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Circuits > Security and other sensors and detectors > Proximity Alarm
Proximity Alarm
Inverters U1a and U1b are connected in a simple RC oscillator circuit. The
frequency is determined by the values of R1, C1 C2 and the internal
characteristics of the integrated circuit. As long as the circuit is oscillating,
a positive dc voltage is developed at the output of the voltage-couple circuit:
C3, D2 and C4. The dc voltage is applied to the input of U1c-the third inverter
amplifier-keeping its output in a low state, which keeps Q1 turned off so that
no sound is produced by BZ1. With C1 and C2 adjusted to the most sensitive point,
the pickup plate will detect a hand 3 to 5-inches away and sound an alert. Set
C1 and C2 to approximately one-half of their maximum value and apply power to
the circuit. The circuit should oscillate and no sound should be heard. Using a
non-metallic screwdriver, carefully adjust C1 and C2, one at a time, to a lower
value until the circuit just ceases oscillation: Buzzer BZ1 should sound off.
Back off either C1 or C2 just a smidgen until the oscillator starts up again-that
is the most sensitive setting of the circuit.
Title: Proximity Alarm
Source: unknown
Published on: 2005-08-27
Reads: 822
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Circuits > Security and other sensors and detectors > Proximity Alarm |
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