Mini Battery Powered CRT Oscilloscope

Hello! In this Instructable I will be showing you how to make a mini battery powered CRT oscilloscope. An oscilloscope is an important tool for working with electronics; you can see all of the signals flowing around in a circuit, and troubleshoot electronic creations. However they aren't cheap; a good one on Ebay may cost you a couple hundred bucks. This is why I wanted to build my own. My design uses a mini CRT that you can find in an old camcorder viewfinder, and a few other fairly common electrical parts. Let's get started!

For this project you will need the following:

For the triangle wave generator:

-2x 10KΩ Potentiometers

-2x 10KΩ Resistors

-2x S8050 Transistors (npn)

-1x S8550 Transistor (pnp)

-2x LM358 Op Amp

-1x 2KΩ Resistor

-1x Diode (I used the 1N4007, but the type isn't super important)

-1x Capacitor (The capacitance affects the frequency of the triangle wave so it's not super critical, but just make sure it's not bigger than 10µF)

There are multiple capacitors and a DIP switch in the picture, but you will only need those if you want to switch the capacitance.

For the LM317 regulator:

-1x LM317 Adjustable Voltage Regulator

-1x 220Ω Resistor

-1x 680Ω Resistor

-1x 0.22µF Capacitor

-1x 100µF Capacitor

For the 7805 regulator:

-1x 7805 5v Regulator

-1x 47µF (Or higher) Capacitor

-1x 0.22µF Capacitor

Additional Materials:

-1x SPST Switch

-1x Push Button Switch (Optional)

-1x 10Ω Resistor

-1x DPST Switch

-1x Mini CRT (These can be found in old camcorder viewfinders, which you can get on Ebay for about $15-20)

-1x 12v Battery Pack With Center Tap

-3D Printer

-Hot Glue Gun

There are two voltage regulators because when I built the first one, it got zapped, so I had to build a second one. You only have to build one voltage regulator! The battery pack has to be able to hold eight batteries and you need to put a wire in the middle. This creates a split power supply: +6v and -6v and the center tap is GND (You need this because the waveform needs to be able to go positive and negative relative to GND.

This project uses a CRT because they are analog screens, and they are relatively easy to convert to an oscilloscope. The CRTs inside old viewfinders vary from company to company, but they will all have the same basic layout. There will be deflection coil wires running to the front of the CRT, a connector/wires leading to the circuit board, and a high voltage transformer. Caution! When the CRT is powered on, the transformer generates 1,000-1,500 volts, this may not be lethal (it depends on the current), but it can still zap you! The CRT is built so that the dangerous parts aren't too exposed, but still use common sense. Build this at your own risk! Before we start building the circuit, we need to find the positive, negative, and video wires for the CRT. To find the ground wire, take a multimeter and set it to the continuity mode. Then, find any metal casing on the circuit board (possibly the transformer housing), touch a probe to that, and test each of the signal wires to check for a connection. The wire that is connected to the metal casing is the ground wire. Now the power and video wires are a bit more difficult. The power wire may be colored, or there could be a large circuit trace leading to it. My power wire is the brown wire shown in the picture. The video wire may be colored or it might not be. You could find these by trial and error (not a very good way to do it, but I used that method and it worked), or by looking up schematics of the CRT. If you provide power to the CRT and you hear a high pitched sound but the screen doesn't light up, you have found the power wire. When you are building the circuit, the power wire and signal wire are both connected to +5v. Once you can get the CRT screen lit up, you are ready to go!

Note: Other CRTs might need 12v, if your CRT isn't turning on at all when you are giving it 5v, try giving it a little above 5v, but don't exceed 12v! Be absolutely sure that the CRT won't run at 5v if this is the case, because if your CRT really does run at 5v but you try to give it more than 5v, you could fry your CRT! If you found out that your CRT works at 12v, you won't need the voltage regulator and you can connect it directly to the batteries.

Important: On my CRT when it is powered on and you remove the plug for the coils, you would expect there to be a little bright dot on the screen because the electron beam isn't being deflected, but the CRT turns off the electron beam. I think it does this as a safety feature so you don't burn the phosphor on the screen by having the beam just stay there, but we don't want this because we're going to be using both coils disconnected from the board. One way you can fix this problem is to put a small resistor (10Ω) where the horizontal coils would connect to the board. This "tricks" the CRT into thinking there's a load there, so it turns the brightness up and shows the beam. In the next step I will provide a design on how to build this. If whenever you are building this, you see an extremely bright dot on the CRT screen, turn off all power to the CRT, if the electron beam stays on the screen too long, the phosphor could burn and ruin the screen.

Once you have gathered all of your parts, I would suggest testing out the circuit first on a breadboard and then building it. Remember to build the coil "trick" circuit mentioned in step 2 so you can see the beam. Look at all of the pictures of the circuit design closely before you build. I soldered my circuit on different boards (one board contained the voltage regulator, another had the triangle wave generator, etc.) I also added a fan and a heatsink to my voltage regulator because it gets hot. If you want to change the value of your capacitor, you can either solder a switch on the pcb and find a way to switch between capacitors, or you can add wires on the pcb where you would connect the capacitor, and connect the capacitor and wires to a breadboard. There are three inputs that will be adjusted when you use the oscilloscope (the two potentiometers and the switch). One potentiometer adjusts the oscillation frequency, another adjusts the amplitude of the triangle wave, and the switch turns on and off the CRT screen.

The "Magic" Resistor: In one of the pictures you will see a resistor labeled "Magic Resistor". When I tested my triangle wave generator it was very unstable, so for some strange reason I decided to put a 10KΩ resistor over another 10KΩ resistor (see picture) and the oscillator worked wonderfully! If your triangle wave generator isn't working, try the using the "Magic Resistor" and see if that helps. Also, during my design, I had to try a couple of different triangle wave oscillator designs. If yours doesn't work and you have some electronic knowledge, you could try some different designs and see if they work.

Once you have everything connected, it's time to test it out! Connect everything to the batteries and turn it on (make sure you have everything connected so it matches the pictures in step 3). Warning! On my first test, I didn't add a power switch, so when I went to test the triangle wave generator I connected the batteries backwards and fried my oscillator. Don't let this happen to you! When powered, the CRT screen should look like it does in the picture (if you connected your triangle wave generator's outputs to the horizontal coils), if it doesn't, there are a few questions you can ask yourself:

1. Check to make sure you have connected everything properly. Are the batteries reversed? Is everything receiving power?

2. Is the triangle wave generator working? Can you hear a constant tone if you connect a speaker to the output wires?

3. Is the CRT coil "trick" circuit working? Try and wiggle the wires a little bit. Does the screen turn on?

4. Is the voltage regulator working?

5. Could you have broken something?

Once the CRT shows a horizontal line on the screen, you can move on to the next step!

For my oscilloscope, I wanted to 3D print a case instead of having to build it out of wood, so I designed my case in Tinkercad and 3D printed it. Depending on what potentiometers and switches you use, your case will look different than mine. I didn't include any room for the batteries in my case (I don't care about portability) but you might want to. Since the 3D printer's bed wasn't level, the case printed a little bit wonky, but it works! Depending on how well calibrated your printer is, you might have to file out the holes so they fit. After it's done printing, fit everything in the case, test it, and hot glue it in.

For this last part, you will need the remaining S8050 npn transistor. Simply connect it so it looks like the picture, and test out your oscilloscope. It is important that you connect the oscilloscope GND and the input signal GND together so the circuits are connected. The square wave output from the triangle wave generator (wire connected to diode in the drawings) goes to the base of the transistor. This allows the signal to flow to the coil when the beam is going to one side of the screen, and doesn't allow the signal to flow when the beam goes to the other side. If you don't use the transistor, you will still see the signal on the screen but it will be "messy" because the waveform will be going in both directions (see the second picture).

After your oscilloscope is complete, I would suggest testing a waveform to make sure it works. If it does, congratulations! If it doesn't, go back to step 4 and look over the different questions, and look over the diagrams again. Now this oscilloscope is nowhere near as precise as the professional ones, but it works well for looking at electronic signals and analyzing waveforms. I hope you had fun building this cool mini oscilloscope, and if you have any questions I'd be glad to answer them.