Hi Guys, I'm trying to build a simple project to get some pratice in analog electronics. The intention is to make an electronic cat flap that only allows entry to a cat wearing a magnetic collar (yes, you've seen them in pet shops). I a 403Ohm coil around the entrance to the cat flap, passing a magnet through this coil generates approximately 1-20mV and varies the resitance of the coil by about 20Ohms in either direction. I need to turn this signal into a digital on or off, with on indicating that a magnet is moving through the coil. I'm not concerned about directionality, and the more sensitivity I can get the better. Neither does the signal need to remain on for very long, as I have a latching circuit in place that will work with a very short signal from the detector.
I attempted to built an A.C. coupled darlington pair as described on the following page (following the resistor and capacitor values exactly):
Can somebody point me in the direction of a circuit that should work for my project? I'm not sure whether I should be using a darlington pair or an op-amp, or whether I need to A.C. couple the coil to the collector in my first transistor, if so, I'm unsure of the reason for A.C. coupling and I'm also unsure of why it does not work for me.
I've tried doing a search of the internet but cannot find anything more than a basic description of a darlington pair and how to wire one to an LDR, presumably a coil is significantly different to an LDR?
Any advice you guys could give me would be greatly appreciated.
Best regards,
~Kieran Simkin Digital Crocus