Step 37: Unlock the House with the Phone & RFID: Implemented
The RFID enabled electronic door strike was by far the simplest subsystem to design and build. The major components came in preassembled modules that just required interfacing through the appropriate power and signal controls.
Some modifications are also provided from the manufacturer and can be found online from ebay sellers if you get the same system as I did.
The RFID reader hardware has capabilities far beyond what is being used in the stock system. The default setup only provides a low current 12 VDC output to signify the presence of a valid RFID tag. In stark contrast to this the reader is also capable of TTL and CMOS output levels, as well as providing the decoded RFID tag data in a serial format. A reader with these features being used is generally sold for many times the cost of this system.
The RFID tag reader was modified to provide TTL output levels as well as display on an external LED when a valid tag was present. The same power connector used in the previous modules was added to the RFID reader to give it a cleaner look.
A solid state relay that triggers from a 5 VDC input was attached to the system and its output was used to sink current through the terminals of the electronic door strike. A diode was place across the terminals of the strike to prevent current spikes from damaging the strike itself.
The addition of the 555 timer circuit made it easier to unlock and open the door when the user’s hands are full. The completed subsystem is shown in above.
All connections made to the RFID reader were kept internal to the reader’s project box and soldering jobs were protected from shorting by a layer of shrink tubing. The results of testing the RFID activated electronic door strike showed a high degree of reliability.
The system only fails to properly open when pressure is applied to the door strikes latch when it is being activated . This pressure is such as would be experienced by attempting to force the door open.
To test the sensitivity of the RFID reader an activated tag was placed within the proper distance of the reader’s antenna and then the path was blocked by various materials. As long as the material did not produce an RF challenged environment (such as a metal ground plane, or a cup of fluid) the RFID reader would respond correctly.