PROXIMITY INDUCTIVE SWITCH
This type
of switches replace the traditional mechanical limit switches, and never
wear out. The basic principle behind this switches is the induction of Fucault
currents on a near by ferromagnetic body. A high frequency
oscilator is the heart of this device. Normally in the AM-SW radio
frequency, the oscillator is tuned. Using a IF transformer, a secondary
ciruit rectifies the high frequency (1-3 Mhz) and feeds this volatge to a
comparator. When a ferromagnetic object aproches the field, circulating
currents (Fucault effect) develop, causing a dropping in the secondary
circuit. The comparator senses this, and triggers the output.
From left
to right: Oscillator, IF Transformer, Secondary Circuit and Comparator.
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Proximity
switch under test.
PROXIMITY SWITCH USING A
CRYSTAL OSCILATOR
Usually PC
motherboards or expansion boards include crystal oscillators. They come in
a large range of frequencies from 1Mhz up to 66 Mhz and beyond. To this
circuit, any frequency in this range will be ok.
Crystal Oscillators
The most
critical component of this circuit is the Intermediate Frequency (IF)
transformer. It can be obtained from discarded radio recivers,
walkie-talkies, or any radio-frequency device. The metal can must be carefuly
removed. I grinded the corners, so I just ‘peeled off’ the can, leaving
the transformer undamaged. I tested many IF transformers, since their
inductance and turn ratio differs from one to another. A good one can be
tested easily. Just conect a Vumeter at the secondary circuit, from the diode’s
cathode to the other terminal. The Vumeter must show a certain voltage,
and if you place a ferromagnetic object on top of the transformer, you
should see a change of voltage, then you are certain this IF transformer will
work for you.
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LED remains
off as the field is not
disturbed.
LED lights when screwdriver is place near IF transformer.