Introduction: Hacking an Old TV Into a Signal Visualizer - Oscilloscope
What does music look like? It is not something that most people think about. Let's connect a music player to an oscilloscope and find out. For non-electronics geeks not out there, an oscilloscope is a piece of electronic equipment that is used to view and analyze electrical signals. It shows the waveform of an input signal and offers other functions such as amplitude control and "sweep". Because oscilloscopes are so versatile, they also do a great job of displaying audio signals. This means that you can effectively see what a song looks like.
Unfortunately, most of us do not have an expensive oscilloscope there. This is where the TV comes in. Because analog televisions and oscilloscope screens work on the same basic principles, it is possible to reuse the previous one within them with some wire and some technical knowledge. The result is a very simple signal visualizer that is capable of displaying audio signals in the range of approximately 100 to 3000 Hertz. As always, I encourage you to try this project out of yourself, even if you don't have much experience with electronics. The construction finished being super simple and should only take about an hour or less if you have everything at your disposal.
Step 1: Materials Needed
- An old analog TV (find one on eBay)
- 3.5mm stereo cable male to male
- Music player with headphones
- Welding or soldering kit
- Screwdrivers (for disassembly)
- Multimeter (not strictly necessary, but quite useful)
Note: For this project, I chose to use a portable television, in black and white because of its low form factor and simplicity. I think the same results are also achievable with larger, color TV, however there may be minor differences. For example, I think you may have to replace the horizontal deflection coil with a "dummy" coil in larger televisions for the circuit to function properly. Since I have not tried this, though, I cannot confirm its accuracy.
Step 2: Disassembly
Before you start, it is important that you realize the risks involved with analog television hacking. The main danger here is the flyback transformer. Located in the image above, this device can store an unpleasant load that can seriously injure you if you are not careful. Be suspicious especially if you use a larger TV, as the transformer will probably store energy even more. Because of this, it is a good idea to wear insulating gloves when handling the circuit board and connections inside the TV.
With this in mind, open the TV housing and locate the deflection coils. It will be the wire coils that surround the back of the cathode ray tube (CRT) screen. The purpose of these coils is to direct the current of electrons (fired from the electron gun in the back) in the appropriate positions on the screen. As you may have guessed, there are two deflection coils, one vertical and one horizontal.
Next to the deflection coils, there will be a group of four connections. Two of these connections correspond to the vertical deflection coil and the other two correspond to the horizontal coil. You will be able to differentiate the cables in the next step through experimentation.
Step 3: Screen Test
Although this step is not strictly necessary, it should provide some insight into how the audio viewer will work. First, take a picture of the coil connections mentioned above, so you don't forget where the wires go. Next, desolder the connections of the 4 coils. Wrap electrical tape around loose wires, keep your hands away from the inside and turn on the TV.
You should see a point in the center of the screen. This is the electron beam deflected without hitting the inside of the screen. Then unplug the TV and reconnect the horizontal deflection coil (you can use jumper cables instead of welding). If you don't know what wires they are, only two of them hook up to their original positions and continue experimenting. Turn on the TV and you will see a horizontal line across the screen. Unplug it and do the same with the vertical deflection coil. As expected, this will produce a vertical line along the screen.
If you have an amplifier, leave the vertical coil connected and the output of the amplifier to the horizontal coil of the hook. When music is played, the waveform appears on the screen as desired. This is where one more stop tutorials on the subject. Unfortunately, many people (including me) do not have an amplifier that you can use for this project. I had originally done to hack an old answering machine, but the sound quality was poor and there was no way I could adjust everything in the case of small TV. Warm up about the problem for a while and then beat me: the TV obviously produces sound, so you should already have an audio amplifier on it! All I had to do was reuse. This is precisely what I will show you how to do it in the next steps.
Before moving on, I should also mention that it is possible to visualize the 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, this only produces a moving static band. The solution is to move the vertical coil cables to the horizontal coil and send the audio signal to the vertical coil instead. Because the screen is not square, however, the resulting horizontal line does not fill the entire screen (see last photo). You can mitigate this problem by adjusting the vertical grip, but I decided that I liked vertical waves better at the end.
Step 4: Modifications
First, locate the audio output connector on your TV. From my experience, more analog TVs have one, but if yours doesn't, you should probably buy and install oneself. Look here if you do. As the name implies, this connector is intended for audio output, usually to a pair of headphones. For this project, we want a signal for the input amplifier. To do this, you first need to disconnect the circuit connector. Use a metal saw or dremel tool to cut the remains going to the socket. Make sure they are completely disconnected. You really don't want the TV trying to send signals on your iPod.
The audio jack has three connection points. On the front of the cat (closest to the outside of the TV) is for the land. On 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. Cut the traces "fools" the TV in the thought that the cat is in use. To solve this problem, weld the traces that went to the second and third connector together. The third photo above should make this clearer.
Now, place the volume control on your TV. It is probably a sphere that turns a potentiometer in the circuit. If so, solder a cable from the signal of the audio connection to the central leg of the potentiometer. Solder the other cable to the ground leg of the audio jack. Plug your music player into the socket and start playing a song. Touch the ground wire to each of the remaining legs of the potentiometer. When you listen to music playing through the TV speakers, you know you have found one. Unplug everything and solder the end of the cable in place. Now that you finished modifying the circuit board.
The only thing left to do is connect the TV speaker to the horizontal deflection coil. Welding two wires in place and close everything. The conversion is complete!
Step 5: Have Fun!
Now is the time to try it! Connect your music player and try your favorite songs. You can also use this online tone generator to play with different frequency, amplitude and 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 good ideas for this project!