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Passive-Optical Person Detector

responds to changes in ambient lighting

Published in issue 318, February 2003

Ordinary proximity detectors use infrared sensors which respond to the warmth of the human body. But there’s a simpler way: we can use ambient lighting and detect changes in illumination.Any move a person makes causes a change in the ambient lighting. This circuit uses an ordinary phototransistor as its ‘eyes’ to detect these changes. The circuit can detect even the smallest change in illumination: it is not even necessary for a shadow to fall directly on the detector. It is enough if just a part of the sky is occluded. The sensitivity of the sensor can be set using a potentiometer.

Components

Relevant products

Resistors:
R1,R6 = 10kΩ
R2 = 470kΩ
R3 = 1kΩ
R4 = 470kΩ
R5 = 4kΩ7
P1 = 10kΩ preset
Capacitors:
C1 = 1µF 16V radial
C2, C4 = 10µF 16V radial
C3,C5,C6 = 100µF 16 V radial
Semiconductors:
D1 = LED, green, low-current
D2 = LED, red, low-current
T1 = BP103
T2 = BC547B
IC1 = LM358P
Miscellaneous:
S1 = switch, on/off, 1 contact
BZ1 = 5V DC (active) buzzer
9V PP3 (6LR22) battery with clip-on lead
Enclosure with battery compartment, size 101 x 60 x 26 mm
PCB, available from The PCB Shop

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