Proximity Switch

This circuit is for an unusually sensitive and stable proximity alarm which may be built at very low cost. If the negative terminal is grounded, it will detect the presence of a hand at more than 200mm. If it is not grounded, this range is reduced to about one-third. The Proximity Switch emits a loud, falling siren when a body is detected within its range.

Resistors:

R1 = 10k

R2 = 4k7

R3 = 1k

R4 = 47k

R5 = 47k

P1,P2 = 100k multiturn cermet, horizontal

Capacitors:

C1,C2 = 22pF

C3 = 22µF 40V radial

C4 = 10nF

C5 = 100µF 25V radial

Semiconductors:

D1 = 1N4148

D2 = LED, red

IC1 = 4093

Miscellaneous:

BZ1 = AC buzzer

PCB, ref. 040219-1