What I do not want is one of those RFID readers which attaches to serial or USB port and writes its input to stdin.

I am looking for cheap & nasty RFID reader which I can query over serial or USB; a bonus is if it is also an RFID writer but doesn't add too much cost.

Coding something like:

fwrite, com1, "hey, you, send me some input");
read(com1, ipit_value);

There are a whole bunch of different RFID types, so it's hard to recommend something without any further information.

I'll assume that you meant EM4100 125kHz tags, those are read only, but very common, so there are many choices.

If all you want is to get the rfid ids into a pc, then pick up an USB reader, which simply shows up as a USB keyboard, which spits out the id and hits return: http://cgi.ebay.com/125Khz-RFID-Proximity-ID-USB-Reader-Free-5-RFID-Cards-/260641368732?pt=LH_DefaultDomain_0&hash=item3caf6faa9c

For your own electronics, the cheapest choice is to hook about $2 worth of jellybean components up to an AVR, like I've done here: http://www.hackaarhus.dk/forum/download/file.php?id=45 Forum thread: http://www.hackaarhus.dk/forum/viewtopic.php?f=4&t=113 Sources: https://github.com/dren-dk/HAL900/tree/master/door-ctrl/hardware-4

Next step up is a module like this: http://www.sparkfun.com/products/8419

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dren.dk has listed some great resources. I'm a bit bummed to see that the RFID IC is $30 alone. I have been using the Parallax RFID read/write module that interfaces via RS232, and have had good results with it so far, and it's only $13 more (currently $43 on sale) than the IC sold at SparkFun, so it might be more immediately integratable for you.

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