press.

A monostable (one-shot) can be connected to an astable (free-running oscillator) so that it gates (or inhibits) the oscillator to produce an output tone for a short duration. The circuit below can be used for an application such as doorbell. It is not suitable for battery operation as the 555 IC's are connected to the supply and draw current at all times.


This circuit can be used for a doorbell.

Pin 2 of the first 555 is HIGH and thus it is "non-operational" as it detects a LOW. Pin 6 is detecting a HIGH and thus the output of the IC is LOW. The output of the first 555 goes to the INHIBIT pin of the second 555. When pin 4 is LOW, the output of the chip is kept LOW.