Well maybe not the smallest but pretty damn close. I dug through my shed and realized that I had an old early 90s GE VHS camcorder that I got out of the trash years ago and was broke. It really wasn't worth fixing but curiosity got the best of me wondering what useful parts could be inside of it.
After ripping it apart I was amazed to find a tiny... and I do mean TINY.. CRT picture tube in the viewfinder. It's 1/2" round style with a square window in front and some optics built into the viewfinder part to flip the image horizontal through a mirror to the magnifying glass that your eyeball goes up to through the rubber cup.
Pulled the CRT and its circuit board out and noted that there was 6 wires. Two of which probably go to the microphone, another two for power, and one for composite video input. The last wire is a mystery. Probed the wires with my multimeter to see which ones went to the obvious ground of the circuit. Then applied low current (200ma) at 5 volts to each wire until something would happen. Found the wire for power that way and low and behold the CRT lit up grey, heard a wining from the tiny 1/2 by 1/2" flyback coil and it was working!
I tried putting some composite video through the other wires and that did work. I was able to watch a DVD through this tiny little tube. I really had no use to watch movies on something so small besides the hilarity of it.
Browsing around on the internet I found a few hacking sites where people have took old B&W TV sets and clipped the wires for the horizontal yoke coil and put audio through it instead of the 15.734kHz scanline oscillator.
Doing so turns the TV into a super cheap oscilloscope with a fixed sweep rate of 60hz (vertical). The problem is that the oscilloscope is sideways making the line go vertical instead of horizontal unless you swap the vertical with the horizontal wires which can disable some TVs completely depending on model. Still cool looking though for a party or dorm room and makes good use of an old useless B&W TV. www.instructables.com/id/Fully-Functional-Television-Oscilloscope/
Well I figured why not do this to the tiny viewfinder CRT? I did exactly that, and since the tube is a roundy all I need to do is turn it sideways and the scope line is horizontal like it should be!
Here is a picture of the tiny tube itself with the yoke coil removed with a AA battery next to it for comparison...
Here is a video of it rigged up temporarily to the audio output of my radio running off of a 9 volt battery for a power supply...
You may not be able to tell from the video but the clarity of this tiny tube is excellent and very sharp. Since the composite wire is not being used the tube normally displays a grey image (no static like a TV) so when using it as a scope the waveform it displays is full and not dotty like on TV sets when people convert/hack them for this use.
Obviously this is not useful for anything realistic besides looks but that's exactly what I intend on using it for. In the future I will update this thread with the finished project in a small box. It should be a cool conversation piece to have connected to my stereo system, or maybe even use it as a tiny modulation monitor! I plan on putting a translucent green or blue screen in front of it to make the color look more like a real scope.
I should mention a few important things here also..
For one even though this is a tiny CRT there are still voltages that can shock the shit out of you. The CRT type is a M01KQM07WB and does require a flyback transformer. Even though it's small (the transformer and tube) it still generates up to 2,000 volts. I learned real quick to discharge the anode of the tube while playing with the circuit because if I don't I can get bit pretty bad from the charged up voltage in the CRT. It's like a nasty bee sting that will throw my arm violently lol.
If anyone here gets the bright idea to do this with a full sized TV keep in mind that there is even more voltage involved with larger CRTs. Some can be downright lethal if you don't discharge their anode to ground after they have been powered up.
Because CRTs use magnetic coils to control the beam they have limited bandwidth unlike a real oscilloscope that uses electrostatic deflection. This makes CRTs have an upper limit of around 200-500kHz which is great for TV or computer monitor use, but very limiting for oscilloscope use. However it is still plenty of bandwidth for audio waveform applications as shown in my video and the link I provided.
I have one of those $10 Ebay "Spy Cameras" that crank out NTSC frame rates. They are also good in near IR (use black glass over it and you can see warmer stuff. like tubes, soldering irons, etc. and with 940nM LED's, the room lights up).
I have one of those $10 Ebay "Spy Cameras" that crank out NTSC frame rates. They are also good in near IR (use black glass over it and you can see warmer stuff. like tubes, soldering irons, etc. and with 940nM LED's, the room lights up).
Image converter
Cheers!
I seen online where a few others were thinking of making a similar project like that. Never seen anyone actually do it yet though. Night goggles would rock!
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- they want nearly 250mA to run (probably because they are full 30fps compared to 10 or 15)