How to Build a Theremin Using Three AM Radios

March 30, 2018 | Author: Cosmin Iordache | Category: Am Broadcasting, Transmitter, Antenna (Radio), Broadcasting, Radio Technology



HOW TO MAKE A BASIC THEREMIN USING THREE AM RADIOS.(Not digital type) Figure 1. Figure 1 shows three AM radios (they do not have to be identical). From here on, we shall name them FIXED TRANSMITTER, VARIABLE TRANSMITTER and RECEIVER, respectively. You can attach temporary labels to the radios with those words, so as not to be confused later. If you cannot find AM only radios, it’s ok to use AM-FM, but we will only use the AM function, totally ignoring the FM capability. First thing you can do to get the feeling of what we are doing is to turn the three radios on. Set the volume of FIXED TRANSMITTER and VARIABLE TRANSMITTER to a minimum, we do not need them to produce any sound at this point. Only RECEIVER has the volume turned up. This radio will be your “output amplifier.” Now rotate the tuning knob of RECEIVER to approximately 1500 kHz. Make sure there is no station there. If there is a station at that spot, you can try going a little bit above or below that frequency. Let us assume you tune to 1500 and there is no station at that spot. All you hear is a little static. Now tune FIXED TRANSMITTER to 1045 kHz (1500 minus 455, which is usually the difference in the local oscillator frequency, it is called the intermediate frequency, or IF). If you do not understand my technical explanations, don’t worry, I’ll walk you through the whole process, anyway. Okay, as you tune FIXED TRANSMITTER to 1045, you should hear a “silence” in RECEIVER, However. the variable capacitor. If it is AM only. We remove the cover of VARIABLE TRANSMITTER and. stop there. your guideline will be the “silence. for the signal is very weak. the static goes away as you turn the dial in FIXED TRANSMITTER (remember. besides a bunch of resistors. That is our next step. This capacitor does the tuning of the radio. they must be almost touching each other). Figure 2. you will see a white (or transparent) plastic square.” That is due to the fact that the two frequencies being generated by FIXED TRANSMITTER and VARIABLE TRANSMITTER are similar. What we must . If your radio is an AM-FM radio. What we need now is to be able to change the frequency in VARIABLE TRANSMITTER without touching the dial. or trimmers. As you approach that frequency. capacitors. If you get that silence. etc.” rather than the exact frequency. they have to be physically close to each other. RECEIVER will start to “whistle. Note: Early receivers had an IF of 175 kHz. and you can hear the frequency difference or “beat frequency” in RECEIVER. you would be able to play a tune by turning the dial of VARIABLE TRANSMITTER back and forth. Basically.that is. one at each corner of the plastic square. also. Now tune VARIABLE TRANSMITTER to 1045. and then they changed to 455 kHz. but that would be kind of odd and cumbersome. In any case. the capacitor will have four small screws. it will only have two trimmers. although some models use 465 kHz. 455 kHz is quite universal. you can use a stiff wire. so the antenna will be vertical. If you wish. by sanding the hanger wire). the equivalent of turning the tuning dial. . so just disconnect it from its normal connector and attach the alligator wire to it. but the factory-made antenna has a better appearance (make sure the contact is good. Right now you can run to the electronics store and buy yourself an alligator clip (shown in figure 3). and put VARIABLE TRANSMITTER back near the other two radios. as you touch it. the little terminals located at the corners of the capacitor. or have no effect. Now we can touch the little screws. Note: If VARIABLE TRANSMITTER is an AM-FM radio. Now you can mount the antenna or the piece of wire from the clothes hanger. or better yet.” Figure 3. the telescopic antenna will already be there. the oscillator frequency goes down. as well as a small telescopic antenna. just look in “more from eltunene. and the other will bring the station out of tune. on the plastic cover. One of them will maybe make the station sound louder. This is the “oscillator terminal. Now use the alligator clip to connect your antenna to the oscillator terminal in VARIABLE now is turn up the volume of VARIABLE TRANSMITTER and tune in an AM station (I posted a video in youtube called “Detailed instructions for preparation of third radio for theremin” or something like that.” where you can see the actual capacitor in a radio). such as a piece of a clothes hanger. That is the one we want! That little terminal will control the local oscillator of the radio. Put the cover back on. outside the radio. . carefully adjust FIXED TRANSMITTER until RECEIVER starts whistling. because its frequency has gone lower because you added the antenna. to prevent interaction. If you need additional help. Keep fine-tuning until holding your hand about 70 cm will produce a very low note (bass) and as you bring your hand close to the antenna or hanger wire.You’ll have to tune VARIABLE TRANSMITTER to a slightly different position than 1045 kHz.” Now. please drop me a message via youtube. again trying not to have your hands close to the antenna of VARIABLE TRANSMITTER. Regards! eltunene. A suggested tuning order would be (remember only RECEIVER has the volume up): Tune VARIABLE TRANSMITTER to approximately 1045 kHz. the pitch will raise until it becomes out of hearing range. tune RECEIVER until you hear the “silence. Very carefully. keeping your hands as far from the antenna as possible.
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