Theremin. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Musician

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"Thereminvox" is the first electronic musical instrument developed in 1921 by the St. Petersburg physicist Lev Theremin and named after its inventor. It is unusual in that it does not have keys or strings. The performance of the melody is carried out by approaching (removing) one or both hands to the antenna.

The theremin diagram of which is shown in Fig. 1 is a simplified version of the theremin and is implemented on three integrated circuits. A generator (multivibrator) of variable frequency is built on elements D1.1 and D1.2, element D1.4 acts as a buffer.

(click to enlarge)

The frequency of the multivibrator depends on the resistance of the resistor R2. the capacitance of the capacitor C3 and the capacitance between the antenna WA1 and the common conductor of the device, which is formed when the performer's hand is brought to the antenna. To obtain the maximum sensitivity of the generator to the capacitance of the arm antenna, the frequency of the multivibrator is chosen relatively high (several hundred kilohertz).

The second fixed frequency oscillator, identical to the first one, is built on elements D2.3, D2.4 with buffer D2.2. The generator frequency can be changed within small limits using the RP1 potentiometer. In the integrated circuits D1 and D2, three logic elements are used (four in total). The inputs of unused logic elements are connected to a common wire. From the outputs of these two generators, the signals are fed to the mixer, implemented on the D3 chip. If at some inputs of elements D3.1 ... D3.4 the signals have a frequency f1, and at others - f2. then at the output of the mixer, signals with a frequency f1 ± f2 are obtained. The elements are connected in parallel to increase the load capacity of the mixer. In this case, the amplitude of the received signal is sufficient for the buildup of the output transformer T1 connected to the output of the mixer. The transformer is loaded on the dynamic head VA1.

The sound volume can be continuously adjusted using the RP4 potentiometer. A telescopic antenna from a portable transistor radio is used as a sensor, but a piece of metal tube 04 ... 6 mm long 350 ... 500 mm can also be used. When using a telescopic antenna, you can additionally adjust the sensitivity of the device by changing its length.

The theremin is powered by a 9 V DC source. The current consumption does not exceed 10 mA, so one 6F22 battery can be used. To prevent the mutual influence of two generators, each of them is connected to the power supply through an RC filter (R5-C5 and R6-C7). The output transformer and loudspeaker are taken from a portable transistor radio. The device is mounted on a printed circuit board, the drawing of which is shown in Fig. 2, and the arrangement of elements is shown in Fig. 3.

The self-assembled theremin is placed in a case measuring 160x90x40 mm, on the front side of which there is a socket for an antenna, axes of two potentiometers, a BA1 loudspeaker and a power switch. The enclosure dimensions given are indicative and depend mainly on the dimensions of the loudspeaker. Integrated circuit CD4011B can be replaced by K176LA7. K561LA7, CM14011R, HEF4011.

Setting up the theremin is done as follows. Using the RP1 potentiometer, the "zero beats" mode is set, i.e. the frequencies of the two generators are aligned like this. so that no sound can be heard from the loudspeaker. When the hand approaches the antenna, a sound should appear. The sensitivity is fine-tuned with the RP1 potentiometer: by smoothly turning its axis to the left or right, the optimal position is found at which the sound appears at the maximum distance from the hand to the antenna. Greater sensitivity can be achieved if one hand of the performer touches the common conductor (mass) of the device. When the hand approaches the theremin antenna, the sound frequency gradually decreases (removal of the hand causes an increase in the sound frequency). If you move your fingers near the antenna, a sound resembling laughter arises. After some training, you can learn how to play simple melodies.

The described device is of interest not only in the field of music. Without major changes, it can serve, for example, as a signaling device for approaching dangerous objects in the dark, as a safety device, etc.

Author: G.Minchev

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