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