This is an
event interruption alarm circuit, or a circuit used for sounding off an
alarm if an expected event does not happen.
The occurrence of the event is represented in the circuit above by
switch S1, i.e., the switch S1 is momentarily closed every time the
expected event occurs. The circuit activates a self-oscillating piezo
buzzer if this event fails to take place again (i.e., the switch is not
momentarily closed again) within a set time limit after the last event
occurrence.
The main
component of this circuit is the 555, a versatile timer IC. It is
configured as a monostable multivibrator, i.e., a circuit which will
output a single pulse at pin 3 every time pin 2 is pulled 'low'.
Each time
the event occurs, switch S1 closes, discharging C1 to ground through
Q1 and triggering pin 2 to force an internal flip-flop to set pin 3
to a 'high' level. At this stage, the buzzer is silent since
both of its terminals are 'high'. Meanwhile, capacitor C1
charges up again through R2 as soon as S1 is opened. The
voltage across C1 will continue to rise unless S1 is momentarily
closed again to discharge it through Q1. Momentarily closing
S1 regularly before the time limit is up will prevent C1 from
charging up to the critical voltage level, thereby keeping the
buzzer silent.
If,
however, S1 is not closed within the set time limit, the voltage
across C1 will become high enough to trigger the internal flip-flop
to set pin 3 to a 'low' level. At that point, the
self-oscillating buzzer will alarm because pin 3 is now pulling one
of its terminals to 'low' while the other terminal is at Vcc.
See
also: 555 Monostable
Multivibrator; Flip-flops
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