Introduction: Make Course Variable Capacitor
This instructable was created in fulfillment of the project requirement of the Makecourse at the University of South Florida (www.makecourse.com). In this Project I created a capacitor than can be varied depending on the position of a servo motor controlled by a potentiometer. A capacitor is a common passive element that is found in electrical systems. It has many uses and applications that can be applied to all sorts of circuits. The capacitance is varied based on how much area of the rotor plates is under the stator plates. The less area under the stator the less capacitance.
Step 1: Create and Print Out Your Parts Needed.
The very first step in this instructable requires the necessary parts for the stator, plates and stem. In my case, I designed the parts on autodesk inventor and later 3-D printed them out. When designing your parts, you must first consider the Stator( The object on the right in the picture depicted above). This will be the base of all the other measurements that you decide on. The stator consists of a block with multiple rectangular prisms cut into it. These prisms need to be able to fit the plates( object on the left) in between them with a very small distance( 1 mm) on both sides between the plates and the rotor. When designing how wide and deep the area of the plates and rotor is depends on the max capacitance desired. The more area of the plates the higher the capacitance can be. My plates were designed to be semi-circles with a diameter of 2.54 cm. The number of plates and rectangular prisms cut into the stator also increases the capacitance. Finally a stem need to be designed to hold the plates as depicted above. I used a simple cork and hole technique but other options such as screws and glue exist.
Step 2: Create a Conductive Surface
In order for any charges to build up in a capacitor there needs to be a conductive surface. The plastic of the 3-D printer will not cut it. The best way to go about this is to go acquire some foil tape and packing tape from a nearby hardware store. The foil tape is used for insulation duct work so it is bound to be there. Now once you have acquired the materials, tape the foil tape on both sides of the plates so that they have at least a little contact between each other. Then place a layer of packing tape over both side of the foil tape. The packing tape allows the plates to touch the stator without shorting out. Next we will perform the same steps on the stator that we just did on the plates. The only difference is this time we will not use the packing tape as it is not needed. Be extra careful to make sure that the surfaces are smooth to avoid any friction between the plates and the stator. Once you add the foil and packing tape, connect your plates to the stem and slide the plates into your stator. This gives you a working capacitor if you just wanted to connect some lead on the top and bottom plates of the stator.
Step 3: Setting Up the Electrical Circuit
Now that the capacitor is already set up and ready to go all that is left is to make a circuit that can control a servo motor with a potentiometer. I threw in an LCD that reads the expected capacitance for a 10 plate capacitor for the outgoing Makers out there. The potentiometer is placed on a bread board. The out side pins need to be powered with the arduino's five volt power supply and grounded. The middle pin of the poteiometer needs to be connected to the anolog pin 0. Next connect the Servo to power,ground and pin 9. Look at the picture above to get a grasp on what needs to be powered and what needs to be grounded. This part may be a bit tricky to new comers but a great reference can be used on the (makecourse.com and arduino websites.) The LCD has specific pins that need to be hooked up. These pins have names written on the back of the LCD as well as the back of the arduino.
Step 4: Download the Code and Put It Into Your Arduino
Below I provided a word document with the a commented code for the servo motor controlled by the potentiometer. In this code are a few libraries that may need to be added to your arduino library collection. You can find these online with little effort. Check the Arduino reference if you need help adding a library. The code also includes the LCD coding if you want to add that function into it. The code essentially sets the position of the servo to the position of the potentiometer. The servo can only go from 0 to 180 degrees. This give the max ranges of the potentiometers effects on the servo. The LCD is a calculated constant that increases with the degrees of the servo motor.
Step 5: Connect the Servo to Your Stem and Include Your Finishing Touch Ups.
In this final step you will need to connect your servo motor to the bottom of your stem. This can be done with many methods, mine was krazy glue. I enclosed my circuit into a box before I attached the the servo to the stem but that is not necessary. Once you have done the final step you are all done. Your potentiometer's position will directly effect the capacitance that your new variable capacitor will output. Now you can connect your leads to the top and bottom of the stator and integrate it into any desired circuits.