What is it?

A pHat that can interface a Thermocouple and two Solid State Relays (SSR's) to your Raspberry Pi. Using your software and Pi coupled with this pHat-Stat board and its associated Python libraries, precisely control your thermal process. Heres whats inside:

• Microvolt Level Analog Input. One input of an MCP3426 A/D converter is made available on a terminal block for low level voltage inputs such as a thermocouples. With its programmable gain amplifier (pga) set to its highest 8x gain setting, this 16 bit a/d can resolve less than 10 microvolts which, with a Type K thermocouple, is a resolution well under a degree Celsius. Although optimized for a thermocouple, this differential input could be used for measuring other instrumentation such as strain gages or pressure sensors with voltages as high as 2.048 volts at its 1x gain setting.

• On Board Cold Junction Sensor. A Vishay NTCS0805E3103JMT (B3570) Thermistor on the other A/D converter input is dedicated to measuring the temperature of the terminal block. The value of this temperature is used by software in the calculation of the measuring junction temperature, the process of which is commonly called 'cold junction compensation'.

• Two SSR Compatible Outputs. A CD40107 buffer/driver with two low side switched outputs, safe to 20V and 25 mA, can easily drive any standard SSR.

• Hardware Watchdog Timer. The APX823 watchdog timer has hardware priority and will disable the outputs if not kept at bay by software. By carefully integrating watchdog management with your control software, the risk of loosing control of the outputs and possible subsequent meltdown is diminished.

• Raspberry Pi Zero Form Factor. This pHat is designed to be installed on the back side of your Pi and to be the bottom most board in the stack. This allows ready access to a real terminal block while still conforming to the Pi Zero form factor, and more importantly, without interfering with a display stacked on the front side of your Pi.

Connect a thermocouple to the input terminal block, an SSR to the output terminal block, and gain control over your process. Start with a simple on/off (bang bang) algorithm, move up to a PID library, add a software timer, maybe even ramp and soak functionality. Use the second output channel to control an inert gas valve, or even a water solenoid for post process cooling. But whatever you do, dont forget to feed your dog!

Why did you make it?

From the seemingly uncontrolled act of firing Raku pottery where serendipity can be the only guardrail, to the precise temperatures needed in the heat treatment of stainless steel, I seem drawn to high temperature endevours. This board, with its careful analog design and hardware watchdog timer, allows control over temperature for the projects where a more disciplined mindset is required.

What makes it special?

• Thermocouple alloy/type agnostic because the thermocouple voltage is converted to temperature in software, any standard (or non standard) thermocouple can be used.
• A 3.3uA bias circuit that saturates the differential input of the A/D if the thermocouple happens to become disconnected or fails in the open mode. The resulting gross negative temperature can then be detected by software and dealt with as desired.
• Hardware watchdog timer to reduce the risk of process runaway caused by lazy programming or errant neutrino hits.

What you get

• One fully assembled pHat_Stat board as shown in the first photo above, ready, willing, and able to connect your thermocouple and SSR's to your Raspberry Pi, all guarded by a hardware watchdog with teeth.
• A Python class for each hardware subsection easing development of custom controls or acting as examples for more specific instances.
• An obedience class for both you and your new best friend.

But really, who wouldn't want a pHat-Stat if only to keep their Pie warm? (studded dog collar and chain not included)