RFID Door Strike

The final subsystem was designed to unlock the front door of a home remotely. This is useful for allowing a friend to enter the user’s home without needing to leave them a key. Specifically for my wife this allows her to unlock the front door when bringing groceries or large packages home without needing to fiddle with her keys. The user can call the house on the way home and unlock the door, or the user can locally open the door with either keys, or by using the Radio Frequency Identification (RFID) tagging system that is installed as a parallel trigger for activating this system. The system block diagram is shown below in the first figure.


This system originally was designed to use an electronic door lock, but a typical keypad door lock costs over $100 and the budget was nearly spent. Instead of the electronic door handle an electronic door strike was purchased for $30 and can be seen in the next image.


An electric door strike has the same functionality as a regular door strike, namely to hold the latch of a door secure. Electronic door strikes are designed so that when a voltage is applied across its input terminals the strike will collapse and the door will be able to swing freely open without needing to be unlocked.

Electronic door strikes can be either normally locked or normally unlocked. An electronic door strike that is normally locked when power is not applied is called a ‘normally open’ type, whereas a normally unlocked door strike is known as ‘normally closed’. Normally open, and normally closed refers the strikes driving electronics and not to the locked or unlocked nature of the door itself.

For this design a normally open strike was chosen so that the door would function in typical fashion without needing to be connected to a power supply. The electronic door strike measures 160 x 25 x 28 mm (L, W, H) and is actuated with 12 VDC and 200-450mA.


As another means of actuating the door a passive RFID tag reading module was purchased on e-bay for $4 and is shown in the image above. The RFID reader operates at 125 kHz and can read tags up to 75 mm away. When a valid tag is identified 12 VDC is placed on an output line for as long as the tag is being read. The 12 Volt output is used to drive a relay. The relay is a solid state relay and will power the electronic door strike.

The RFID reader comes with a master RFID tag which is used to program up to 25 additional tags into the reader’s non-volatile memory. The reader unit can be run from 9 -15 Volts direct current. The reader was modified to light an external LED when a valid RFID tag has been read. This will provide the user with feedback to know if their RFID tag has been read or not. Instructions on performing this modification to the RFID reader can be found online, or downloaded from the distributor. Any passive 125 kHz RFID tag can be used in this system, and a variety of shapes and sizes are available from online distributors.

RFID Electronic Door Implementation Details

The RFID enable 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.

The major tasks involved in this subsystem were reverse engineering and modifying the existing RFID hardware. While reverse engineering is not design in the strictest sense it provides real world applicable knowledge that may be required to understand a future project already in the late stages of development when a person is first introduced to it. This also demonstrates the many required problem solving skills necessary to perform in this profession. This is why this subsystem is included in this design report.

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 inside of the system and its output was used to sink current through the terminals of the electronic door strike. A 10k Ohm resistor was places across the terminals of the door strike to generate the required voltage drop, as well as prevent current spikes from damaging the strike itself.

The setup described in the section above will be used for this subsystems demonstration. A future version of this product will incorporate a 555 timer operating in monostable mode which will hold the door latch open for a few seconds after a valid tag is read. This addition will make it easier to unlock and open the door when the user’s hands are full. The completed subsystem is shown in the next image.


This was an extra subsystem added as time and funds permitted. As a result test requirements were not designed for the subsystem other than to ensure that the system behaved as expected, and to prevent harm from befalling the system or its users. 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. This system is considered a success and when properly interfaced with the remote access system will become a great boon in the automated home project.

The electronic strike is great for quickly entering a home without needing to fiddle with keys. The system could still benefit from an electronic door lock because the electronic strike does not allow the user to lock the door when they away. Any door using a deadbolt would also require two electronic strikes. This will be an excellent update to make to this subsystem in the future. Although these updates are planned for the future this does not mean that the electronic door strike will not be used. The electronic strike will still be exceptionally handy to allow access to locked drawers or other buildings such as a tool shed or to open a gun safe.

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