## Introduction: Digital Theremin: Touchless Musical Instrument

In this experiment with Digital Electronics, I'll show you how to generate music (close to it :P ) without touching the musical instrument, using Oscillators & Op-amp. Basically this instrument is called as Theremin, originally constructed using analog devices by a Russian scientist Léon Theremin. But we'll design this using IC's which generate digital signals and later we'll convert them into analog for music.I'll try to explain the every stage of the circuit also. I hope you'll love this practical implementation of what you have studied in your college.

I have also designed this circuit on www.tinkercad.com & performed its simulation of components. You can see try it & manipulate it as you like, because there's nothing to loose there, only Learning & Fun!

## Step 1: Components

Here's the list of all the essential components needed to build this circuit:

1) MCP602 OpAmp (Differential Amplifier) x1

2) CD4093 IC (4 NAND Gates IC) x1

3) Resistors: 6x 10k, 1x 5.1k, 1x6.8k & 1x 1.5k

4) Potentiometer : 2x 10k Pot

5) Capacitors: 2x 100pF, 1x 1nF & 1x 4.7µF Capacitor (Electrolytic)

6) Breadboard/PCB board

7) Telescopic Antenna (Minimum Req: 6mm diameter & 40cm+ length) OR it's better to use Copper tube with the given dimensions for better sensitivity

8) Power DC Jack (5.5mmx2.1mm)& Audio Jack (3.5mm)

9) Other components like wire and soldering parts

Note: You can find all these components easily on a Radio shack or Online on amazon/ebay. Also note that in tinkercad circuit, the op-amp & Nand gates are different, but they'll work also. Still If you find any difficulty in getting any component, let me know.

## Step 2: Let's Understand Circuit Working

Above you can find the circuit layout image for reference.

Working: Basically theremin works on the principle that we generate two oscillatory (sine wave in analog) signals from two different oscillators- 1) One is Fixed oscillator 2) Second one is variable Oscillator. And we basically take the difference of those two frequency signals to get the output signals in Audible frequency range (2Hz-20kHz).

* How we are doing?

As you can see, below NAND gate (U2B) circuit is a a Fixed oscillator and the above NAND gate circuit (U1B) is a variable oscillator circuit, whose overall frequency varies slightly with the hand movement around Antenna connected to it ! (How ?)

* How does hand movement around antenna changes frequency of the oscillator?

Explanation: Actually, Antenna is connected in parallel with C1 Capacitor here. Antenna acts as One of the Capacitor plate and our hand acts as the other side of the capacitor plate (which is grounded through our body). So basically we are completing the additional (parallel) Capacitive circuit and hence adding overall capacitance to the circuit. (Because Capacitors in parallel are added).

* How does oscillations are generated using NAND Gate?

Explanation: Initially, One of the inputs of the NAND gate (take U2B for example) is at HIGH level (1) and other input is grounded through C2 (i.e 0 ). And for ( 1 & 0 ) combination in NAND GATE, we get output HIGH (1).

Now when the output gets HIGH, then through feedback network from output (through R3 & R10) we get HIGH value to the previously grounded input port. So, here is the actual thing. After feedback signal, the Capacitor C2 gets charged through R3 and after that we are getting both inputs of NAND Gate at HIGH LEVEL (1 & 1), and output for both HIGH logic input is LOW (0). So, Now the Capacitor C2 discharges back and again the one of the input of NAND Gate gets LOW. Hence this cycle repeats and we get the Oscillations. We can control the oscillator frequency by changing the value of resistor and Capacitor (C2) because the charging time of capacitor will vary with different capacitance and hence the oscillation frequency will vary. This is how we are getting oscillator.

* How do we get musical (Audible) frequency from High frequency signals?

To get audible frequency range, we subtract the two frequency signals from each other to get lower frequency signals which is within audible range. Here we are using Op-amp as in differential amplifier stage. Basically in this stage, it subtracts the two inputs signals to give the Amplified difference (f1 - f2) signal. This is how we get audible frequency. Still to filter the unwanted signals, we are using LOW pass filter to filter noise.

Note: The output signal we get here is very weak, hence we need additional Amplifier to amplify the signal. You can design your own amplifier circuit or just feed the signal of this circuit to any amplifier.

Hope, you understood the working of this circuit. Still any doubts? Feel free to ask anytime.

## Step 3: Design the Circuit

Please first design the whole circuit on breadboard first and check it. Then only design it on PCB with proper soldering.

Note1: This is a high frequency circuit, hence it's advisable to keep the components as close as possible.

Note2: Please use only +5V DC power supply (Not Higher), due to IC voltage restricitons.

Note3: Antenna is very crucial in this circuit, hence please follow all the instructions given strictly.

## Step 4: Circuit Working and Software Simulation

Please see the circuit simulation and its Video.

I have added the Multisim Circuit File, you can directly run the circuit using that and design your own and do manipulations.

Hey, I have also added the Tinkercad (www.tinkercad.com/) Circuit link also, there you can design your circuit OR manipulate my circuit also and perform circuit simulations also. All the best with learning and playing with it.

Tinkercad Circuit Link: https://www.tinkercad.com/things/8pQo6T7XgwX-exqui...

Hope You liked this. I'll try to improve it further and add its analog version & Microcontroller based(using VCO) soon which will have better linear response to the hand gesture movements about antenna. Till then, Enjoy playing with this theremin.

Update: Guys, I have also designed this another theremin using LDR & 555

### Attachments

Participated in the
Audio Contest 2018