Reading RFID-badges operating at 125 kHz is very easy. You just have to put a carrier
(125 kHz) into the air and have the badge do the work. The badge will do the ASK
(Amplitude Shift Keying) and you only have to monitor the envelope of the carrier.
This can be done by a simple diode rectifier.
The carrier is generated by a 74HC4060 and a 4 MHz crystal. Using "Q4" delivers the desired 125 kHz signal.
The AM Demodulator is build of a AA113 (for historic reasons) and R5, C5. The demodulator is followed by a limiting amplifier.
The voltage across the coil is about 14 Vpp.
The envelope of the carrier, when a tag / badge is brought close to the coil ...
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