💻 HS402 3.0 ANDROID STM32 Oscilloscope PCB and assembling

This article describes how to build android oscilloscope HS402, includes a few ways to flash firmware with PC and Android. 

Electrical circuit

K1, K2 – Solid-state (PhotoMOS) relay for DC/AC choose;
R2, R3 – voltage divider;
D1, D2 – input protection (GND-0.3V; GND+0.3V);
U1, U3 – input signal gain with different coefficient;
U2 – virtual ground (3.3V/2 = 1.65V);

📄 SCH_HS402-WIFI_V3.0_210327.pdf
📄 SCH_HS402-WIFI_V3.1_220108.pdf

Components

Full list of components you can look in the .PDF document.

Ceramic capacitors C8 and C11 (47 uF) could be rare and expensive, so it can be replaced by 10uF 16V X5R/X7R.

📃 HS402_Component_List_V3.1.pdf

Power supply options

Microcontroller

Both modules are pin-to-pin compatible development board, but STM32F411 is more powerful than STM32F401, but it twice as expensive.

I strongly recommend don't solder module on the board, because if you want to take devboard comeback – it'll be super-hard, so just use PBS connector:

BNC (RF connector)

Presented plastic white BNC connectors are really bad, if you can, replace it with more reliable.

Programmable Amplifier

MCP6S21 is rail-to-rail amplifier with SPI-programmable gain, that allows change voltage range by application.

Solid-state relay

AQY210 is a solid-state relay (PhotoMOS), that have internal LED (input), solar panel (convert light emitting into voltage) and transistor (output).

Voltage Regulator for Virtual Ground

Voltage regulator  AMS1117-1.2 give a chance to get 1.65V (half of 3.3V) for virtual ground, what is require for take input signal polarity in both sides.

PCB

This is 3.0 version of the board. Now is available 3.1 newest and better version.

The Gerber files is available for download and order with PCB factories (current ver. is 3.1)

    ? PCB_HS402-WIFI_V3.1_220108.zip

Assembling

Before assembling find minimum required components for the oscilloscope by the list and put in case.

You can use soldering iron, but the easies way is using hot air gun. Before mounting you have to prepare PCB pads. If you have solder paste put it on pads manual or with help SMT-stencil, I don't have it, so I must coat pads by using soldering iron.

Don't forget to shortcut J1 jumper, if you use Option #1 power supply.

Then mount leftover THT components with soldering iron.

Settings

Visual inspection

After soldering thoroughly inspect soldering quality and components values.

Check input and output voltage

I use on-board voltage regulator 3.3V (Option #1).

Check input voltage ( GND and 5V) and output (GND and 3V3).

Flashing firmware

There are many ways to load firmware into microcontroller's flash-memory:

USB-TTL converter

Android

The easiest way for you it's to use USB-TTL. I use CH340, you can use: CP2102, FTDI232RL, PL2303HX) converter. CH340 Driver for Windows 10, 11 (must be installed manually): 

? Download driver for CH340:  ch340_driver_win10.zip

Connect it to  STM32 board according to the wire diagram and then to android phone:

Download and install STM32 Utils (from Play Store):

Check connection again and connect it to the phone:

Press both buttons NRST and BOOT0 and connect it to the phone, release first NRST and then BOOT0:

Open application and init chip. Press on blue cube, choose vendor Martinloren and select the last version of the HS402 oscilloscope firmware, press FLASH FW.

Wait for erasing data and writing chosen firmware, that's all.

PC

Also, you can use computer software STM32CubeProgrammer. Download it on official site (need registration), or in telegram.

After connection to the PC open Device Manager and check, that your converter is properly identificated (drivers properly installed).

Press both buttons NRST and BOOT0 and connect it to the phone, release first NRST and then BOOT0:

Open Cube Programmer, choose UART and select COM-port (in my case it's COM5, we have seen it before), press Connect. (If you do previous step wrong, you will see that, disconnect power and press buttons again)

Boot error

Download firmware binary file (available on  GitHub):

HS402_V1.6_F401.bin

HS402_V1.8_F411.bin

Press Open file and choose it.

Now we have opened binary file, press Download. Wait about 20 seconds and that's all!

Programmer STLINK

PC

The easiest way for me is using programmer-debugger STLINK, connect it appropriate to table:

Check connection and connect it to the PC.

Android

By the way, you can use android application ZFlasher STM32.

USB (in DFU mode)

Another simple way is using DFU (Device Firmware Update) mode.

Connect board to the PC (using USB cable), then press both buttons NRST and BOOT0, release first NRST and then BOOT0 (within 0.5s) to enter load mode:

Choose USB connection and select USB1 port:

Press Connect:

Open Open file and select firmware file (see above where to get), press Download and wait some time.

Android

By the way, you can use android application StmDfuUsb.

First connection

Download and install HScope app from Google Play. The most convenient connection way is is USB Type-C—USB Type-C cable and Android phone with USB-C socket.

Virtual ground tune

Most likely, even if no signal on input you will see non-zero value, don't worry.

Just tune line to zero (must no input signal!) by rotation variable resistor.

Fronts tune

Connect first channel to rectangular waveform signal (use can use PWM output pin), if angle is no acute rotate trimmer capacitor (better use plastic/ceramic screwdriver).

The same tune do with second channel.

Calibration Zero Level

Shortcut ground and first channel input and do zero calibration.

Do the same with second channel.

Ready to use

Generally, the oscilloscope is ready to use, I want to test it on the generator based on comparator.

Test

I have got another STM32, let's generate PWM-signal and observe it with android oscilloscope.

PWM-signal (50% duty cycle)

10 Hz, 100 Hz

10 kHz, 100 kHz

200 kHz, 400 kHz

666 kHz, 1 MHz

Comparator-based relaxation oscillator

RC (Differentiator)

Previously generated signal connected to the input (yellow CH1), output on CH2 (green).

LC (Resonant circuit)

We can see the damped oscillations.

Level trigger (red line) is set, then battery (power supply) connected for a moment.

PWM generator

You also have got a PWM-signal generator with frequency and width regulation.

It can be used for tests or like voltage regulator, if transistor is connected.

Video

Conclusions

As you can see, assembling an HS402 oscilloscope is not difficult at all, but in any case the effort and time invested are considerable.

The most worth of this oscilloscope is possibility to get wireless data transmission to your phone, so you can observe electrical values remotely. To add this feature you need add ESP32 Wi-Fi microcontroller module, I will do this in next part.