Step 1: Decide on a Kind of Secret Compartment
Pictures above are from:
Stryker @ https://www.instructables.com/id/The-Mysterious-Bookcase/
The Goose @ https://www.instructables.com/id/BOOKCASE-DOOR-THAT-REPLACES-YOUR-DOOR/
flaming_pele! @ https://www.instructables.com/id/Making-a-Motorized-Secret-Entrance/
Step 2: Select a Latch to Keep the Compartment Closed
Step 3: Use a Candlestick As a Door Knob
Most door knob spindles will be too long to mount the candlestick flush against the wall. So you can either cut it shorter, or you can just replace it. For prototyping, I replaced the spindle with a 1/4 inch square wooden dowel. However, I highly recommend using steel in your finished product. Turning a door knob involves a lot of torque, and a wooden dowel will wear out quickly.
To attach the spindle to the candlestick, I made a small bracket out of a scrap piece of sheet metal. I traced the spindle onto the center of the metal. Then I cut an X across across opposite corners of the square. Then I bent the sides down to create a square opening that the dowel could be pressed through. You will probably also need to add a screw and washer to the back side to help keep the candlestick mounted to the dowel.
Once you have your new spindle mounted to the candlestick, you can insert it through the latch and into the other door knob. You can then use the candlestick to open the latch just like a regular door knob. You can easily mount the latch and candlestick in a standard door that is covered or disguised (although you may wish to mount them higher on the door to be at a more reasonable height for a candlestick). Alternatively, you can mount the latch and candlestick to the wall next to the door and place the strike plate on the door. It should work the same. You will just need to have a separate handle so that you be able to pull the door open.
Step 4: Use a Candlestick to Activate a Door Latch at a Distance With Cables
You can make these brackets the same way that you did for connecting the spindle to the candlestick. Cut an X in a piece of sheet metal, bend the tabs out slightly and press the spindle through the center. Then drill a small hole in the metal that is a little bit larger than the wire. This is where we will attach the wire to the bracket. Orient the brackets so that when the candlestick bracket is turned clockwise, the wire will turn the second bracket clockwise as well (see the picture above). Feed the wires through the holes and bend them down into a hook so that they will not come loose when they are pulled on by the brackets.
After mounting the candlestick onto the second spindle bar, you should be able to activate the latch. Just turn the candlestick. This turns the spindle, which turns the brackets, which pulls on the cable, which turns the first bracket, which turns the first spindles which finally activates the latch. A simple setup like this will let you position the candlestick a short distance away from the latch and still be able to activate it mechanically. However, this process is greatly complicated if you need to run the cable through studs. So choose your locations carefully.
Step 5: Use a Candlestick to Activate an Electrical Switch
To activate the switches, I made another bracket for the candlestick. This one just had a flat piece that was bent out and fit between the buttons. When the candle stick was turned one direction, it would press the first switch. When it was turned the other direction, it would press the second switch.
To help avoid shorting the wires on the metal candlestick, I applied a piece of masking tape over the switches and wires. Then I cut a slit in the tape so that the bracket could still press the two buttons.
Lastly, I ran the wires from the switches to the power supply and to the motor. This setup will let you activate a wide variety of circuits by slightly turning the candlestick. It can be something small like a car door lock motor or something large like a garage door opener. Use your imagination and have fun.