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Completing, testing & adjusting the . . . Opto-Theremin In Pt.1 last month, we described how the Opto-Theremin works and gave the assembly details for the two PCBs. This month, we complete the construction and describe the test and adjustment procedure. Pt.2: By JOHN CLARKE T HE OPTO-THEREMIN’S main PCB is housed in a black UB1 plastic utility box measuring 158 x 95 x 53mm. This box is supported on a timber plinth (or base) using threaded rods and three 50mm lengths of 10mm ID aluminium tubing. The first step is to prepare the box by drilling the various holes. We’ve prepared a template (in PDF format) to make this job easy. This can be downloaded (no charge) from the SILICON CHIP website and printed onto plain paper – browse to www.siliconchip.com.au and then mouseover ‘Shop’, click ‘by Year/ Month’ and select the month. While you are there, download the front panel artwork and the drilling templates for the timber plinth and the smaller UB5 case. These can again be printed onto plain paper but for a better result, print the panel artwork onto photographic paper. 82  Silicon Chip Next, cut the case template sheet into its various sections, then attach the templates to the case (eg, using adhesive tape) and drill the holes to the dimensions indicated. Use a small pilot drill to start the larger holes, then carefully enlarge them to their correct sizes using larger drills and a tapered reamer. Be careful not to over-enlarge the 10mm-diameter the hole for the antenna. The aluminium antenna tube should be a tight fit into this hole. Once the holes have all been drilled, the main label can be affixed to the lid using silicone sealant or a suitable adhesive. Allow the adhesive to dry, then cut out the various holes using a sharp craft knife. The speaker can then be secured to the inside of the lid by smearing a suitable adhesive (eg, super glue) around its outside metal frame. Once the speaker is in place, it can be fitted with a short figure-8 connecting cable terminated in a 2-way header plug at the far end. The main PCB is fitted into the box by first tilting it down at the front, so that the pot shafts and the switch can be slid into their respective holes. The M3 x 6mm SCREW PCB M3 TAPPED 9mm SPACER M3 NUT BOX M3 x 10mm SCREW Fig.7: here’s how to install the rear spacer assemblies. No spacers are required at the front of the case, since the PCB is supported along this edge by the two pot shafts. siliconchip.com.au rear of the PCB can then be pushed down into the case, after which the assembly should be secured in position by attaching the nuts to the pots. Do the nuts up firmly, then mark out the positions of the two rear mounting holes on the base of the case (eg, by hand-twisting a 3mm drill through the PCB holes). That done, remove the PCB and drill these holes in the base out to 3mm. There’s no need for corresponding front mounting holes, since the PCB is supported on this side by the pot shafts. The rear spacer assemblies can now be installed as shown in Fig.7. First, an M3 x 6mm screw is inserted up through the bottom of the case. This is then secured with an M3 nut, after which an M3 x 9mm tapped spacer is fitted. Don’t reinstall the PCB yet – that step comes later, after attaching the case to the timber plinth. (UB1 BOX & LID) M5 OR 3/16" NUTS 3 x 50mm LENGTHS OF 10mm OD ALUMINIUM TUBING M5 OR 3/16" THREADED RODS M5 OR 3/16" NUTS Making the timber plinth A piece of 151 x 90 x 19mm DAR (dressed all round) pine timber is used to make the base – see Fig.8. Note that Fig.8 is not to scale, so you should download the full-size diagram from the SILICON CHIP website and print it out to use as a template. Cut the timber plinth to size, then round off the edges and the corners using sandpaper. The paper template can then be attached to the base and the three holes drilled to accept either M5 or 3/16-inch threaded steel rod (zinc-plated). Countersink the holes on the underside to allow the nuts to be recessed. Fig.8 shows the cross-sectional view (two rods only shown). The timber base is now used as a template to mark out the corresponding holes in the bottom of the case. Drill these to suit the threaded rod, then cut the threaded steel rod into three 75mm lengths. You will also need to cut three 50mm lengths of 10mm-diameter aluminium tubing, to serve as spacers. It’s a good idea to paint the timber base black to match the box colour. After that, it’s just a matter of attaching it to the case using the 75mm threaded rods, 50mm aluminium tube spacers and nuts as shown in Fig.8. Take care to ensure that the threaded rod protrudes no further into the box than the nut, otherwise it may later short against the tracks on the underside of the PCB. siliconchip.com.au HOLES COUNTERBORED TO INSET NUTS PINE TIMBER PLINTH MEASURING 151 x 90 x 19mm 50 mm ME D IA NOTE: NOT TO SCALE T C ER LE IRC CL CL Fig.8: this diagram shows how the timber plinth is attached to the base of the case using three 50mm lengths of 10mm-diameter aluminium tubing and M5 or 3/16-inch threaded rods. Note that this diagram is not to scale. A full-size version can be down­loaded from the SILICON CHIP website and used as a drilling template for the plinth. The PCB can now be reinstalled in the case and secured to the previously installed rear spacers using M3 x 6mm machine screws. Tighten these screws down firmly, then install the pot nuts and fit the two knobs. If the knob pointers are in the wrong positions, prise the end caps off and refit them so that they are correct. Volume control case The volume control PCB is housed in transparent blue UB5 plastic utility box measuring 83 x 54 x 31mm. A rectangular cut-out has to be made in the base (which becomes the top) to accept the distance sensor, while five holes have to be drilled in one end for the external wiring connections and two threaded mounting rods. As with the larger case, it’s just a matter of attaching the drilling template downloaded earlier and then drilling the holes to the sizes indicated. The rectangular cut-out is made by drilling small holes around the inside perimeter, then knocking out the centre piece and filing to shape. October 2014  83 The PCB is installed in the case by first angling it down at the front and sliding the pot shafts and the switch actuator into their respective holes. The rear of the board is then slid down into position and the PCB secured by doing up the pot nuts and fitting the screws to the rear spacer assemblies. Be sure to make this cut-out in the base (not the lid). The case is later attached to the main case with the base facing upwards and the lid on the bottom. Refer now to Fig.9 to see how the volume control case is attached to the main case. The first job is to cut two 62mm lengths of M5 or 3/16inch threaded rod plus two lengths of 10mm OD aluminium tube. These aluminium tube pieces should be 50mm long minus the width of the nuts used (eg, if the nuts are 4mm wide, then cut two 46mm tube lengths). Once you have all the pieces, attach the two threaded rods to the volume control case as shown in Fig.9; ie, for each rod, use a nut inside the case and another outside the case. We used Nylon lock nuts (metal nuts with a Nylon thread insert) because they each have a rounded end that the aluminium rod fits over and because they don’t come undone. The next step is to fit three straight 88mm lengths of 1mm-diameter steel wire to CON5 on the volume control PCB. These wires are then slid into the holes in the end of the case (between the nuts securing the threaded rods) and the PCB clipped into place (ie, into the integral ribs). If you can’t get steel wire, use 1mmdiameter tinned copper wire. This can be straightened by clamping one end in a vice and then stretching it slightly by pulling on the other end with a pair of pliers. Final assembly The speaker is secured to the inside of the lid by smearing super glue or silicone around its outside metal frame. 84  Silicon Chip The volume control case assembly can now be attached to the main case as follows: (1)  Cover the front threaded rod (ie, at the bottom in Fig.9) with a length of 6mm-diameter heatshrink tubing. This heatshrink layer should cover the entire length of the thread and can be trimmed to size after shrinking it down. (2)  Cut another length of heatshrink tubing about 3mm shorter than the aluminium tubing and add this to the rod. Push it all the way up against the nut at the volume control case end siliconchip.com.au Fig.9: the volume control case is attached to the main case using two lengths of 10mm-diameter aluminium tubing and M5 (or 3/16-inch) threaded rods and nuts (see text). Three 88mm lengths of 1mm-diameter steel wire are then fitted to CON5 on the volume control PCB, after which the board is clipped into the case, with the wires exiting via three holes between the threaded rod assemblies. These wires feed into matching holes in the main case and are terminated in CON2. It’s a good idea to insulate the middle wire with heatshrink tubing. 50mm M5 OR 3/16" NUTS (VOLUME CONTROL PCB INSIDE UB5 BOX) (MAIN PCB INSIDE UB1 BOX) 1mm DIAMETER WIRES GP2Y0A41SK0F CON5 CON2 SHARP M5 OR 3/16" THREADED RODS 10mm DIAMETER ALUMINIUM TUBING (LID OF UB5 BOX UNDERNEATH) THREAD COVERED IN SLEEVING TO FIT TIGHTLY IN HOLE – NO NUT USED M5 OR 3/16" NUTS This close-up view shows how the end of one of the threaded rods is covered in heatshrink tubing so that it is a tight fit into its matching hole in the main case – see Fig.9. before shrinking it down. (3)  Repeat step 2, adding more heatshrink layers until the aluminium tube is a firm fit over this threaded rod. (4)  With the aluminium tubes in place, insert the three wires and the threaded rods into the main case, with the ends of the wires going into CON2. The heatshink-covered rod should be a tight fit into its hole. (5)  Secure the other threaded rod with a nut on the inside of the main case. (6)  Tighten CON2’s screws to secure the three wires in place. Making the pitch antenna The pitch control antenna is also made from 10mm-diameter aluminium tubing. You also need an M4 x 10mm Nylon (or polycarbonate) screw siliconchip.com.au The volume control PCB is clipped into the UB5 plastic case with the three 1mm-diameter wires exiting through holes at one end. and two M4 Nylon (or polycarbonate) nuts. First, cut a 450mm length of the tubing and clean up the ends with a file to remove any metal burrs. That done, gently file each corner of one of the M4 nuts until it fits tightly into one end (ie, the top) of the antenna. Once it’s in position, wind the second M4 nut all the way onto the screw and then screw this into the captive nut in the antenna. This translucent ‘top piece’ provides the blue glow at the top of the antenna when lit by LED3 on the main PCB (ie, the blue LED that shines up the antenna tube). The other three blue LEDs (LEDs1, 2 & 4) light the base of the antenna. As an option, these three LEDs can be covered with a translucent, halfhemispherical, hollow ball that’s slid over the antenna and pushed down onto the lid of the main case. A ball salvaged from an empty can of roll-on deodorant is suitable. All you have to do is cut the ball in half using a fine-blade hacksaw, file the ends to October 2014  85 An M4 Nylon (or polycarbonate) nut is pushed into the top of the pitch antenna after which a nylon M4 screw with captive nut is fitted. This translucent assembly glows blue when lit by the LED shining up the aluminium tube. The pitch antenna is pushed into the two fuse clips on the main PCB assembly (usually after the lid is in place). LED3 is between the two fuse clips and shines up the antenna tube to light the translucent screw assembly at the top. a smooth finish and drill a 10mm hole in the top. It’s then simply slid over the antenna. As previously stated, the bottom end of the antenna is connected into circuit by sliding it into the two fuse clips on the main PCB. It may be necessary to squeeze the lugs of these fuse clips together slightly so that the antenna makes a good contact. Rotating the antenna a few times will also clean the contacts if they oxidise over time. For the time being, leave the lid off the case and simply support the antenna in its fuse clips. You are now ready for the setting-up procedure. Setting up The adjustment procedure is as follows: Step 1:  fit link LK1 (near the equalising coil) to the TEST position and LK2 to the MAX position. Step 2:  connect a 9VAC plugpack or a 86  Silicon Chip 12V DC source, switch on and check that all the LEDs light. If they don’t light, check that they are orientated correctly. Step 3: connect a DMM set to read DC volts between TPS (near IC2) and TP GND and adjust trimpot VR4 for a reading of 1.7V. Step 4:  connect the DMM between TP1 and TP GND and adjust the slug in transformer T1 for a minimum reading (note: do not use a screwdriver as this could crack the ferrite core. Either use the correct plastic alignment tool or grind down an old screwdriver so that its blade is thicker than normal and snugly fits the slot in the slug). If you are unable to find the minimum, then either coil L1 has been incorrectly wound or its leads haven’t been soldered. Check the solder joints and check also that the Nylon washer spacers have been installed to provide the required 2.5mm gap between the two core halves. Step 5:  move your hand very close to the antenna (but don’t touch it) and adjust T1’s slug so that the voltage slightly increases. When it does, move your hand away from the antenna and check that the voltage increases even further. If the voltage decreases instead, then the slug needs to be rotated the other way. On the prototype Opto-Theremin, we adjusted T1’s slug for 1.1V with the hand close to the antenna and 1.7V with the hand away from the antenna. Step 6:  move jumper LK1 to the NORMAL position, connect the loudspeaker to CON4, set VR1 to mid-position and set VR2 fully clockwise. Step 7:  adjust transformer T2 until a tone is heard and set it for a low frequency. This tone should then change if you move your hand away from T2 (and away from the antenna), so this may take some trial and error. Step 8:  rotate VR2 anticlockwise and check that the pitch can be adjusted to just reach a point where there is no sound. The frequency should then become audible again and increase as a hand is brought close to the antenna. If not, reset VR2 fully clockwise again and repeat Step 7, this time adjusting T2’s slug in the opposite direction. Note that these adjustments require patience and you may need to repeat the process several times before you get it right. Step 9: adjust VR1 fully clockwise, then adjust trimpot VR3 to limit the volume so that it isn’t high enough to cause spurious vibrations or noticeable distortion. Voicing adjustment Trimmer capacitor VC1 must now be adjusted to set the voicing. It’s just a matter of tweaking it to obtain the required sound from the Theremin. Note that there will be a point where, at the lowest frequencies, there’s a ‘snap-on’ effect whereby either no frequency is produced or the tone suddenly snaps on and becomes audible with hand movement. This occurs because inter-coupling between the pitch and reference oscillators causes both oscillators to track together and if there’s no frequency difference between them, there’s no audible output from the mixer. However, as a hand is brought closer to the antenna, the pitch oscillator’s tuning changes and it is eventually ‘pulled’ far enough to suddenly produce a different frequency to the reference oscillator. Hand volume adjustment The hand volume adjustments are all done on the main PCB as follows: Step 1:  move jumper LK2 back to the NORMAL position, then check that the volume control has a suitable handmovement range. The volume should increase as the hand is moved away from the sensor and vice versa. Step 2:  if you want to change the range, connect a DMM between TPS and TP GND and adjust trimpot VR4 for a reading that differs from the 1.7V set earlier. Note, however, that if VR4 is set to give maximum volume too away from the sensor, the volume will rise again at close range (ie, as the hand is brought below 40mm). This is a quirky effect of the sensor itself and is cured simply by backing off the setting for VR4. That completes the adjustments. You can now complete the unit by attaching the lid to the main case and reinstalling the antenna, with the translucent dome slid all the way down so that it covers the three LEDs. Take care when fitting the lid to ensure that the four LEDs go through their corresponding holes. You will find it easier to do this if you apply power so that the LEDs are lit. Finally, fit the lid to the underside of the volume control box and your Opto-Theremin is ready for action. SC siliconchip.com.au