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. A wide range of metal objects may be used
for the sensor, including a metal plate, a doorknob, tin foil, a set of
burglar bars — even a complete bicycle. Not only this, but any metal
object which comes within range of the sensor, itself becomes a sensor.

For example, if a tin foil sensor is mounted underneath a table,
metal items on top of the table, such as cutlery, or a dinner service,
become sensors themselves. The touch plate connected to the free end of
R1 detects the electric field surrounding the human body, and this is of
a relatively constant value and can therefore be reliably picked up. R1
is not strictly necessary, but serves as some measure of protection
against static charge on the body if the sensor should be touched
directly. As a body approaches the sensor, the value of C1 effectively
increases, causing the frequency of oscillator IC1.A to drop.

Consequently capacitor C2 has more time to discharge through P2,
with the result that the inputs at IC1.B go Low, and the output goes
High. As the output goes High, so C3 is charged through LED
D2. D2 serves a dual purpose —namely as a visual indication of
detection, and to lower the maximum charge on C3, thus facilitating a
sharper distinction between High and Low states of capacitor C3. The
value of R4 is chosen to enable C3 to discharge relatively quickly as
pulses through D2 are no longer sufficient to maintain its charge. The
value of C3 may be increased for a longer sounding of the siren, with a
slight reduction in responsiveness at the sensor.

When C3 goes High, this triggers siren IC1.C and IC1.D. The two NAND
gates drive piezo sounder X1 in push-pull fashion, thereby greatly
increasing its volume. If a piezo tweeter is used here, the volume will
be sufficient to make one’s ears sing. The current consumption of the
circuit is so low a small 9-V alkaline PP3 battery would last for about
one month. As battery voltage falls, so sensitivity drops off slightly,
with the result that P1 may require occasional readjustment to maintain
maximum sensitivity. On the down side of low cost, the hysteresis
properties of the 4093 used in the circuit are critical to operation,
adjustment and stability of the detector.

In some cases, particularly with extremely high sensitivity
settings, it will be found that the circuit is best powered from a
regulated voltage source. The PCB has an extra
ground terminal to enable it to be easily connected to a large earthing
system. Current consumption was measured at 3.5 mA stand-by or 7 mA
with the buzzer activated. Usually, only P1 will require adjustment. P2
is used in place of a standard resistor in order to match temperature
coefficients, and thus to enhance stability. P2 should be adjusted to
around 50 k, and left that that setting.

The circuit is ideally adjusted so that D2 ceases to light when no
body is near the sensor. Multiturn presets must be used for P1 and P2.
Since the piezo sounder is the part of the circuit which is least
affected by body presence, a switch may be inserted in one of its leads
to switch the alarm on and off after D2 has been used to check
adjustment. Make sure that there is a secure connection between the
circuit and any metal sensor which is used.

R1 = 10kΩ
R2 = 4kΩ7
R3 = 1kΩ
R4 = 47kΩ
R5 = 47kΩ
P1,P2 = 100kΩ multiturn cermet, horizontal
C1,C2 = 22pF
C3 = 22µF 40V radial
C4 = 10nF
C5 = 100µF 25V radial
D1 = 1N4148
D2 = LED, red
IC1 = 4093
BZ1 = AC buzzer

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