A Homemade Variable Tuning Capacitor for tuning in your FM stations...

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From a range of 5pF to 27pF, this multi-turn capacitor can be made using only 6 items...

My main interest has always been in FM Transmitters and FM Receivers...and always looking for unique ways of achieving the same end results, by making homemade devices in place of store-bought components.

The Radio Shack Special project calls for a typical 6-lead variable tuning capacitor. This item has to be mail-ordered, as Radio Shack does not carry it. Should you have trouble finding your variable tuning capacitor for the Radio Shack Special Project, this homemade device can be substituted for it. In fact, the tuning range for this device is much less critical then the conventional 6-lead variable tuning capacitor...as it has the capability of multi-turns.

The above picture is the homemade variable capacitor. With just 2 nuts and a machine screw, soldered to a very small PCB (1/2" X 1/2")...then taking 22 gauge enamel-coated solid copper wire (this wire can be purchased at your local Radio Shack Store)...a homemade capacitor is achieved.

This homemade device can be used for the Radio Shack Special Project or the TDA7000 FM Receiver Project or any RF device that would need a variable capacitor. So my friend, should you fancy this project, shall we begin?

2 machine nuts	1 Single Sided or Double-Sided PCB (1/2" X 1/2")
1 machine screw	12" of 22 gauge magnet wire (must be enamel-coated)
1 hacksaw blade	A piece of plastic...explained further in this webpage.

The machine screw and nuts can be purchased at your local Lowe's Store. Make sure the length of the screw is longer then 20mm's. The width of the screw is 4mm's. The screw will be cut to proper length further down in the instructions.

Radio Shack sells a double-sided Printed Circuit Board (PCB). Cut out a portion of the PCB that is exactly 1/2" X 1/2". Then buff up one side of the PCB...in order for solder to cling to the board real good.

Radio Shack sells magnet wire. As shown in the picture to the left. 3 spools of wire come in a single package. One spool of 22 gauge (gold-colored), one spool of 26 gauge (green-colored) and one spool of 30 gauge (red-colored). It is the 22 gauge (gold-colored) that you will need for the project. The wire is 'enamel-coated'. This is a must for the project. You will need approximately 1 foot of this wire for the project.

Begin by screwing the 2 nuts on the screw itself...as shown in the picture. Notice in the picture that each hex nut has been scratched up with a one-sided razor blade. Only scratch one side of each nut (as each nut has 6 sides). The remaining 5 sides of each nut does not have to be scratched up.

The scratching is for the soldering to adhere to the nut. These nuts, although steel, have been zink-plated. So scratch off the surface, to get down to the steel itself. This way you will have no trouble in having your solder adhere to each nut.

Once you have scratched-up your 2 nuts, go ahead and place a large dab of solder onto each side of each nut, as shown in the picture to the left. Put enough solder to cover the whole side. Make sure you hold your soldering gun onto the nut for a good 30 or so, for the solder to adhere well to each nut. You will actually see the solder start to grab onto the nut, after a good-many seconds...so make sure this happens...otherwise, your dap of solder will not be good and secure. You may want to try to scrape it off the nut, once you have soldered the dab into place...making sure that you have a good binding hold.

Once you have put your good 'dab' of solder onto each side of each nut...go ahead and take your 1/2" X 1/2" PCB and place it under your two nuts...as shown in the picture to the right.

Adjust your 2 nuts so that each nut rest on each side of the PCB...also shown in the picture. When this is done, you should have approximately 1/4" between the 2 nuts.

Place some type of heavy form over your setup, so that it doesn't move. Then take your soldering gun and heat up the solder on one of the nuts. When the solder becomes soft, go ahead and start applying more solder to that area. Once the solder has accumulated enough, the weight of itself will fall directly on the PCB. When this does, continue to give it a little more solder and let the solder adhere to the PCB...as shown in the picture to the left.When you're through with one of the nuts, do the same for the other nut.

When all is finished, you will have 2 nuts that are adhered 'rock-solid' to the PCB!

Go ahead and make 12 wraps of wire around your screw. Once you have made your 12 turns, cut the excess off...leaving about 1/2" of wire on the ends...as shown in the picture to the right.

Now go ahead and unscrew your 'coil' from the screw. We will now cut the screw for proper length using your hacksaw...or just a hacksaw blade.

Measure where 16mm's will be on your screw...that is...from the bottom of the head of the screw...to 16mm's out...as shown in the picture.

Then take a new single nut and position it where 16mm's is. Then take your hacksaw blad and cut at that point.

Remember, the overall distance of the threads themselves...will be exactly 16mm's in length.

The new nut is there to keep an accurate measurement while cutting. Always keep the nut secure and steady...while making your cut. This way, once the screw is completely cut, you will then noitce how 'easily' the new nut will come off. If it does not come off easily...you may have to take a small file and shimmer whatever 'gouge' you may see. Keep shimmering your screw, until the nut can come 'off and on' very easily...as if the screw had never been cut. This is important...because the end of the screw will be running through your newly made coil...when all is done

Once you have finished making your cut, take the screw and screw it into your 2 soldered nuts that is now attached to the PCB.

Screw the screw all the way, and then take your newly-made coil and screw that onto your 'freshly-cut' screw...as shown in the picture to the right.

The picture below shows the last step by putting a 'folded' piece of plastic underneath the screw...between the 2 nuts. This is needed to 'secure' continuity between the screw and nuts while turning your screw.

If we did not put a folded piece of plastic underneath the screw, there would be no tension on the screw...and hence...when you would turn the screw, you will get spuradic 'dis-continuity' between the screw and nuts...being that the screw and nuts are not 'snugly' fit. So...with the plastic, it will give an 'upward push' on the screw...thereby having to cause the screw itself to push upward on the 2 nuts...thus having 'continual' continuity between the screw and nuts...when turning your screw. You can adjust the pressure on the screw, in relation to the nuts, by using a thin or thick piece of plastic.

The picture above and to the right shows various types of plastics in your home. I have used part of the plastic tube that comes with 'silicon'...purchased at Lowe's...but any type of plastic will do the job...so find whatever you may having lying around your home and just experiment...that's the FUN part!

The next and final step is to bend the outside end of the coil wire toward the inside end wire. Then snip off the excess.

Take your single-edged razor blade and scrap off the 'enamel-coated' insulation where the two wires come together...then solder them in place as shown in the picture to the right.

Next, take a small piece of your 22 gauge wire and scrape off all of the enamel. Then solder one end to the PCB...in between the 2 nuts...as shown also in the picture to the right. Snip off the end so that both terminals are equal...being around 1/2". Now you have your two terminals that can be soldered into place...on whatever RF project you are working on.

The wire soldered to the PCB acts as the rotor (which is the turnable portion of the device) and the wire coming from the coil acts as the stator. Should one of your terminals on your project be ground, always connect the 'rotor' end wire to ground...as this will eliminate 'hand' capacitance when your hand turns the screw for adjustment. I advise adding a plastic knob to make your turns...or a non-metallic screwdriver will also do the job just fine.

All the best, to you and your project, my friend. It was a pleasure to share this project with you...and may all your endeavors challenge you...to give your best!