Dual Tone Multi Frequency Circuit

Dual Tone Multi Frequency Circuit


Designing a touch tone signal generator for #1 by using oscillators. In this project, I need to design a touch tone signal generator for a specific button (for only one button #1). The output signal is the sum of the two corresponding sinusoids (high&low frequency components). In my design, you need to use your own components. Final generated signal will be the sum of these signals and a loud speaker is needed to hear the final signal.


Dual Tone Multi Frequency signals are using electronic world. Each button is represented by an identical set of two frequencies. This provide ID for each button and It provide easier the phone communication with ther devices such as computer software


In this experiment, we are supposed to design a Dual Tone Multi Frequency signals for play the specific frequencies with a buzzer. Wien-bridge oscillators will be used in order to generate 1377 Hz and 697 Hz (for button 1).

At the Wien bridge, it is found in oscillations with a combination of two different resistors and capacitors. Parallel combination, parallel connection, parallel combination. Parallel combination. Theoretically, oscillation is produced in this way. The amplitude along this path is scaled to 1/3. At this point, the OPAMP gains re-scales the signal with 3 factors to prevent losses and losses. For a non-rotating opamp, this section is shown as Rf / Rg = 2. However, this is not the case to produce oscillations. The signal produced is a measure of the nanovolts, so it must be amplified. An easy way for this case is to increase the earning factor a little. For example, Rf / Rg = 2.01. Thus, it reaches the voltages that the signal will rise and cut during the turn.

After the previous arrangements, it is necessary to summarize the signals using a gain amplifier of only 0.5 gain (it is possible to generate the sum of the sines in the offset voltage range of the opamp with this gain).

Producing the desired frequencies depending on the following calculations:

Assuming that the combined resistance values of the capacitors are equal and the capacitors are equal,

And for the gain of the opamp (in theory):

The circuit is as follows: 1


For #1, equations results as follows:

foscillator,1 = 1209 Hz →When C1 = C2 = 100nF fixed, R1 = R2 = 1377Ω

foscillator,2 = 697 Hz →When C3 = C4 100nF fixed, R3 = R4 = 2285Ω


  • 2 X ua741 operational amplifiers
  • 4 X 104j63 100nF capacitors
  • 1 X 1W 8Ω speaker
  • 4 X 0 – 5k potentiometer
  • 7 X Ohmic resistances, while 4 of them 2R=200ohm and 3 of them are R.
  • Jumpers
  • 2 Board
  • 2x9V bataries


The setup of the oscillators is simple and is like the previous picture. The important parts are that the gain resistance of the opamps should be 2.1 ratio of a degree; This is the way it works to have a ratio of 5% to 5% of the resistances. One thing to be aware of is the installation of the pickup amplifier. An input signal should be scaled by 0.5 gain. The signal obtained in this way will be in the range of + – 9V, ie. It is smaller than Opamp’s supply voltages. A potentiometer is available to carefully adjust the resistance of the oscillating path. The calibration of the resistors is important for producing the desired signals. Finally, the circuit is ready to be heard. When the negative probe of the loudspeaker is connected to the ground, the positive value is connected to the output of the summing amplifier to emit the desired loudspeaker for the loudspeaker number 1. It seems a bit noisy at this point, in fact the exits from the oscillator are almost perfectly shaped and the frequency only has an error rate of around 0.3%.


This project reflects the difference between analogue and digital atmospheres perfectly. Theoretically, ie Calculations and PSPICE simulations, the experiment is straight forward and requires 2 oscillators and a summing amplifier. However, due to the noise of the UA741 during the entire operation, a very high noise ratio is encountered for this problem solution is can be filtering but I can’t do this because I don’t have enough equiment


As a result, it was satisfactory regarding a complex circuit. Many mistakes were observed for various reasons, and the various tools were reasonably handled. PSPICE simulations and oscilloscope views of the “same” episode were very different at first, but solved. Many practical applications of information gathered from this experiment have been learned. The oscilloscope screen images are appended to the end of the report.

Circuit Board

C:\Users\cceem_000\AppData\Local\Microsoft\Windows\INetCache\Content.Word\IMG_9225.jpg C:\Users\cceem_000\AppData\Local\Microsoft\Windows\INetCache\Content.Word\IMG_9224.jpg




First input tone (697 Hz)

Second input tone (1209 Hz)

Final Output:

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