Fast, Portable and Affordable Oscilloscope and Inductance Meter

When turned off, it looks like an ordinary toy car that would entertain a kid for hours, but actually it is an oscilloscope kit in disguise!

The idea and also part of the code for this project came from another scope called STM-32-O-Scope (aka pigScope), which uses a more powerful Arduino cousin called STM32. I originally posted my version on hackaday, but now I've deciced to bring it here too, so it may help more people who don't have a commercial oscilloscope and also becase there's a contest happening ;)

I use this tool daily to watch signals or as a second voltmeter and also to measure inductance of home-wound inductors. As the code is open, you may add, remove or improve features according to your needs.

As it's more than just an oscilloscope and "oscilloscope and inductance meter" is too cumbersome, I named this little guy multiScope. Here's what it currently does:

• Measure inductances greater than 100uH with 10% precision
• Display analog or digital signals with up to 1.7MS/s sampling rate
• Temperature and pressure sensing (an extra feature)
• Displays voltages, frequency and duty cycle

The core parts for the oscilloscope:

• STM32F103C8T6 minimal dev. board (aka "blue pill"), buy one like this STM32F103C8T6 min. dev. board

• 3.3V USB/Serial adapter (FT232RL or something like that)

• 2.4 TFT LCD touch screen display (the display library I used was adapted to the model of the picture, but with some changes in code, you might be able to use others). Example link: 2.4" TFT LCD Shield on eBay

• A box or enclosure (may be something 3D-printed, an old box you have or even a plastic toy)

• 6-9V battery (I used 7.2 li-ion as it lasts very long)

• Small switch (main power switch)

• (2x)2.2K Resistor

• LM7805 (5V regulator)

Parts for the inductance meter:

• LM339 (comparator IC)

• Resistors: 330R, 150R

• 1uF ceramic capacitor (the higher the precision, the better)
• 1N4007 (common diode)

Parts for attenuators :

• 2x3 female header socket

• Resistors: how many will depend on how much attenuation you need, but they are regular resistors and trimpots.

Before building our hardware, let's setup our development environment. Here, standard Arduino IDE was used with an add-on called Arduino STM32. The guys who made it also host a forum called Arduino for STM32 that is not so beginner oriented as the Arduino forum, but people can discuss things related to the STM32 board family and also share their projects. There's even a page with other oscilloscopes based on pigScope too, from which I've learned many things and maybe you will too.

OK, so do the following:

• Make sure you have a compatible IDE version. They recommend 1.6.9, but I used 1.8.1 without any problem

• Install Arduino SAMD Boards package with the Arduino Boards Manager (in case you don't know, it's a tool inside the Arduino IDE)

• Download or clone Arduino STM32 into the Arduino/hardware folder

• Open the IDE, select your board (Generic STM32F103C series for blue or red pill) and Serial as Upload Method to program using the USB-Serial adapter

• Connect the board by following the little sketch. You may test it with the ordinay Blink code, just note that the onboard led is on PC13. Every time you upload code, you will have to change the position of the first jumper to switch between Run and Program modes before powering or resetting the board

Once we're sure the blue pill is working, let's proceed to the display. I won't cover the placement of parts in detail because it will depend on the enclosure you've chosen. Be creative and put them where you think it's best.

The patched TFT library contains code that might not work well with the default Wire library due to pin conflict, so let's edit a few lines to avoid trouble:

• Go to Arduino\hardware\Arduino_STM32\STM32F1\libraries\Wire
• Open Wire.h with a text editor you like and look for #define SDA
• Make it like this:

#define SDA PC15
#define SCL PC14

• Save the file. Now SDA is on PC15 and SCL is on PC14.

Now, do the display wiring as described in my drawing. After it's finished, download multiScope_patched_librarires.zip and extract to your libraries folder. To test it, upload lcd_touch_paint.ino. The pins are already adjusted. All you need to do is set your inner artist free.

Now, we're going to build the inductance meter circuit. It's a very simple one, and works in a somewhat similar way to ultrasound sensors: STM32 sends a "trigger" pulse to pulse_in and receives another trough pulse_out, from which we extract frequency (and thus inductance) information. This site explains the schematic and the physics principles in detail if you're interested.

You should connect pulse_in to PB3 and pulse_out to PB4. The unknown inductor of the schematic is the one we want to measure. Solder some alligator clips or female jumpers if you want.

In this step we'll to cover the scope main signal input (CH1), test wave output (WAVE_OUT) and the attenuators. You will need at least a 2x3 female header socket and two single male of female jumper tips. Each attenuator requires a 2x2 male header.

This project uses both 3-resistor and 2-resistor attenuators. The 2-resistor ones are ordinary voltage dividers and allow us to reliably scale down a positive signal. They're simpler to design and build, but won't handle negative voltages.

The 3-resistor ones are a little bit more complex, but will scale down and offset signals, being capable to take positive and negative input voltages.

To design a voltage divider, just follow the simple equation in the pictures, replacing Vout with 3.3V and Vin with the max. voltage you want to measure. For no attenuation just connect CH1 directly to ADC and ignore GND and 3.3V.

About the 3-resistor attenuator, I've made a small python terminal script to calculate rough values. For a deeper understanding you should see this StackExchange answer.

To keep things simple though, I've provided the schematic to the attenuators I currently use, so you don't have to worry about any math for now.

That schematic also contains the wirings to STM32, whose pins are defined in the code: PIN_TEST_WAVE is PB1; PIN_CH1_ANALOG is PB0; PIN_CH1_FALL is PA15; PIN_CH1_RISE is PA12.

The ambient sensing is kind of a bonus feature and in case you're here just for the essential, just skip this step.

The sensor I used was bmp180, and it's capable of sensing pressure and temperature. It could be used to detect current altitude or even predict the weather, it's all up to you!

Besides power, you just have to connect SDA to PC15 and SCL to PC14, since we changed I2C pins before.

All the wiring is finished and we are ready to upload the final code. If everything is fine, you should see a splash screen and a screen with options just like in the images. By shorting the test wave and CH1 a nice square wave will show up on the screen!

Some things about the scope:

• Currently it only detects voltage divider probes. It means frequency counting and everything else may work for any probe, but voltages shown on the screen won't be accurate for 3-resistor ones.
• The 3-resistor attenuators have trimpots that will allow you to offset the signal
• Frequency counting will work for both analog and digital signals, as long as they cross the falling edge, which I marked with a dotted line
• The fastest changing signal I tried was a 62.5kHz square wave, so there's definitely more room for exploration!

Well, that was all. I hope you enjoyed.