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The hobby circuit below was designed to indicate if a key held over the small coil of wire contains a 125KHz RFID chip. These chips are often
used in smart car ignition keys. The left side circuit forms a 125KHz power oscillator, which drives the 680uH air core inductor. The magnetic field generated excites the RFID chip
inside the key. The AM signal produced by the key is stripped off of the 125KHz carrier, is filtered and is routed to the clock input of a flip/flop. The flip/flop is configured as a
one-shot pulse generator. With the component values shown, the clock signal only has to be about 100 millivolts peak to peak to toggle the one-shot action. If the flip/flop is
toggled, a red LED is turned on. Try about 250 turns of 30 gage magnet wire with a diameter of one inch for the air core coil. Tune the electronic circuit for 125KHz by adjusting the
value of the series 0.002uF cap. |
