TV Oscilloscope
A quick update on a random and fun evening. A hall mate and a few friends from around Senior Haus showed up to tinker around with a TV we were disassembling. One cool thing found online to do with a spare CRT is to drive the deflection coils with audio frequencies and make a cool visualizer out of the thing. We did just that and had a grand old time messing around with Audacity and some music files.
The basic premise is pretty straight forward. Take a TV cover off and check out the big tube inside. Be careful not to break the tube (it can implode) or to short the capacitors on the flyback circuit (the one with the transformer and a line running to the back of the tube, where the electron gun lives) or touch the anode running into the tube near the screen, with the big high voltage line and the rubber coating and what not. Now that you’re looking at the tube, find the terminals for the deflecting coils. They’ll look like this:
Close up on the deflection coil terminals (Large posts with green wires attached).
These coils control the horizontal vertical deflection by generating a magnetic field and literally pushing the focused beam around. Now that you have these terminals identified and some leads so you can do something fun with them, grab yourself an amplifier and run the leads into the output posts. Join some speakers into the same posts, and find an input source for your computer. Here we’re using a spare low quality amp and small computer speakers with a USB DAC hooked up via RCA, but an 1/8″ to RCA wire would work just fine as well.
With this, you’re ready to drive the channels with your laptop and some nice free software like Audacity to make some really spectacular things happen. By driving the left and right channels, you can independently drive the horizontal and vertical deflections of the beam. So you can do something simple like have the left and right channels be driven by a track, and you’ll get circles for in phase balanced music, and fun shapes otherwise. Even more interesting is to drive the horizontal axis with a sawtooth wave and the vertical with a signal, say a square wave, and generate an oscilloscope image of what you’re dealing with. Neat! You also still get music output from the speakers so the whole thing makes for a cool system visualizer that is live and a bit different from what you’ll normally see. When you don’t drive it, the beam sits centered on the screen at fairly high intensity, meaning the phosphor will be burned out fairly quickly. In fact, the TV screen isn’t designed for this heavy of a load, so you’ll burn the phosphor fairly fast this way. But CRTs are old and cheap, so no big loss.
Below you’ll see a picture of my hall mates watching the image kick on, the initial stand still beam, and a tracking sine wave. Try it out!
Electron Beam sitting still on screen center.
Electron beam etching a sine wave across the TV Tube.
Take care and enjoy kids!