Capacitive proximity switch circuit

It can be used for lighting control, and can also be used in various
places such as anti-theft, alarm, limit, and positioning.
    When the human body is close to the metal plate M, the capacitance of Co increases,
the oscillation frequency of the self-excited multivibrator composed of the 555 time base integrated circuit Ai, etc. decreases, and the output pin 6 of the comparator composed of the operational amplifier A2, etc. is low. level, the transistor
VT is turned on, the relay KA is closed, and its contacts control the lighting or alarm system circuit, lighting the lighting or sending out an alarm
signal.
    Adjusting the potentiometers RPi and RPz can change the sensitivity of the device, which can be determined according to actual needs.
In situations where the environment is poor and there is a lot of dust, photoelectric control switches are not suitable, and inductive non-contact switches can be used.