I am be very interested to see how this works out.
I am currently making an RFID controlled lock for a door and I know from my work on that,that RFID have range limitations(depending on what type of reader and chip you use) so the position of both reader and chip may be very important especially because you're planning on using it on an unpredictable living thing. That might be a big challenge and may require more than one RFID reader.
As far as a locking mechanism you can make one really cheap by taking apart an old cd drive and using the slots drive system.
Most small microcontrollers will be capable of doing what you need. You could even ditch the Arduino "wrapper" and use a USB capable micro in it's place.
Microchip, Atmel, TI, ST, etc all have 8, 16, 32-bit uCs of varying RAM/FLASH/EEPROM sizes to pick from. All the modern uCs come with at least UART, SPI, I2C peripherals that can be used for your communications.
There is not a lot in them really, I'd just pick one and see how you like it.
I (currently) use ST's 32-bit ARMs and Microchip 8, 16 ,32-bit PICs.
I'd probably use a few PIC12F or 16Fs for the slave uCs and a PIC18F or PIC24F for the master.
You mention needing ~10kbits of memory (not quite clear what type or which uC needs it from your description to me though)
It's easy to determine what is suitable though, just check the RAM/ROM/EEPROM specs of each uC you look at.
For example the PIC16F1938 has:
Parameter Name Value
Program Memory Type Flash
Program Memory (KB) 28
CPU Speed (MIPS) 8
RAM Bytes 1,024
Data EEPROM (bytes) 256
So 28KB of program memory is more than enough to store non-volatile data if your program is small enough (on the newer PICs you can also read/write to program memory at run time) 10kbits will not quite fit into the RAM though, at 1024 * 8 = 8192 bits.
The 16F1527 has 1536 bytes of RAM though, so you could use this if necessary.
For the master (alternatives to Arduino) there is something like the 18F25J50 or similar, which has a USB 2.0 peripheral. Microchip provide a USB stack an plenty of example firmware to get you started with USB.
If you need something more powerful for the master, have a look at the PIC24 series with up to 256K of Flash and 96K of RAM. Or even the PIC32 which is 32-bit and up to 80MIPS.
The PICKit3 is a low price programmer that will program all the above mentioned PICs, and MPLAB (or MPLABX) is a free IDE for firmware development.
Communication can be done with I2C, which deals with the master/slave configuration and addressing easily. All you have to worry about is sending the data. 7 meters should be no problem with a reasonably quiet environment and the right setup (low value pullups - say 2.2k, low capacitance cable)
Best Answer
The biggest risks are that they destroy the equipment, not that they get hurt. At least not until you introduce them to soldering irons!
I would recommend not using lithium ion batteries for power - since they can explode if shorted. Regular AA batteries will get hot if shorted (enough to cause burns/fires after awhile) but arent likely to explode. The best power supply would be one with an adjustable current limit which helps avoid damaging equipment or people.
Some components like capacitors can explode if run over their voltage rating or with polarity reversed - sometimes with a very scary bang. But even that isnt too risky unless it gets in your eyes.
As long as they dont do anything incredibly stupid they will be fine. Electricity at the voltages used in microcontrollers isnt going to hurt anyone.
(As an example of "incredibly stupid" - in our high school electronics lab someone decided to find out what happens if you short out a 120v electrical outlet with a piece of solder. The answer is that it glows for a second, then violently explodes and sprays molten solder a good 10-15 feet. I recommend not repeating that experiment!)