The circuit below is part of a sensor that detects intruders - detects motion. As motion is detected a signal (current) is sent to the controlling computer. The controlling computer automatically activates the alarm, contacts the police and locks internal doors. However, during testing a problem is found. The signal from the senor is too brief as it only last a fraction of a second and it is not detected by the computer. The signal duration/length is not long enough. Consequently potential intruders will not be detected.

A time delay is the obvious answer and this can be achieved by adding a capacitor in parallel to the relay in the sensor circuit. If the relay is held closed for 0.75 seconds or more the computer program will have time to detect the signal - A capacitor provides the time delay.

The capacitor is placed in parallel to the relay and is charged by the current/power supply. When the sensor detects motion the relay energises (is held closed) and the capacitor discharges its current. The discharge from the capacitor ensures that the relay stays energised for approximately 0.75 seconds. This is enough time for the controlling computer to detect the signal from the sensor circuit.

The 555 circuit shown above is more sophisticated than the circuit above and is composed of several components included the integrated circuit (NE555). When switched on the buzzer sounds for a certain amount of time.
Some of the components are resistors and capacitors. It is often the combination of resistors and capacitors that control the time delay - in this case the length the buzzer sounds for.
If the capacitor C1 is changed for a higher value capacitor then the buzzer sounds for a longer period of time. The variable resistor VR1 can also determine the length of time.
 

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