Pax Instruments T400 temperature datalogger Kickstarter campaign

[charlespax]

Hello, everyone! There has been a discussion in the Open Source hardware section of the EEVblog about the Pax Instruments T400 temperature datalogger. I'd like to announce here that you can support the T400 on Kickstarter right now.

The T400 campaign was launched about twelve hours ago and we're at 56.7% funding. This is truly an exciting time.

The T400 simultaneously displays all four thermocouple temperatures, the ambient temperature, and a graph of the last 100 readings. It also saves the readings to a MicroSD card and prints the readings to the USB serial port.

The main processor is the ATmega32U4 running at 8 MHz and 3.3V. The processor was chosen to give Arduino compatibility. Since it runs at 8 MHz and 3.3V it would be programmed as an Arduino Lilypad USB through the Arduino IDE.

The MCP3424 analog-to-digital converter measures the voltage produced by each thermocouple.

The MCP9800 temperature sensor is used for cold junction compensation.

The DS3231 real time clock is used to trigger readings. Between readings the device goes into low power sleep mode. The RTC wakes up the unit to take a reading, giving a longer battery life.

The T400 uses a standard BL-5C battery, great for battery replacement in the field.

As you can see in the attached screenshot, the device can display all four thermocouple temperatures while graphing the most recent 100 readings from all four channels. The graph can display each channel individually or all four together. If the ATmega32U4 has enough programming space, the T400 will also be able to display each channel in large numerals for easy viewing.

If you find this project interesting, learn more at Pax Insruments or head directly to Kickstarter and lend your support. Thank you.

[miguelvp]

Not for me, but I did spread the word a little at work about it because it's not horribly expensive and looks customizeable.

A couple of peeps at work are into home automation so I told them about it.

[Kean]

Overall very nice, although it seems to me that the ATmega32U4 is really limiting the usefulness of such a device.  The MCP342x are nice ADCs (I use them in some of my instruments) but I'm not sure I'd use it for measuring a thermocoupe without some external gain, due to the very small signal (even with the PGA at max).  These are the two primary things that put me off backing your project.

A few questions:
- Have you considered switching to an ARM Cortex to remove the memory limitations?  Much nicer to do it "properly" than a kludge like offloading some data to an I2C EEPROM.  Arduino support has some complications, but is possible (e.g. see Teensy 3).
- How are you going to manufacture the enclosures and buttons?  Surely you don't want to 3D print 550 sets of them.  You likely wont be able to afford injection moulding (maybe via Protomold for those qtys, or if you have separate funds put aside)
- Why is the RTC battery directly connected to the main battery?  Normally if rechargeable it would be diode isolated, so it only powers the RTC.  I'm looking at schematic for v0.7 which may be old (and I see it is missing many component designators...)
- Are the humidity and pressure sensors seen in some PCB photos going to be populated?