Introduction: Hack an Old TV Into an Audio Visualizer
What does music look like? It's not something that most people think about. Luckily for us, somebody decided to hook a music player up to an oscilloscope and find out. For the non-electronics geeks out there, an oscilloscope is a piece of electronic equipment that is used to view and analyze electrical signals. It displays the waveform of an inputted signal and provides other functions like amplitude control and "sweep". Because oscilloscopes are so versatile, they also do a great job of displaying audio signals. That means you can effectively see what a song looks like.
Unfortunately, most of us don't have an expensive oscilloscope lying around. This is where the TV comes in. Because analog televisions and oscilloscope displays operate on the same basic principles, it is possible to repurpose the former into the latter with a bit of wire and some know-how. The result is a very simple oscilloscope that is able to display the audio signals in the range of about 100 to 3000 Hertz. This is why I have dubbed my creation an "audio visualizer" instead of an oscilloscope.
As always, I encourage you to try this project out yourself, even if you don't have much experience with electronics. The build ended up being super simple and it should only take about an hour or so if you have everything at your disposal.
Step 1: The Bare Necessities
- Analog TV (Find one at a fleamarket/yard sale or buy one on eBay)
- 3.5mm Male to Male Stereo Cable (Make one yourself from two pairs of old headphones, or buy one at Walmart, Amazon, etc.)
- Music Player with Headphone Jack
- Wire (nothing specific, a few scrap pieces will do)
- Solder Flux
- Electrical Tape
- Screwdrivers (for disassembly)
- Soldering Iron
- Dremel or Hacksaw
- Multimeter (not strictly necessary, but quite helpful) (Get one for free at Harbor Freight with a coupon)
Note: For this project, I opted to use a portable, black-and-white television for its low form factor and simplicity. I believe the same results are also achievable with larger, color TVs, however there may be minor differences. For example, I believe that you may have to replace the horizontal deflection coil with a "dummy coil" in larger TVs for the circuitry to function properly. Since I have not tried this myself, though, I cannot confirm its accuracy.
Step 2: Disassembly
Before you begin, it is important that you realize the risks involved with hacking an analog television. The main danger here is the flyback transformer. Located in the image above, this device can store a nasty charge that can seriously hurt you if you aren't careful. Be especially wary if you are using a larger TV, as the transformer will probably store even more energy. Because of this, it is a good idea to wear insulating gloves when handling the circuit board and other connections inside the TV.
Keeping this in mind, open the outer casing of the TV and locate the deflection coils. They will be the coils of wire surrounding the back of the cathode-ray tube (CRT) display. The purpose of these coils is to direct the stream of electrons (fired from the electron gun at the back) to the appropriate positions on the screen. As you may have guessed, there are two deflection coils, one vertical and one horizontal. If you would like to know more about how the system works, I encourage you to go here.
Next to the deflection coils, there will be a cluster of four connections. Two of these connections correspond to the the vertical deflection coil, and the other two correspond to the horizontal coil. To find out which are which, trace the wires back to the board. In my case, there were abbreviations on the board indicating the role of each wire. From this, I surmised that the blue and red wires went to the horizontal coil, and the yellow and gray wires went to the vertical coil. If this information is not on your board, don't worry. You will be able to differentiate the wires in the next step through experimentation.
Step 3: Testing the Display
Although this step is not strictly necessary, it should provide some insight into how the audio visualizer will work. First, take a picture of the coil connections mentioned earlier, so you don't forget where the wires go. Next, desolder the four coil connections. Wrap electrical tape around the loose wires, keep your hands away from the inside, and turn on the TV. You should see a single dot in the center of the screen. This is the un-deflected electron beam hitting the inside of the display. Next, unplug the TV and reconnect the horizontal deflection coil (you can use jumper leads instead of soldering). If you don't know which wires are which, just hook two of them up to their original positions and keep experimenting. Turn on the TV, and you will see a horizontal line across the screen. Unplug it and do the same thing with the vertical deflection coil. As you might expect, this will produce a vertical line along the screen.
If you have an amplifier, leave the vertical coil attached and hook the output of your amp up to the horizontal coil. When music is played, the waveform appears on the screen just as desired. This is where most tutorials on the topic stop. Unfortunately, many people (including me) don't have an amplifier that they can use for this project. I originally made one by hacking an old answering machine, but the sound quality was poor and there was no way I could fit everything into the small TV case. I mulled over the problem for a while and then it hit me: The TV obviously produces sound, so it must already have an audio amplifier in it! All I needed to do was repurpose it. This is precisely what I will show you how to do in the next steps.
Before moving on, I should also mention that it is possible to display waves horizontally on the screen. Unfortunately, it is not as simple as sending a signal to the other coil. Because the horizontal deflection coil operates at a much higher frequency than the vertical one, doing this just produces a moving band of static. The workaround is to move the vertical coil wires to the horizontal coil and send the audio signal to the vertical coil instead. Because the display is not square, though, the resulting horizontal line does not fill the whole screen (see last picture above). You might be able to mitigate this problem by adjusting the vertical-hold, but I decided I liked the vertical waves better in the end.
Step 4: Modifications
First, locate the audio output jack on your TV. From my experience, most analog TV's have one, but if yours doesn't, you will probably have to buy and install one yourself. Look here if you do. As the name implies, this jack is meant to output audio, usually to a pair of earphones. For this project, we want it to input a signal to the amplifier. To do this, you first need to disconnect the jack from the circuit. Use a hacksaw or dremel tool to cut any traces going to the jack. Make sure that they are completely disconnected. You really don't want the TV trying to send signals into your iPod.
The audio jack has three connection points. The one at the front of the jack (nearest the outside of the TV) is for ground. The one at the back of the jack is for audio signal. The remaining connection is used to turn off the external sound when the audio jack is in use. Cutting the traces "fools" the TV into thinking that the jack is in use. To resolve this issue, solder the traces that went to the second and third connector together. The third picture above should make this more clear.
Now, locate the volume control on your TV. This will probably be a dial that turns a potentiometer on the circuit board. If so, solder a wire from the signal leg of the audio jack to the center leg of the potentiometer. Solder another wire to the ground leg of the audio jack. Plug your music player into the jack and start playing a song. Touch the ground wire to each of the remaining potentiometer legs. When you hear music playing through the TV speaker(s) you know you have found the right one. Unplug everything and solder the end of the wire in place. You are now done modifying the circuit board.
The only thing left to do is connect the TV speaker to the horizontal deflection coil. Solder two wires in place and close everything up. The conversion is complete!
Step 5: Having Fun
Now its time to test it! Plug in your music player and try out your favorite songs. You can also use this online tone generator to play around with frequency, amplitude and various waveforms. I used it to create standing waves at approximately 320 Hz. Thanks for reading and let me know if you come up with any other cool ideas for this project!
P.S. If you enjoyed this instructable, I would appreciate your vote in the Tech Contest and the Formlabs Contest. Winning a 3-D printer would totally make my year, and it would allow me to create even cooler projects to share with all of you in the future!
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