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Small proximity sensors with short range and alarm either IR or RFID Answered

I would like to make two key fob sized units that if brought within ~20cm of each other would sound an alarm from one of the units. I've seen a pair of key fob sized RDIF alarm tags that sound an alarm in one if moved more than ~10m apart. They sell for ~$50 US but I need something that accomplishes just the opposite from this device. I want to keep two pets away from each other (one bullies the other) and such a device would I believe accomplish this task.
It seems like it should be relatively simple to design but I'm an artist and machinist and know little about electronics. I purchased a book entitled Practical Electronics for Inventors that has given me some knowledge about the basics of transformers, relays, capacitors, resisters, etc. but nothing on IR or RFID transmitters/receivers. Everything I read about RDIF is geared towards one stationary reader (or large hand-held unit) and one or more transmitters. I made one collar with an IR receiver from an R/C car and a small piezo alarm that would sound when I pushed a button on a remote. This works but only if I'm in the same room with the offending animal. I need something that works when I'm away or the animals are upstairs or in another part of the house. They are indoor only pets so the device need not be weather-proof. Anyone with knowledge of such things want to lend me a hand?


You could build some low-power inductive sensors where one pet's sensor would interfere with the field of the other. Unfortunately, for a distance of 20cm you'd be looking at needing more power than would likely be safe to put around an animal's neck.

NFC/RFID or some variation is probably your best bet. Design a signal to be transmitted from one colour and received by the other, much like a radio hard-coded to that one frequency. Just make sure to transmit at a frequency not bothersome to pets. I believe that RF signalling is also the technology used in those invisible pet fences. Of course, all you need to do is wire a sound device into the collar, instead of a shocking system.

With more wireless networks being designed, I feel there should be a way to do this with bluetooth or xigbee technology as well. I can't offer much guidance in this field, but it's worth looking into.

No matter what, I think you'll have to get your hands dirty in a bit of the wiring and programming of this.

Did you ever get this figured out? I'm wanting to do something similar.

Wow - me too! I need a way to keep one cat away from the other cat's food. I'm going to keep researching and I'll post what I find! (Unless anyone else can do it before I finish teaching myself!) :)

Maybe you could use NFC (~10cm I think), putting a fob on one pet and the sensor on the other and then some kind of basic electronics (not sure what) that finds a way to trigger if the sensor starts detecting Maybe you could circumvent the need for a processor by looking for some change of state of one of the pins when the sensor detects the NFC fob. Like with this hardware: