What??? How can a house plant be a touch switch?
Or a glass of water:
Or a vase full of flowers:
Or even an ice cold beer:
Anything that conducts electricity, or contains a liquid that conducts electricity, can be used to make a touch switch.
How is it possible? It works through the magic of capacitive touch sense technology. It’s the same tech that smart phones use to know where you’re touching on the screen. But here we’re only detecting when an object is touched to toggle a relay on or off.
The capacitive touch sense circuitry creates an electrostatic field on and around an electrically conductive medium, and then it constantly watches the field looking for extremely tiny changes in the capacitance measured on the medium. When the circuit detects a sudden change in capacitance, like when a human touches it, a relay is flipped to turn on the lamp.
In this project, the capacitive touch sensor is connected to a circular aluminum foil plate that allows the touch sensor’s electrostatic field to project up into the body of the bamboo plant. The fibers and leaves of the plant contain water, which is a very good electrical conductor, allowing the electrostatic field to travel up through the entire plant, and out to the furthest leaf.
This project is pretty easy to build with moderate soldering and wiring skills. Parts cost is about $50.
Chances are you don’t know anybody with such a strange light switch as this one!
Check these links to learn more about capacitive touch sense technology:
Add a Teacher Note to share how you incorporated it into your lesson.
Here are the parts and materials you'll need:
- A plastic project box, about 4” x 2.75” x 1.75” high, about $3 at polycase.com
- 12V DC Power Supply, Mean Well Part No. EPS-15-12, about $7.00 at arrow.com, or $8.90 at mouser.com
- Relay, SPST, 12V DC coil, 230VAC rated contacts, Fujitsu Part No. FTR-K3AB012W or similar, about $2.70 at mouser.com
- Three Prong 6’ Extension cord, must have 3rd prong for earth ground connection, available at any hardware store or online for about $4
- Phone cord extension, Leviton Part No. C2406-25W or similar, must be 4 conductor type (some have only 2), must be male on one end, female on the other. About $7 on Amazon.
- Qty 4 each of 4-40 x ½” screws, 4-40 nuts, 4-40 x 1/8” standoffs, available at most any hardware store for cheap.
- Capacitive Touch Switch Module, Xkitz Electronics Part No. XCTS-1M, $27.95 at Xkitz.com
- A piece of copper sheeting, or aluminum foil, or any conductive metallic material, about 4” x 4”, cut round if you like.
Tools You'll Need:
- Soldering iron
- Wire cutters and strippers
- Needle-nose pliers
- X-acto knife
- Small straight screwdriver (0.1” wide head)
Step 2: Build the Relay Box
The relay box holds the 12V power supply and a relay capable of switching AC loads. Here are the steps to build it:
- Cut the power cord in half. The box will be inserted inline with the power cord so that the relay can switch the load on and off. You can cut it closer to one end or the other if that works better for your application, but leave at least two feet on either end.
- Cut the phone cord in half also. This cord is used as a convenient plug connector to connect the relay box to the capacitive touch switch box. Again, you can cut it closer to one end or the other, and you may not need the full 25’ length, so cut it as long as you need.
- Cut off a 12” section of the phone cord wire and save it for later use.
- Drill three holes in the project box as shown in the photo. Two holes big enough to pass the power cord through, and one big enough to pass the phone cord through.
- Mount the power supply and relay in the project box. I used ½” 4-40 screws to mount the power supply on 1/8” stand-offs. I just epoxied the relay to the bottom of the box with the pins facing upward.
- Strip about 5” of the outer sheath from one of the cut ends of the power cord. Strip 10” of sheath from the other end. From the 10” end, cut and save 5” of the wires. You should see three wires which are usually black (for load), white (for neutral), and green (for earth ground). If your colors are different, make sure you identify which wires are load, neutral and earth ground before proceeding. Check Wikipedia to see how your outlets are wired.
- Insert the ends of the power cord through the holes, use tie wraps to strain relieve the cords so they stay in place.
- Connect the load and neutral wires from both cords to the power supply AC inputs, as shown in the photo. Note that the load wire from the female cord is routed through the relay. Use the extra 5” black wire for this.
- Connect the green earth ground wires from both cords to the ground (-V) output of the power supply, see the photo.
- Strip about 4” of the outer sheath from the female side of the phone cord. Use an X-acto knife to cut the circumference of the sheath, then just slide it off.
- Cut off and save the 4” green wire from the phone cord.
- Insert the end of the phone cord through the smaller hole in the box, use tie wraps to strain relieve the cord so it stays in place.
- Connect the yellow wire from the phone cord to one of the coil pins of the relay.
- Use the 4" cut off extra green wire from the phone cord to connect the ground (-V) terminal of the power supply to the other side of the relay coil. Connect the phone cord black wire to the same relay pin also.
- Connect the phone cord red wire to the 12V DC output (+V) from the power supply.
That’s it for wiring the relay box. Next step, test that you have it all wired up correctly.
Step 3: Test the Relay Box
Steps to test the relay box:
- Plug the male to the female side of the phone cord
- Strip about 1” of the outer sheath from the male side of the phone cord.
- Strip about ¼” of insulation from the red and yellow wires
- Plug the power cord into the wall, and plug a lamp or something into the female side of the power cord.
- Now short the stripped red and yellow wires together, you should hear the relay snap on and the lamp should light. If so, then you’re done with the relay box. You can screw the top on the project box.
Step 4: Wire Up the Capacitive Touch Switch Module
Now we just need to wire up the XCTS-1M capacitive touch switch module:
- Disconnect the phone cord jacks to cut power to the touch switch.
- Set the two jumpers in the XCTS-1M as shown in the diagram. This selects toggle switch mode, and direct touch mode.
- Cut off and discard the 1” green wire from the phone cord
- Insert the phone cord through the hole on the side of the XCTS-1M box, use tie wraps to strain relieve it.
- Strip about ¼” from the red, black and yellow phone cord wires. Tin the wires to make it easier to insert into the terminal blocks.
- Connect the wires to the terminal block as shown in the wiring diagram. You’ll probably want to use needle nose pliers to grip the wire when inserting it into the terminal block.
- Red wire goes to terminal #3 (terminal #1 is closest to the fuse)
- Black wire goes to terminal #4
- Yellow wire goes to terminal #5
- Strip the sheath from the 12” section of phone cord saved from above.
- Use the green 12” wire to connect to your touch plate. This could be a sheet of copper or aluminum foil about 4” x 4”. Strip and tin ¼” from one side of the green wire and connect it to terminal #1 of the terminal block. Strip and connect the other end to the touch plate.
Almost done! We just have to put it all together and calibrate.
Step 5: Put It All Together and Calibrate
Here are the final steps:
- Find the small trim-pot on the XCTS-1M board, close to the jumpers. Using a small screwdriver, turn it full clockwise, and then back it off about 1/8 turn.
- If not done already, plug the relay box into the wall and plug your lamp into the relay box cord.
- Plug the phone jacks together. You should see an LED on the XCTS-1M board flash a few times red and green. If you don’t see it flash, check the wiring of the terminal block. You should see 12V DC between the red and black wires.
- If your XCTS-1M is up and running, you should be able to toggle the lamp on and off by simply touching the touch plate.
- Now, set your plant, or glass of water, or ice cold beer on the touch plate. You’ll probably see that touching the object doesn’t switch the lamp. If so, just turn the trim-pot on the XCTS-1M clockwise, a little at a time, and retest. You’ll see the LED flash while you’re turning the trim-pot (and the lamp will turn off if it was on). Keep adjusting and re-testing in this way until you can switch the lamp just by touching the object. If you’d prefer to hide the plate, it will still work with a coaster between the plate and the object. If you do this, you may need to go a little higher on the trim-pot.
That’s it. Have Fun!