We wanted to do a hardware project because most projects at hackathons are software based.

What it does

Our original aim was to create a voice modulator using Double Sideband Suppressed Carrier (DSB-SC) modulation. The input would be your voice, which would go through a high pass filter and then a mixer where the input would be multiplied by a cosine for DSB-SC modulation. The DSB-SC output would then be amplified with an op amp. However, due to many difficulties, we had to downscale our project significantly. Instead, we built the op amp to amplify audio from a computer. The signal comes from the computer, goes into the op amp, and is output by the speakers. The op amp is a simple inverting amplifier, meaning that the input comes from the negative terminal of the op amp. The resistors in series from the input are what determines the gain, or volume in this case.

How we built it

We first found the gain of an inverting op-amp by taking a nodal equation at the terminal between the two resistors in series. This gave us the equation we would need to calculate the resistor values needed to acquire a desired gain of around 100. Then, we prototyped the circuit on a breadboard and tested it. Once we verified it was working, soldered it to a solderable breadboard.

Challenges we ran into

There were many challenges, mainly due to the lack of electrical equipment. None of us owned a function generator. We drove all the way to Leavenworth to buy a function generator from Facebook marketplace, but then found out that it did not function by the time we got back to Lawrence. So instead, we decided to use our computers to mimic the function generator which would create the sinusoidal signal for us.

The next challenge was the oscilloscope. One of our members has a CRT oscilloscope from the 1960's, but it was not properly calibrated, so we couldn't see the correct waveforms on it. We located the calibration point and then adjusted the square wave so that it could be shown accurately on the display. After that, we were able to see the signals a lot better.

When making the amplifier on a breadboard, there was a lot of noise and distortion coming from the amp into the speakers. However, we realized that we needed filtering capacitors on the voltage supplies and on the output of the op amp. Once we did that, the audio quality got much better, but when we transferred it to a solderable breadboard, the audio quality went down once more. At the time of writing this, the audio quality is still not very good.

Accomplishments that we're proud of

We can finally hear audio! ... except that now it's distorted and deep fried.

What we learned

  1. Filtering capacitors are very important in op-amps.
  2. We learned how to calibrate an oscilloscope from the 1960's
  3. We learned and refreshed our knowledge on how DSB-SC works.

What's next for Transvox

The audio quality is still not very good with the amplifier. There is a lot of noise and distortion that needs to be filtered out, so the next step would be to determine exactly what is causing the noise and how to fix it.

Built With

  • capacitors
  • op-amp
  • resistors
  • solder
  • speaker
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