Variable Capacitor for a Crystal Set

I have been interested in constructing a crystal set radio and making as many of the parts as possible. A key component is the variable capacitor used to tune the radio. A 500 pf variable capacitor is typically used for this purpose and the one described here should have a maximum capacitance of at least that value.

The materials required are:

1. 0.2 mm phosphor bronze sheet [20 cm x 20 cm] (1)
2. 2.0 mm acrylic sheet [25 cm x 12 cm] (1)
3. M3 knife handle round coupling nuts [6 mm diameter and at least 10 mm long] (2)
4. M3 brass round head bolts [10 mm] (5 + a few spares)
5. M3 form 'A' brass washers [7 mm x 0.5 mm] (6 + a few spares)
6. M3 brass nuts (6)
7. M3 countersunk head brass bolts, 10 mm (2)
8. A4 125 micron Mylar (stencil) sheet (2)
9. A4 150 micron clear PVC binding cover [anything up to a maximum of 350 micron would be OK ] (1)
10. Round acrylic disc for a tuning knob (about 50 mm diameter and 2-3 mm thick) - or make this.
11. Some metal polish and a little acrylic medium or brass sealer (optional)

The tools required are:

1. Stanley knife (with plain blade and preferably also a hooked blade for scoring acrylic sheet). Some means of clamping parts will be required when splitting parts along scored lines.
2. Circle cutter compass
3. Scissors
4. Steel rule, 12 inch
5. A hand drill (an electric drill with a half inch chuck and a drill stand is also desirable but not essential)
6. 3.2 mm drill bit (a fine e.g. 1.5mm drill bit is also useful for drilling pilot holes)
7. A quarter inch (6.3mm) drill bit
8. Countersink bit
9. 8 mm hollow punch
10. Screw driver (for the M3 bolts)
11. Pliers
12. Junior hacksaw
13. Flat file
14. Wet and Dry 240 grade sandpaper (for metal) and a small block of wood.
15. Conical rotary rasp or rounded file (about 6-8 mm diameter)
16. Low tack masking tape (to temporarily hold parts together)
17. Fine permanent marker pen (useful)

This is set of templates showing the measurements required for the various parts of the variable capacitor. Downloadable files in Microsoft Word format (.docx) and PDF format are also given.

Two rotor plates are required. These are constructed from 0.2mm phosphor bronze sheet. For each plate, a section of the sheet is marked as shown in the first photo following the measurements given in the template. An arc is scored out on the plate using the circle cutter compass. Cut round the outside of the scribed arc about 6mm away from the line using a pair of scissors. The required section can then be cut out by working carefully round the arc using a set of pliers and bending up and down till the plate splits along the section being bent. It may be worth doing a bit of practice on a spare piece of sheet to get a good feel of how deep the scoring needs to be and to get a feel for the technique (see video clip). Two rectangular sections are cut out by scoring the sheet with a Stanley knife and then using the pliers as just described. Even though there is a corner involved, the sections can be removed with a bit of care. The edges can be smoothed a bit with a piece of the wet and dry sandpaper wrapped around a small block of wood. The corners of the lug are also rounded using the wet and dry sandpaper.

Three stator plates are required. Two of these are simple rectangles but the third has a lug attached to it. Mark up a piece of the phosphor bronze sheet as shown in the first photo and following the dimensions from the template. Cutting lines are scored using a Stanley knife and a steel rule. Cuts may be completed by securing the plate with the cutting line at an edge (e.g. as shown in the second photo) and bending the plate up and down carefully until the sheet splits along the scored line. For the plate with the lug, a pair of pliers may be used to do the bending and splitting for the inside corner as described in the previous step when removing a rectangular section from a rotor plate. Edges may be smoothed with sandpaper.

Two phosphor bronze washers are required. These are 7 mm in diameter and made from the 0.2 mm thick phosphor bronze sheet. They have a central hole with a diameter of 3.2 mm. To make a washer, cut a small piece of phosphor bronze sheet and drill a 3.2 mm hole in it. Put an M3 bolt through it with an 7 mm form 'A' washer on either side of the plate and secure the assembly with a nut. Cut round the outside of the 7 mm washer with a pair of scissors. The phosphor bronze washer is then filed down till it also is 7 mm in diameter. This could be done by holding the bolt in a pair of pliers and working round the washer with a file. Alternatively, if an electric drill in a stand is available, secure the assembly in the drill chuck, start the drill and apply a file to the rotating edge of the phosphor bronze washer until it is reduced to the size of the enclosing 7 mm washers (see video clip).

The rotor contact is make from a 30 mm by 20 mm piece of phosphor bronze sheet as shown in the templates. Three 3.2 mm holes are drilled as shown. Cuts then need to be made using a Stanley knife from each of the four corners of the 10 mm square to the centre. In this case the scoring needs to be continued until the sheet is cut through. The four lugs produced are then bent back along the sides of the 10 mm square as shown. The points may be tidied up slightly, if desired, using a small file if one is available. The 30 mm by 20 mm section is cut out from the sheet as usual using scoring and bending and the corners are rounded using the wet and dry sandpaper on a wooden block. One of the knife handle couplers may be pushed through the central hole to expand it a bit whilst still ensuring that the lugs will press against the coupler.

The two millimetre acrylic sheet may be cut by scoring it quite deeply using a Stanley knife (preferably with a hooked blade) against a steel rule (see video clip). The sheet is then clamped very close to the score line and the sheet bent evenly and firmly so that is snaps along the scored line (see video clip). Two pieces are cut from the sheet as shown in the templates. One is 88 mm by 88 mm and the other 108 mm by 88 mm.

In this stage and some subsequent pages, some low tack masking tape is used to hold pieces together while holes are drilled. Most of the holes require a 3.2 mm drill bit but it is a good idea to drill pilot holes with a finer (e.g. 1.5 mm) drill bit to help position them as accurately as possible.

Start by taping the front and back acrylic plates together as shown and drill a central hole through them (at this stage a 3.2 mm hole). Tape the three stator plates together and then tape them in position between the front and back acrylic plates and secure the assembly with a nut and bolt through the central hole. Mark the positions for drilling the corner holes.

Drill a 3.2 mm through one of the corner holes with the sandwiched stator plates and then secure that corner with a nut and bolt. Drill the other three corner holes and the two mounting holes at the ends of the top plate.

Disassemble the pieces and, as you do so, mark them to ensure that everything can be reassembled later with all the parts the same way up and the same way round. The drilled set of pieces together with the two rotor plates are shown in the final photo for this step.

Drill a 3.2 mm hole in the lug of the stator plate. Round off the corners of each plate using the wet and dry sandpaper on a block of wood. On each plate mark out an area 10 mm by 5 mm as shown. The aim is to remove a semi-circular area as shown with the aim of minimising the capacitance of the variable capacitor when the rotor and stator plates are not engaged. I did this by cutting out a little bit using scissors and a hand file and then using a rasp bit in an electric drill on a stand to complete the process (see video clip). There is a tendency for the rotating rasp bit to drag the plate to one side, so some sideways pressure needs to be applied to prevent this. The process could be done more manually using a suitable rounded file.

The variable capacitor uses four separator films, each 88 mm by 88 mm. Three are made from 125 micron Mylar film an one from 150 micron clear PVC film. The main reason for using clear PVC film for the top separator is aesthetic so that the top stator and rotor plates are not obscured by a semi-transparent film. The thickness of the PVC film is not critical and any thickness up to 350 microns would be OK.

A template is given for the separator films. It is a good idea if you can print copies of this to scale, one for each separator film.

The first photo shows one of the face plates on top of the standard template. It can be seen that there is a slight misalignment between the holes in the plate and the positions of the hole centres on the template. As I was able to use Microsoft Word, I was able to make a slight adjustment to the template before using it so that the hole centres were more closely aligned (see the second picture). For each separator film, a piece of film 88 mm by 88 mm was cut out (Stanley knife and steel rule) and taped on to a cut out template. The holes were made using an 8 mm hollow punch. I did this using the punch inserted into a half-inch chuck of an electric drill on a stand (see video clip). It can be done using the hollow punch and a hammer but I was able to get cleaner holes by the other method. The fifth photo shows a separator film with the first two holes punched. I marked all the separator films to indicate their required orientations when the capacitor is assembled.

At this stage I put a bolt and couple of washers through each hole in one of the face plates and checked that each separator film could be put in place round the washers. Small adjustments can be made to the holes on the separator films if they cannot be placed cleanly around the washers.

This step may be omitted as it is only done for aesthetic reasons to show polished plates when the variable capacitor is viewed from the top. I polished the top faces of the top rotor and stator plates with a bit of metal polish and then sealed them with a thin layer of acrylic medium. Any form of protective lacquer could be used.

At this stage, the central holes of the acrylic face plates are enlarged to a quarter inch (6.3 mm) in diameter using a drill bit. This needs to be done carefully to avoid cracking the plates. If you have a set of drill bits, I would recommend working through them in increasing size until the quarter inch hole has been achieved.

A knife coupling nut is placed through the central hole of the back plate and the rotor contact fitted over it to position it. The contact is placed on what will be the outside face of the back plate. Two 3.2 mm holes are drilled through the acrylic back plate corresponding to the holes in the rotor contact.

The contact is temporarily removed and the plate turned over. A countersink bit is used on the two 3.2 mm holes such that the countersink headed bolts will not protrude into the capacitor when fitted. Note that this countersinking is done on what will be the inside surface of the back plate when the capacitor is assembled. The rotor contact is then attached using two countersink headed M3 bolts and nuts.

Cut out one large and two small Mylar washers and one large PVC washer. The small washers are 7 mm in diameter with central holes of 3.2 mm diameter. It will not be a problem if they are a bit larger than this as they do not need to fit within any 8 mm separator film holes. The large washers are about 18 mm in diameter with central holes a quarter inch (6.3 mm) in diameter. Two M3 nuts are used as spacers at the opposite side of the case from the stator plates. They need to fit within the 8 mm holes in the separator films and be close to 2 mm thick. A minimum thickness of 1.925 mm is needed. I used what seem to be standard nuts which are about 6 mm across with a thickness of 2.4 mm. A little extra thickness at this side of the case should not cause a problem. If you have a sufficient number of form 'A' washers, four of those could be used instead of each nut.

As alternatives to this step, an acrylic disk of about 50 mm in diameter and 2-3 mm in thickness may be purchased or an old style knob for a quarter inch spindle may be used. To make make a disk, put a couple of pieces of masking tape on a piece of 2 mm acrylic sheet and draw a circle of 50 mm in diameter. Drill a 3.2 mm hole through the centre of the disk and cut round the outside of the circle with a hacksaw. If you have an electric drill in a stand, secure an M3 bolt through the disk with a nut and clamp the assembly into the drill chuck. With the drill running apply a file to the rotating disk until reaches the required size. Be aware that there is a tendency for the plastic to melt when doing this and for a raised edge to form. This edge may be trimmed off with a knife if that happens. If a drill assembly is not available, it will be necessary to manually sand down the disk edge.

The first diagram gives the details of how the variable capacitor is assembled. As shown, it is very much expanded in the vertical direction so that details of the components can be seen. Note also that the front and rear acrylic plates have been split apart horizontally (dotted lines) so that all parts can be seen. In reality, the rotor blades will extend nearly to the outside edges of the base of the case when engaged with the stator plates..

When I assembled my capacitor, I did so with the whole assembly turned upside down so that I started with the top face of the top acrylic plate lying down on the assembly surface.

In any case, the first stage is to assemble the rotor parts as shown in the second picture (also expanded in the vertical direction, though to a lesser degree than in the first diagram). Before this is done it is necessary to cut the head off a M3 bolt to give a section of M3 screw rod a bit less than 10 mm in length. The steps are as follows:

1. Insert the M3 screw rod part way into one of the knife nut couplers.
2. Put on the first rotor plate.
3. Put on two Mylar separator films and a form A washer.
4. Put on the second rotor blade.
5. Put on a form A washer.
6. Put on the second knife nut coupler.
7. Align the two rotor blades and tighten the assembly firmly ensuring that the separator films are not trapped between the washer and the rotor blades.

Carry out the rest of the assembly with the rotor blades turned away from the stator blades when the rotor assembly is added. Assembling the capacitor upside down as suggested, the steps are as follows:

1. Place the top acrylic plate top face down on the assembly surface with four M3 bolts inserted and pointing upwards (two on the stator blades side and two on the other (open) side.
2. Put nuts on the two M3 bolts on the open side and tighten them moderately.
3. Put on a plain stator blade followed, on each of the two bolts, by a 0.2 mm phosphor bronze washer and then a form 'A' washer.
4. Put on the PVC separator film and then insert the rotor assembly in the direction indicated (exposed washer downwards).
5. Add the second plain stator blade between the two Mylar separator films attached to the rotor assembly.
6. Add a form 'A' washer on each of the two bolts.
7. Add the third Mylar separator film which will go around the rotor spindle as well as around the washers on the stator bolts.
8. Add the stator plate with the connection lug. Make sure that no separator films are trapped between the stator plates and the washers.
9. Add a small spacer Mylar washer to each of the stator bolts.
10. Add a large spacer PVC washer and a large Mylar washer to the rotor spindle.
11. Add the pre-assembled base plate with the rotor contact.
12. Put nuts on the four bolts and tighten them, again checking that no separator films are trapped between the washers and stator plates.

This completes the assembly. If desired, the acrylic disk knob may be screwed into the rotor spindle with an M3 bolt. It might de necessary to trim a bit off the end of this bolt depending on exactly where the central screw rod was positioned within the rotor assembly.

The first three photos show the completed product with the rotor blades disengaged, partially engaged and fully engaged with the stator plates. Finally, there are photos of two crystal sets set made using the variable capacitors. In the last photo, it can be seen that the same basic construction method can be adapted to different forms of capacitor. As well as there being a variable capacitor as described in this instructable, there is a fixed capacitor, a variable butterfly capacitor and a trimmer used in conjunction with the butterfly capacitor.