Introduction: How to - Make a $25 Oscilloscope

I bought this $25 oscilloscope DSO 138 Kit online to use in electronics projects. Many electronics enthusiast
recommend getting a cheap oscilloscope for a first time user.

Step 1: $25 Oscilloscope DIY Kit - DSO 138

Picture of $25 Oscilloscope DIY Kit - DSO 138

This particular scope is both a kit that you can solder yourself and in the end a usable tool. Unfortunately, it only has 1 channel and the LCD screen is rather small. However for $25, shipping included, it was worth trying.

If you’re looking to buy your first DSO and don’t yet have that Dave Jones YouTube money, the DSO 138 might just be something to consider.

Step 2: Adding Solder Paste / Placing SMD Components

Picture of Adding Solder Paste / Placing SMD Components

I decided to buy the without the SMD components soldered on the board so that I can do some surface mount soldering. Once I got the package unpacked, I started to apply solder paste to the pads. Afterwards, I put some extra flux in areas where I thought it was needed. With the solder paste in place, it was time to place all of the surface mount resistors and the chips using tweezers.

Step 3: Reflowing the SMD Parts

Picture of Reflowing the SMD Parts

I started by slowly preheating the board with my hot air gun. Some of the resistors decided to fly about. In retrospect, I probably should have reflowed the board in an oven. With the board hot enough, I reflowed the components and adjusted accordingly.

Step 4: Adjusting Pieces and Shorted Pins

Picture of Adjusting Pieces and Shorted Pins

Any component which tomb stoned or any pins that were shorted I corrected using my soldering iron. With the SMD part complete, I set about to prepare the through hole components. I soldered these in place and subsequently clipped the leads off.

Step 5: Soldering LCD Headers and Verifying Voltages

Picture of Soldering LCD Headers and Verifying Voltages

After most components were soldered, I then completed the headers for the LCD screen and various larger connectors. It was now time to solder the headers to the LCD screen.

With the soldering complete, it was time to plug in the DSO 138 and verify that the build was successful by verifying the voltage at test pads on the board.

Step 6: Cleaning the Solder Flux Residue

Picture of Cleaning the Solder Flux Residue

Once everything looked like it was working, it was time to give the board a good clean using isopropyl alcohol to remove any excess solder flux.

Step 7: Veryfing the Functionality of the Oscilloscope

Picture of Veryfing the Functionality of the Oscilloscope

Finally, it was time to test the functionality. I quickly put together a PWM circuit using a 555 Timer and a potentiometer. The oscilloscope seems to work ok, it displays the PWM signal fine. I also tested the scope using the built in sine wave signal.

Comments

haltenfelder (author)2016-03-18

Would you recommend the kit? Are there any limits/disadvantages?

made2hack (author)haltenfelder2016-03-19

I would recommend it if:

A - You have no oscilloscope and want a really cheap one to use

B - No one in your area is selling a used analog oscilloscope up to $50 that you can pick up.

The disadvantages are:

- Small screen difficult to see from a distance. Software is kind of crappy since you use buttons as opposed to knobs to adjust time base and divisions.

- Limited signal you can use (I think max 50V)

- Only 1 channel and the fact it needs 9V to run. I don't know why they have a mini USB connector. I don't know why it can't use 5V.

Advantages:

- $25 dollars, shipped.

- It's a kit so you get to solder the components yourself.

- It's an oscilloscope with one channel that really works for a beginner

- It runs on 9V battery and fits in your pocket. You can take it anywhere

LongJ (author)made2hack2016-11-05

The reason they need 9V is because they have an switching voltage inverter so they can feed the input op amps a bipolar DC voltage so they can be DC coupled. You can't do that with a single supply. Since the are using classic 78/79Lxx linear regulators they need at least 7 volts and since they use a series diode as reverse polarity protection you need at least 7.7 v and in practical use 8 v ..... In other words only 6 NiMH rechargeables (7.2 v) or 2 Li-on batteries (7.4 v) in series isn't going to cut it. It's either one more battery or disposable alkalines

Dig out the schematic and then use the scope to explore the waveforms around that switching voltage inverter circuit. Since everything on the board is already grounded to the same ground as the input you only need to use red lead or just the probe tip if you are using real o-scope probe which I would recommend. It would likely get rid of some of the noise on the display especially above a few 10's of khz plus you can use the probe's x10 switch to increase the scope's range to 400V. Inversely to increase the sensitivity you can put a quality x10 or x100 (or both switchable) AC coupled battery powered preamp with a quality op-amp in front of the scope and have a rig to look at noise on DC power lines and see just how "DC" they really are ...... If the scope measures 1 V spikes then you are seeing a 100 mV or a 10 mV of noise on your DC lines. The reason you use a battery powered preamp is so you aren't introducing extra noise from the opamp's power supply. You can't get cleaner DC than directly from a battery.

made2hack (author)LongJ2016-11-06

That makes sense. Thanks.

magkopian (author)haltenfelder2016-03-22

Also, one thing that the author forgot to mention, is that it has a sample rate of only 1Msps which is going to make it unusable for frequencies above 100 Khz, as you need at least 10 samples per cycle to accurately represent the basic shape of a waveform.

Furthermore, I don't have a good degree of confidence that this cheap DSO is going to keep persistent during its entire operation and always give you the same measurements. After all, it's just a microcontroller using its internal ADC to sample the signal, running from an 8 Mhz XTAL. It's guaranteed that it's going to drift with time. Not to mention that according to the datasheet of the STM32F103Cx in order to achieve a sample rate of 1Msps the ADC clock needs to run at 14 Mhz. So, in order to achieve that with just an 8Mhz XTAL, it must be using an internal PLL which might lead to jittering issues preventing you from triggering a signal properly, especially on the faster timebases.

Especially when you are a beginner your main goal is to learn, so you need to have tools that you can trust. Otherwise, every time you have an issue with your circuit you will think that there must be a problem with your design, when in reality it might be an issue with your oscilloscope. The last thing you want when you are a beginner is to fight with your own tools.

The user interface also doesn't seem as easy to use compared to a real oscilloscope. Sure, it's simple and easy to understand but it's full of momentary switches instead of knobs, and after using it for a while it's going to get quite annoying. But the most annoying thing off all is going to be the momentary switch used for adjusting the trigger lever, as the trigger level is something you will need to adjust all the time. Why they didn't just used a rotary encoder to adjust the trigger level? I have absolutely no idea. There is also no AC trigger coupling which only makes things worse when you don't have an easy and quick way of adjusting the trigger level. Just by adding a small DC offset to a signal your triggering is going to get off and you will have to set the trigger level again.

To summarize, if you absolutely cannot afford to get a real digital storage oscilloscope like Rigol DS1052E for example, which you can even find brand new for less than 300€ nowadays, try your best to get a used analog scope. A used analog scope is still a real scope and the cheapest one you can find will give you at least 10 Mhz of real bandwidth. Especially if you live in US you should be able to find one for less than $50 on eBay. I'd say if you are absolutely desperate and you are unable to find a cheap used analog scope and you absolutely cannot effort investing 300€ on a brand new oscilloscope then it might worth getting this DSO kit. After all, any scope is better than no scope. But, please before you decide to buy it, make the effort to try finding a used analog scope, trust me you want regret it.

LongJ (author)magkopian2016-11-05

It's a $21 dollar o-scope that can measure and display volts, frequency, time period, etc.... Get real for cripes sakes ..... 20 years ago this would have cost several hundred bucks.....

Sure there are some that are faster for under $100 but they are only 8 bit and thus have horrible vertical resolution at any frequency .... Below 200 khz this thing will eat their lunch since you are talking about a vertical resolution of 256 levels vs the 4096 levels this device gives you .... I have a 50 Mhz storage scope and a classic 20 Mhz Hitachi V-212 (dollar for dollar one of the best analog o-scopes ever made) but I'm getting one of these, printing a nice case with battery compartment and use it as a portable ..... It also looks pretty hackable to me and if you bork it you aren't out much and still have a STM32F103C

For instance like most digital based o-scopes you should be able to make it work as a 200 khz spectrum analyzer ..... You should also be able to send the data to a computer via the USB port or one of the USARTs hooked to a RS-232 to USB converter ...... the main limitation is the amount of program memory available

JoeG17 (author)2016-07-17

How do you use the built in sine wave signal?

made2hack made it! (author)JoeG172016-07-18

You connect the red clip lead to the 1k test ring that is close to
the power input jack. You leave the black lead unconnected or perhaps
connect it to GND.

See attached photo

rring883 (author)2016-05-24

Very nice video. You can also get the kits from gearbest.com. They have kits with and without the SMD components already attached. I thought it was a fun project.

rgrenier1 (author)2016-03-18

I bought the sameand, I did a 3d printed case. It look good but I haven't suceed too use it inbig project. I took a partial image from my cellphone galery. If you want I could send you my stl files.

made2hack (author)rgrenier12016-03-19

Sounds good. Are STL files what you use to 3D print? Or can i import them in SketchUp?

BeachsideHank (author)2016-03-18

This looks interesting, and yes you are right, $25 is cheap to get a basic 'scope up and running, how long did it take for your build?

made2hack (author)BeachsideHank2016-03-19

Well, you can build it in 2 hours I think if you are organized. Maybe less. It took me a bit longer since I was recording a video at the same time using 3 cameras.

MightyNozzle (author)2016-03-19

It is also an acrylic case from 6-7 USD available. Found some on banggood and aliexpress.

MightyNozzle (author)2016-03-19

It is also an acrylic case from 6-7 USD available. Found spme on banggood and aliexpress.

Dav22 (author)2016-03-18

you can get the kit here http://insanedeals4u.com/?s=oscilloscope this where i bought mine.

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