Someone left an old CRT TV on the laundry room swap table, so I decided to try and hack it. Following this instructable, I converted it into an extremely basic oscilloscope in about 30 minutes. Basically, you just disconnect the horizontal scan (which is too fast for most inputs), connect the vertical scan up to the horizontal coil (since oscilloscopes are normally have time across the x axis), and snake some wires from the vertical coil out of the case as an input. Setting the channel to static noise gave a nice bright line.

Oscilloscope with 0V DC

I next tried hooking up some DC. Four AA batteries gave a good signal, but I haven’t measured how much current they’re supplying. Tapping the leads together sporadically gives some cool traces as the current starts and stops:

Trace while connecting a 6V battery
Trace while connecting a 6V battery

Closeup of traces from 6VDC spikes. Note oscillations towards the right of the screen.
Closeup of traces from 6VDC spikes

The horizontal oscillations near the right of the screen were unexpected. It would seem to indicate that the vertical oscillator (now driving the horizontal motion) has some feedback near the end of the trace. The TV didn’t show any distortion before I took it apart, so I’m not sure where this jiggle is coming from.

Other thing that one should be able to deduce from the above photo is the direction of the x-axis. All the spikes have very sharp left sides, with more gradual declines on the right. Although I’m not entirely sure about this, my intuition is that briefly tapping the leads together would result in a rapid surge as the leads contact, followed by a slower drop-off as the circuit breaks due to the energy stored in the magnetic field of the coil. If that’s the case, the oscilloscope is correctly wired with time increasing to the right.

The last thing I tried was attaching a small 12V AC transformer I salvaged. This results in a single wavelength of sinusoid. The AC should oscillate at 60Hz, so a single trace must take 1/60 sec. I could also have found this out by looking up the NTSC spec of 59.94 Hz per frame. Had I left the horizontal oscillator attached, I would have had a 15750 Hz oscillation (30 fps*525 lines/frame), or 64 µs per trace. The phase of the wave drifts slightly due to the slight difference between NTSC frame rate and mains power.

With 12V 60Hz AC

As an oscilloscope, this setup leaves much to be desired. It needs circuitry for changing the scan frequency, triggering scans, amplifying signals, etc. However, it should be cool for visualizing music. I have a couple old iPod speakers lying around, so hopefully I can cannibalize one for an amplifier. That should give some cool videos for a future post!


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