Friday, February 4, 2011

Project - Simple RFID access system

Hello Readers

The purpose of this project is to prototype a basic RFID access system. Although it is not that complicated, this article is my response to a kit reviewed in the Australian "Silicon Chip" (November 2010) electronics magazine. Their article describes the kit in detail - operation, schematic, use and installation. However the code for the microcontroller (PIC16F628A)  is not published due to the kit manufacturer holding copyright over the design. This is a shame, as many organisations have been quite successful selling open-source kits. So instead of moaning about it, I have created my own design that matches the operation of the original, instead using the ATmega328 MCU with Arduino bootloader. Consider this a basic framework that you can modify for your own access system, or the start of something more involved.

original article 

There are pros and cons with the original vs. my version. The biggest pro is that you can buy the whole kit for around Au$40 including a nice PCB, solder it together, and it works. However if you want to do it yourself, you can modify it to no end, and have some fun learning and experimenting along the way. So let's go!

The feature requirements are few. The system must be able to learn and remember up to eight RFID access tags/cards, etc - which must be able to be altered by a non-technical user. Upon reading a card, the system will activate a relay for a period of time (say 1 second) to allow operation of a door strike or electric lock. Finally, the RFID tag serial numbers are to be stored in an EEPROM in case of a power outage. When a tag is read, a matching LED (1~8) will show which tag was read. There are also two LEDs, called "Go" and "Stop" which show the activation status. The original kit has some more LEDs, which I have made superfluous by blinking existing LEDs.

This is a simple thing to make, and the transition from a solderless breadboard to strip board will be easy for those who decide to make a permanent example. But for now, you can follow with the prototype. First is the parts list:

When selecting a relay, make sure it can handle the required load current and voltage.

If attempting to switch mains voltage/current - contact a licensed electrician. Your life is worth more than the money saved by not consulting an expert.

And here is the schematic:


Here is the prototype on the solderless breadboard. For demonstration purposes an LED has been substituted for the transistor/relay section of the circuit, the power regulator circuitry has not been shown, and there are superfluous 4.7k resistors on the I2C bus. To program the software (Arduino sketch) the easiest way is by inserting the target IC into an Arduino Duemilanove/Uno-compatible board, or via a 5V FTDI cable and a basic circuit as described here.

The Arduino sketch is also quite simple. The main loop calls the procedure readTags() to process any RFID tag read attempts, and then monitors button A - if pressed, the function learnTags() is called to allow memorisation of new RFID tags. Each tag serial number consists of 14 decimal numbers, and these are stored in the EEPROM sequentially. That is, the first tag's serial number occupies memory positions 0~13, the second tag's serial number occupies memory position 14~28, and so on. Two functions are used to read and write tag serial numbers to the EEPROM - readEEPROMtag() and writeEEPROMtag(). The EEPROM is controlled via the I2C bus. For a tutorial about Arduino, I2C bus and the EEPROM please read this article. For a tutorial about Arduino and RFID, please read this article. The rest of the sketch is pretty self-explanatory. Just follow it along and you can see how it works. Sketch file download hereAnd finally, a quick video demonstration:

[Youtube=http://www.youtube.com/watch?v=H9dtUZAjfP8]

So there you have it. I hope you enjoyed reading about this small project and perhaps gained some use for it of your own or sparked some other ideas in your imagination that you can turn into reality. If you would like to discuss it further, please do so in our Google Group. If you are having trouble sourcing the required parts, let me know via john (at) tronixstuff.com.

Otherwise, have fun and be good to each other!

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