This simple but complete Theremin with Inverter Gates is
constructed using only two inverter chip plus one regulator IC. This
Theremin circuit consist of five functional blocks: power supply
regulator, hand controlled oscillator, null oscillator, mixer, and
filter. Here is the complete schematic diagram.
Theremin with Inverter Gates schematic diagram
Voltage Regulator and Circuit Protector
The power supply regulator consist of LP2950 regulator IC, which
stabilize the voltage from battery to 5V. You can use more popular 7805
IC for this, but since the power consumption of this Theremin circuit is
very small, then you can use 78L05 which is smaller. CR1 diode is used
to protect from inappropriate battery polarity, shorting the battery
voltage together with R8 100 Ohm resistor which prevent the large
current when the battery is installed in wrong direction. Although the
inverter chip will work well for 9V battery, there is a benefit of using
voltage regulation to regulate the battery voltage at lower voltage
level, that the voltage will remain constant for until the end of
battery life. This will avoid frequency drift of the Theremin’s null
oscillator which should be carefully adjusted to zero the output
frequency, which can be affected by the supply voltage.
Hand Controlled Oscillator
The hand proximity sensor is an oscillator which has antenna
extension which shift the capacity coupling in the loop. This
capacitance shift occur when we move our hand approaching the antenna.
Since this change is very small in percentage, we need this oscillator
to be high enough to produce notable frequency difference. This
oscillator block is built around U1A, U1B, and U1C. This oscillator give
oscillation at around 73kHz. This frequency is not directly audible, we
have to process this signal further to produce audible signal.
Null Oscillator
Null oscillator is employed to produce a constant frequency
oscillation that will be used to produce differential frequency which is
audible. This oscillator block is built around U2A, U2B, and U2C. This
null oscillator should be adjustable to set the null point where the
Theremin should produce no oscillation at certain hand position. At
this point, the null oscillator should be adjusted to have same
frequency with the hand controlled oscillator since the audible Theremin
output is the product of the difference between hand controlled and
null oscillator frequencies.
Frequency Mixer (Differentiator)
The mixer is used to mix the signal from two oscillators, the hand
controlled and the null oscillators. This mixer produce an output which
contain many frequency components, not only the difference but also the
original and the sum, since the amplifier U1F is basically a digital
inverter which has non-linear transfer function. Fortunately, all the
frequency components, except the difference, will be much higher than
the needed signal and inaudible. This make it easy to remove by simple
low pass filter to obtain only the differential frequency component.
Low Pass Filter
As stated before, we need to obtain only the audible frequency
component by low-pass filtering. Although the high frequencies is
inaudible, we still have to remove it since it can cause damage in audio
power amplifier is the level is too high. It can distort the audible
signal, and it can eat the electric power like ghost, frying your
amplifier or at least make it overheated. This Theremin circuit use
simple low pass filter consist of C4, R5, and R7 for the passive stage,
and C2 inside the inverter amp loop.
+Power+Amplifier+Circuit.gif)
