Magnetic Reed Switches to Unlock Door

Thread Starter


Joined Mar 26, 2007
Hello everyone. This is my first post. In browsing around a bit today I thought that this community would be a good place to ask a question (or two) about a project I've been dreaming up for a while now.

For a bit of background, my only experience with circuits (besides minor home repairs) is from a couple of under-grad classes I had to take as part of my mechanical engineering degree. In other words, I know enough to recognize a term or two, and as far as I know, Ohm's Law can solve every problem know to man about electricity.

So, here's what I've dreamed up...

I want to build a secret door to the space under our stairs. After watching a show on HGTV, I've gotten a bug to use magnets as keys to unlock the door. I've attached a pdf of the simple circuit I've come up with. Basically S1 would be a magnetic switch in the door to turn everything off if the door is open. S2 is a normally closed switch that could be opened up to re-set the circuit. S3 is some sort of switch to turn the circuit off if it is left on too long. Step one would be triggering reed switch S4 with a magnet. This would open up the first transistor (R2 would keep the circuit from frying). Then S5 would be triggered (again with a magnet) to turn the solenoid on which would unlatch the door. Once the door is unlatched S1 would open up, turning off the circuit.

So, my questions are: is this a practical and workable circuit? If not, what would you recommend for something like this? What could I use for S3? I was thinking a thermal switch (replaced one in a blender once, boy did that prove the little missus wrong for once!) Or would some sort of timer be easier/more reliable?

Thanks for any help you can offer!




Joined Jan 17, 2007
I was thinking a thermal switch ...
What were you thoughts about the thermal switch? Were you going to use a thermal lag to make a delay? This sounds like a neat idea, although I've never seen anyone actually use this approach to make a delay.

Or would some sort of timer be easier/more reliable?
It's common to use 555 timer in one-shot configureation for delay switch. For example, see the 3rd circuit from the bottom here
Googling "555 one shot" will bring up lots of schematics.


Joined Jun 30, 2006
Have you considered a magnetic mechanical solution?

Each has a lever that will either engage or disengage based on where you put the a magnet. Eight positions could be 'programmed' thus
Latch = 0 and Unlach = 1

A magnet on a latch postion causes the lever to latch, a magnet on an unlatch postion cause the lever to unlatch. I chose 1 for unlach as it takes 1 magnet.

So 10111001 would require five magnets to unlock the door but if you put a magnet where a zero is the lever latches.

You can make it more complex by using magnets on the levers in such a way that the wrong polarity effects them.

The magnets can control switches and a solenoid "bolt" without one bit of semiconductor material. I leave it to you to figure that one out. :)


Joined Jan 10, 2006
One of our local suppliers had a kitset RFID setup designed just for this sorta thing...... if you wanted to use a small RFID tag, or RFID credit card and wave it across the pickup.
I have one set up on the Cat door at home, as those magnetic ones where useless when the neighbours cat also has a magnet on its collar.
Now the cat has a uniquely coded RFID tag in its collar, and can come and go as she pleases locking all other cats out.


Joined Feb 20, 2011
Going to try to help install a hidden door bookcase in my parent's home for them. I'm a newbie, but would like to use a chess board on the bookcase, and when you move a certain chess piece to a certain spot, the door would open. How could I approach this? Hidden magnet under chess piece or chess board? :confused:
What kind of materials would you recommend using? For a latch?
Could you please also explain how I could make it safe for anyone in the room, to not to be stuck inside. A failsafe in case of power failure etc.?
I have a college degree :eek:, but not in electrical anything, could you please try to describe in the simplest terms possible :)
Thanks in advance to anyone who takes the time to respond!


Joined Jan 17, 2007
Could you please also explain how I could make it safe for anyone in the room, to not to be stuck inside. A failsafe in case of power failure etc.?
On the inside, have a handle (or a push-plate, depending which way the door opens) which is connected to the latch mechanically.

P.S. Mechanical design and carpentry - not electronics - are the tough part of this project, if you want to do it right You can try various electronics designs even in the late stage of the project.