Picture of How to Make a Voltage-Controlled Oscillator
The objective of this Instructable is to show you one method of turning DC values, such as those from a thermometer, pH sensor, or pressure sensor into a frequency which can be used to send information over the microphone band of an audio jack to a smartphone.

We assumed an input DC voltage as our sensor and built a voltage-controlled oscillator (VCO) to turn different DC voltages into correspondingly different frequencies, which the microphone input into the smart phone’s audio port reads as different tones.
Remove these adsRemove these ads by Signing Up

Step 1: What You Need

Picture of What You Need
Oscilloscope & lead wires
Frequency generator
Device that plays audio and takes microphone over a 3.5mm audio jack (we are using a Samsung Galaxy Vibrant).

3.5mm TRRS Audio cable
Resistors (two 100kΩ and one 100Ω)
1μF Capacitor
OPA 551 [] (1)
10Ω-10kΩ Potentiometer

Assumed Knowledge
You can use a breadboard. If you’ve never used one before, see
You can use an oscilloscope. If you’ve never used one before, see

Step 2: Build an Oscillator

Picture of Build an Oscillator
Why: There are many designs of VCO, but we chose a hysteretic oscillator, which is a type of relaxation oscillator. If you're interested in oscillator design, this website has good information.

Theory: The comparator (an op-amp, in our case) generates a positive feedback loop between the positive and negative voltages. This feedback charges the capacitor when it draws from the positive voltage, then once the capacitor fills, it discharges, switching the power draw from positive to negative. This process repeats to oscillate continuously.

The frequency of the oscillation is thus dependent upon the size of the capacitor and on the input voltages.

Practice: Values should be selected so that the frequency output will be in the audio range, approximately 16 Hz to 20 kHz.
Power the op amp according to the values on its data sheet.
gaieb1 month ago

In the old days, before digital they were used to make signals that could be recorded on tape. Industry used 9 channel tape for recording data using different freq. VCOs so that several could be recorded on each channel, if I remember right you could record 24 different data streams on each channel. They were also used to tune radio transmitters and receivers.

Interesting project. This turns anything that can control a voltage into a signal that can be used as sound right? I'm guessing it's not 1 octave per volt or anything particularly musical, but the idea of making analogue events generate audible tones or digital signals has all kinds of interesting applications.