Why did I make it?

After discovering that my T-962 reflow oven had uneven heating inside the oven I wanted to investigate this further and I needed a way of measuring & logging the temperature inside the oven in multiple points at the same time. I designed the Voltlogger™ to be capable of recording 10 simultaneous thermocouple measurements.

What are the features?

An ESP32 is the processor for this board which means you not only get lots of processing power but also a Wifi/BT interface. There are 10 x MAX31855 (K-Type) thermocouple interface chips which feature built-in cold junction compensation and each MAX31855 features input filtering circuitry to improve noise immunity.

There is built-in USB>Serial converter chip (CP2104) with the required IO0 and EN switching circuitry to put the ESP32 into firmware download mode so you can flash new firmware over the built-in USB Type-C connector.

Data logging is done on a microSD card using the built-in card slot which is interfaced to the ESP32 using the MMC interface thus providing good bandwidth for the read/write operations.

There are two on-board LEDs shared which share the same GPIO with chip-select for thermocouple interface chips #1 and #2 but since the LEDs are active high and the chip selects are active low, with careful coding, one could signal the state over the board on the LEDs.

Power can be supplied via the on-board USB Type-C connector (5V 0.5A DC) or via the 0.1 inch pin header marked EXT_5V. Do not connect both options at the same time.

How do you write software/code for this board?

You can write code in pretty much any development environment that supports the ESP32 but you can checkout the example code which is written in Arduino using PlatformIO and Visual Studio. This is just an example code and is provided AS IS without warranty of any kind.

How do you load code to the ESP32

Because there is built-in USB to serial chip, all you have to do is to connect the board to your computer and it will automatically install and present itself as a COM port. Select that in your favorite IDE and upload code to the ESP32.

Safety Warning

Depending on what type of thermocouples you plan to use (shielded or not) and depending on where you are performing the measurement, the effect of ground loops or floating voltages needs to be considered. Having the board connected to a computer while doing the measurements means that any potentially dangerous voltages traveling through the shield of the thermocouples could travel through the GND connection up to your computer. These issues can be avoided by powering the board from an isolated 5V external supply like a powerbank or 1S lithium-ion battery and keeping the board disconnected from the computer while performing the measurements. If you need live broadcasting of the measured values, the wifi connectivity could be used instead.

Specs

• Input channels: up to 10 K-type thermocouples.
• Power: DC5V 0.5A via the USB Type-C connector or via the 0.1" pin header for EXT power.
• Connectivity: USB, 2.4GHz Wifi/BT.
• Storage: via FAT formatted microSD card. (tested up to 64GB)

What you get with your order

You will get 1 x fully assembled Voltlogger board.

What else do you need to get started recording data?

You will need some K-type thermocouples and a microSD card. Personally I use the cheap thermocouples from Aliexpress from which I remove the small yellow connector. Here are some examples of K-type thermocouples: Link1, Link2, Link3. As for the microSD card I tested with several different brands up to 64GB and they all worked fine so there are no special requirements as longs as it's formatted as FAT.

Known issues in revA

There is one important known issue in PCB revA: the chip select pins for the MAX31855 channels #1 & #2 are tied to GPIO34 and GPIO35 which on the ESP32 are input only pins. This means the ESP32 cannot drive the chip select signals low to activate those channels. As a workaround revA boards will contain a bodge to re-route the chip select signals to GPIO22 and GPIO23 shared with the LEDs. Since the LEDs are active high and the chip selects are active low, with careful programming one should be able to use both. Although the bodge is done clean and the wires are secured with some epoxy, there is a risk this becomes a reliability issue if one of those wires gets ripped from the board.

Shipping delay & exception notice

Because of the current global situation I will not be able to ship to a list of countries as decided by the postal operators. Long delays for delivery are expected especially when shipping overseas to places like the US, Canada & Australia.