When you start out on your amazing discovery of electronics the second most important tool after your multimeter must be an oscilloscope. An oscilloscope literally gives you insight into exactly what is happening in an electronic circuit. Not only is it an indispensable troubleshooting tool, but you can learn so much about circuit design and function from just probing around and looking at the various waveforms.
You can pay a lot of money for an oscilloscope, but when starting out on a budget why not build your own from a kit? Not only will you then have this wonderful tool but you will have gained more electronics experience and knowledge.
Step 1: Video of Kit Build and Test
In the video I show the complete build and test procedure. You can skip to parts of the video by clicking on the time in the description; this may only work when viewing on a PC.
In the next steps I will give additional information and tips to help with the build.
Step 2: DIY Oscilloscope Kit DSO 068
The kit comes complete with everything you need except for a Lithium battery. I found one in my “packrat” supplies where I keep all manner of parts I salvage from old or broken electronics.
You do not need the battery as the unit can be powered by the supplied USB cable. I think using a battery is a good idea as there will be enough cables on your bench already! The battery can be charged by the USB cable.
I will put links to all the parts and tools I use in a later step.
Identifying the resistors is a real challenge and I strongly advise the use of a component tester. Most of the resistors are 1% with a 5 band colour code, these are especially difficult to read but a breeze to identify with the component tester. It is a good idea to mark off the components on the instruction sheet as you fit them. Fitting the small ceramic capacitors is best done by bending the legs to the correct spacing with pliers. If you just force these capacitors down onto the PCB, being made of ceramic they could easily break or crack. The small crystal has a glass seal at the bottom so only use tweezers to handle this.
It is a good idea when placing each component to look around the area for any errors in the components that came already fitted to the PCB. For example, the switches are mounted but not soldered. Also check the orientation (Dot on Pin 1) of IC’s. Look for any missing or bad solder joints. I discovered on my board that one of the trim capacitors was misplaced.
Before fitting the LCD I soldered in the programming header that lies under it. This would be impossible to solder after the LCD is in place. I decided to space the LCD panel off the main PCB a little by putting a piece of cut down credit card underneath.
Finally Solder on the "Bob" boards. Generously tin both the Bob and main PCB pads and simply tack down with your soldering iron well loaded!
Step 3: Calibration
Calibration is done by adjusting the compensation "trim" capacitors using the built-in signal generator.
1. Connect test signal output to scope input and set SW1 to DC.
2. Power up. Set test signal to 10kHz and 5V. Set scope timebase to 20us.
3. Set SW2 & SW3 to 1V & X5 respectively. Adjust trigger level to make display stable if necessary. Adjust C8 to obtain square waveform with minimal “ringing”.
4. Keep signal frequency unchanged and set amplitude to 1V. Set SW2 & SW3 to 0.1V and X5 respectively. Adjust trigger level to make display stable if necessary. Adjust C5 to obtain square waveform with minimal “ringing”.
5. Remove connection between test signal output and scope input. Adjustment is finished and you’re good to go!
When I tried the above procedure at first I could not get a clean square wave. On investigation the problem was with the value of C7, you can see in the circuit diagram. In the diagram it is shown as 100pF but the kit came with 120pF. I found by experiment I had to make it 147pF. I did this by putting a 100pF and 47pF in parallel. This gave a much better result.
Step 4: Final Assembly
In the photo above you can see the Oscilloscope finally assembled. In the bottom cover I have stuck the mobile phone cell down with double sided tape. The BNC connectors were too close to the PCB for my comfort so I cut some spacers from black plastic irrigation pipe. The kit comes with only one small piece of screened cable to connect the BNC’s to the PCB, I quickly found another piece in my “packrat” supplies. A feature I found not well documented is that if you “long” press the level button the backlight can be switched off and on. Also, at first my unit would not switch off. This was because I forgot to remove the jumper J2 after the first set of voltage checks.
Overall I am extremely pleased with the unit. It works better than I had anticipated and has other features like the frequency counter that I was not expecting.
It will prove a valuable asset to any aspiring electronics enthusiast!