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We wanted to lock up the cabinet where we store medications and other things we keep away from the kids. But adding any type of standard lock would ruin the look of the cabinet. We didn’t want to see any visible sign of the locking mechanism.

We decided to use an electric solenoid type of lock that would be triggered by a hidden capacitive touch switch. A piece of conductive copper tape stuck on the inside of the cabinet is monitored by a capacitive touch switch module. Whenever you touch that spot on the outside of the cabinet, the touch switch module detects the touch and energizes the solenoid lock, which releases the door to open.

So now, we have a totally hidden lock on our medicine cabinet that only we adults know how to open (as long as we don’t let the kids see how it works)!

1. Position and Mount the Solenoid Lock

The solenoid lock is placed inside the cabinet with the flat part of the plunger facing outward so that the striker plate will push it open when the cabinet door closes (Figure A). The lock was oriented 90° from what we needed for proper mounting (Figure B), so we removed it from its steel housing and attached it to a piece of aluminum angle iron that would let us easily mount it to the inside of the cabinet.

We decided to add two extra “fail safe” wires from the solenoid to be hidden in the adjacent (unlocked) cabinet, just in case the touch sense mechanism fails at some point. With this, we can simply connect a 12V DC voltage to those wires to activate the solenoid and open the door.

2. Run 12V DC to the Cabinet

The touch sense module and the solenoid run on 12V DC. We used an old wall wart we saved from some discarded device for our supply. We tucked the wall wart into the recessed light compartment above the sink so that the wiring would be hidden (Figure C). Just a few holes drilled inside and between the adjacent cabinets allowed us to run the 12V wire into our cabinet to be locked (Figure D).

3. Build and position the Touch Sense Plate

Two 5″ lengths of copper tape are used for the touch sense plate. These should be affixed parallel and close to each other, but not touching. One of them will be the touch sense plate and the other will be the “ground plane,” which improves the touch sensitivity. We used Scotch tape to hold the two pieces together prior to affixing them to the inner wall of the cabinet. Solder (or just tape) a 6″ length of thin wire to each of the pieces of copper tape (Figure E). These wires will connect to the touch switch module and allow the touch detection. Peel off the paper backings and affix the tapes to the inside of the cabinet behind where you want the touch sense area to be.

Figure E

At first we tried using the small touch plate PCB that came with the touch switch (Figure F). But it turned out to be too small to allow reliable touch sensing through the ¾” thickness of the wood. We had to set the sensitivity of the module very high in order for it to detect touch. That worked pretty well, but sometimes it had false activations because the sensitivity was set so high. Switching to the copper tape allowed us to turn down the touch sensitivity and now it’s working rock solid. Presumably, the small PCB would work if your cabinet wall is thinner.

Figure F

4. Wire the Touch Switch Module

You now have all the wiring ready to connect to the touch switch module. Pass the wires from the power supply, solenoid, touch plate, and ground plane through the small hole in the side of the touch switch enclosure, and connect them to the small green terminal block as shown in Figures G and I. Be careful not to wire the power supply backward, it will blow the module!

5. Configure, Test, and Adjust the Touch Switch Module

Set the two jumpers on the touch switch module to “Indirect Touch, Momentary Contact Mode,” as shown in Figure H. Use a small screwdriver to turn the sensitivity pot all the way counter-clockwise to the lowest sensitivity setting and plug in the 12V power supply. You should see the LED on the module blink a few times. Gradually turn the pot clockwise while repeatedly touching the touch sense area until you get to a workable setting.

Figure H. Designed by Juliann Brown

Important: You must remove your hand from the area between tests, as the module “learns” the ambient capacitance of the touch plate. Keeping your hand on the plate lets it become accustomed to its capacitance so that it no longer detects any difference when you touch it.

6. Mount the Touch Switch Module

Once the module is adjusted and working, you can mount it to the inside of the cabinet with two small wood screws through the mounting flanges (Figure J).

Figure J

7. Position and Mount the Strike Plate on the Door

We fabricated a strike plate from a small T bracket (Figure K). To find the proper alignment, temporarily put pieces of masking tape on the edge of the cabinet and the door so you can mark the vertical position of the lock and transfer it to the back of the door (Figure L). Then measure the horizontal distance and the depth of the solenoid lock. Bend the T bracket to the proper depth (Figure M), cut the excess if necessary, and mount it on the door (Figure N) to allow it to engage the lock.

You can now keep things safe in your hidden-lock cabinet, and open it with a secret touch.