First and foremost, this project is a derivation of the fine work done by Mike Lawrence with his RPi-pHat-Thermocouple project. His implementation has a different use-case, and I adapted his great design for my own purposes.
The major changes from his design to this one are:
- Change the form-factor from a Raspberry-Pi Zero sized board to a "full size" version intended for a Raspberry Pi 2 or Raspberry 3.
- Add an isolated RS-232 to TTL interface.
- Drop the on-board regulator to 5V, and associated bridge rectifier and "ideal diode".
- Drop the SPI EEPROM for automated "Hat" configuration.
- Drop the alarm buzzer.
- Add screw terminal connections for power, thermocouple, off-board 1-wire bus peripherals and RS-232 connections.
- Add pin headers for driving LED annunciators.
- Add pin headers (with configurable pull-ups) for end-user supplied I2C peripherals.
- Add a fuse to protect connections to off-board peripherals.
IMPORTANT
Please note that all of the work described here is on the genmon branch
of the git repository. Make sure that you git checkout genmon if you
clone the repository to get this modified version.
I'd like to extend my thanks to Mike for sharing his work with the community under an MIT license. This derivitive project is likewise available under the same license.
This is a Raspberry Pi PCB that supports:
- Three MAX31850 1-Wire Thermocouple Converters for remote temperature sensing
- DS18S20 1-Wire Thermometer for local Hat temperature
- Electrically isolated RS-232 interface to the Raspberry Pi's serial port UART.
The goal of this board is to piggyback on a Raspberry Pi running the genmon package. That software expects to communicate with the 9600 bps RS-232 serial MODBUS connection on a Generac generator controller.
Additionally, there are 3 type K thermocouple interfaces which can be used to monitor critical temperatures in the generator, such as the oil cooler temperature, exhaust air temperature, body temperatore of the generator, etc.
I have a 22kW generator manufactured by Generac, propane fueled. The addition of the wonderful genmon has really added significant value, and allowed me to integrate the generator with my Home Assistant home automation platform.
On one occasion, I was experiencing an extended power outage, and of course, the generator automatically started and switch the home loads to it. About 6 hours into this power outage, I was wondering how it was working. The genmon software provides a bunch of data on the current operation, indicating no alarms or faults. However, after an extended run time, I was wondering if the generator was overheating or otherwise being stressed?
Measuring some of the relevant components had to be done with, e.g., thermocouples since the usual Dallas Semiconductor/Maxim 1-wire bus temperature sensors wouldn't work in a high temperature environment. It was at that time that I stumbled upon Mike Lawarence's work and decided to adapt it.
The first thing that caused me to want to layout a new board was that my use case was different. I had no need for a buzzer since my Raspberry Pi was buried inside the generator enclosure and wouldn't ever be heard. Further, I really wanted to have screw terminals for the thermocouple wires since the metal alloys used in the thermocouple wires don't take solder easily. Due to the vibration inherit to the environment, I wanted to have a really robust mechanical solution for those connections.
Finally, The Raspberry Pi running genmon requires a conneciton to the RS-232 signals on the generator controller's MODBUS interface. At present, I was using an external TTL/RS-232 level convertor for this purpose. However in connecting this up, I create a ground loop -- there's a ground path from the generator controller through the RS-232 level converter, though the Raspberry Pi, through the 12V to 5V USB power support, to the negative battery terminal, and finally through the cable from the battery to the generator starter and control electronics. I shudder to think how this all behaves when the starter kicks in during a generator start-up sequence.
So this led to an isolated RS-232 to TTL level converter. There is a separate power support for this part of the circuit, intended to be supplied by power available on the generator controller's MODBUS accessory connector. There is an high-speed isolation device (apparently transformer coupled?) rated at approximately 20MBit/s, substantially higher bandwidth than most common optoisolators. This isolation device has separate power/ground connections for each side, and in excess of 1kV of isolation.
A schematic for this circuit is available in PDF format so you don't need to use KiCAD to casually examine what's going on.
- Rev 3.1 of the PCB fixes pin assignment problems on the Maxim MAX3221
RS-232/TTL level converter device.
- You can order parts from Mouser using this shared BOM link.
- Rev 3.0 PCB was the first version of the whole redesign, manufactured by JCLPCB.
Please note that all of the work described here is on the genmon branch
of the git repository. Make sure that you git checkout genmon if you
clone the repository to get this modified version.
Older versions described below are Mike Lawrence's original design in the smaller Raspberry Pi Zero sized form factor.
- Rev 2.0 PCB has been ordered from OSH Park and has been fully tested.
- Removed one of the four MAX31850K's.
- Increased thermocouple input filtering buy adding ferrite beads and a larger capacitor.
- Added more filtering to thermocouple side of MAX31850K's.
- Added DC Power connector and switching power supply that powers both the Raspberry Pi and the Hat.
- The noisy reading the MAX31850K's occasionaly get are reduced in this version but not gone.
- You can order parts from Mouser using this shared BOM.
- You can order the PCB from OSH Park using this link.
- Rev 1.2 PCB has been ordered from OSH Park and tested.
- Discovered the MAX31850K's get a noisy reading about 0.1% of the time. This noise is typically within 5 C but sometimes is greater than 30 C. Adding capacitors did little to help the Rev 1.1 design so most of the 3.3V power was switched to a 3.3V LDO regulator in an attempt to reduce the noise on the 3.3V power seen by the MAX31850K's.
- You can order parts from Mouser using this shared BOM.
- You can order the PCB from OSH Park using this link.
- Rev 1.1 PCB was never built.
- Added a pulldown on the alert signal to prevent the Alert buzzer from sounding on power on.
- Added an on board DS18S20.
- Rev 1.0 PCB has been ordered from OSH Park and tested.
- Discovered the linux kernel doesn't seem to support MAX31850K devices without a DS18S20 present. The kernel detects the MAX31850K devices but does not create a w1_slave file to read the temperature. This is most likely a bug in the Linux W1 driver.
- This PCB design uses some libraries available here Mike's KiCad Libraries.
- This PCB was designed with KiCad 5.1.5.
- The MAX31850K parts have an exposed pad on the bottom which requires either a reflow oven or hot air to solder properly. Be careful to not apply too much solder paste to this pad.
This board has screw terminals to which you can connect a regulated 5VDC power supply. This is is connected directlly to the +5V power pins on the Raspberry Pi GPIO connector.
You should never connect 5V DC power to the screw terminals if the Raspberry Pi is being powered via it's USB connector!
If the Raspberry Pi is powered in the usual way via its micro USB connector, then the Raspberry Pi will supply power to this PCB and power the components on the board. Alternatively, if it is more convienient to provide regulated +5V DC power from another source, then the screw terminals are available for that purpose.
The components on the board draw well under 100mA of power at 5V, so there ought to be no concern when powering from the Raspberry Pi that the PCB is plugged into.
Separately, there is a 4 pin screw terminal connection for RS-232 transmit and receive as well as generator controller ground and generator controller 5VDC. The ground and power on this 4 position screw terminal is isolated and distinct from the Raspberry Pi power and the rest of the PCB. You should not tie the ground on the RS-232 screw termainal block to any other ground connection on the genmon PCB.
Although the MAX38150K datasheet typical application circuit doesn't show the use of ferrite beads many designs seems to include them. That in combination with the larger than normal capacitor across the input will hopefully improve sampling errors even further.
This setup makes several assumptions. First you are using Raspbian Buster. This
software and instructions most likely work on other versions of Raspbian but
they have not been tested. Second Python3 is the target programming environment.
It is also assumed that you are using the standard pi user. Otherwise you will
have to edit the commands by replacing /home/pi with your user's home
directory. Install everything needed by executing the following commands.
sudo apt update
sudo apt -y install git python3-rpi.gpio python3-w1thermsensor python3-paho-mqtt
Clone this repository from Github with the following commands.
cd /home/pi
git clone https://github.com/lmamakos/RPi-pHat-Thermocouple
git checkout genmon
For this Hat you will need to enable the 1-Wire interface. From the command line type
sudo raspi-config and follow the prompts to install support in the kernel.
It's time to reboot your Raspberry Pi with sudo reboot.
Python3-w1thermsensor is a nice 1-Wire python library that also supports command line reading of temperatures from 1-Wire devices. You should have already installed this package in the Raspberry Pi Setup section.
Test the single on-board DS18B20 temperature sensor using w1thermsensor all --type DS18S20.
pi@studio-fridge:~ $ w1thermsensor all --type DS18S20
Got temperatures of 1 sensors:
Sensor 1 (00080372f2c4) measured temperature: 28.06 celsius
Now test the three MAX38150Ks temperature sensor by using w1thermsensor all --type MAX31850K.
pi@studio-fridge:~ $ w1thermsensor all --type MAX31850K
Got temperatures of 3 sensors:
Sensor 1 (000000181928) measured temperature: 23.75 celsius
Sensor 2 (00000018192b) measured temperature: 24.0 celsius
Sensor 3 (000000181d59) measured temperature: 2047.81 celsius
The 2047.81 celsius reading is what you get when there is no thermocouple connected to the MAX31850K.
Integration with genmon
The primary motivation for this board's design is to support the use of genmon running on a Raspberry Pi 2 Model B, Raspberry Pi 3 Model B, or a Raspberry Pi Zero. It includes two major functional elements:
- Three (3) connections for Type-K thermocouples, using a Maxim 1-wire bus attached controller.
- A TTL to RS-232 level converted to connect the Raspberry Pi serial port to the MODBUS connection on the generator controller.
These are intended to enable monitoring of high-temperature points within the Generac generator, such as the temperature of the engine oil cooler, engine block, generator temperature or perhaps exhaust temperature.
The use of Type-K thermocouples permits the measurement of very high temperatures, as high as 700 °C. As a practical matter, the maximum temperature is limited by the insulation and jacket on the cabling connecting to the thermocouple junction. Typically, thermocouple products with cables rated to 400 °C are easily available and inexpensive.
VERY IMPORTANT - the MAX 31855K thermocouple interface devices does not support grounded thermocouples - where the junction of the thermocouple is electrically connected to the power supply ground. In particular, the sensor device does a diagnostic test checking for an "open" connection at the thermocouple terminals, as well as a short to ground or the Vdd power supply on the board.
It's common to see either exposed junction thermocouples, or where the actual thermocouple junction is within a metal tube or other mechanical arrangement. The junction is deliberately tied to the case (which presumably is in contact with the thing you're trying to measure) for better thermoconducivity, making it more responsive to temperature changes. Additionally some thermocouple probe arrangements have a woven metalic armor shield on the cable, which is also electrically tied to the thermocouple junction. If these armored cables come in contact with each other, this will also cause a fault.
It is recommended that you choose a Type-K thermocouple with a high (enough) temperature rated cable, and either have the thermocouple junction be in free space, or insulated from the chassis of the generator.
One possibility if you use the inexpensive generic thermocouple probes is to form a small bead of high temperature expoxy around the junction, or affix the the junction with non-conductive expoxy to the surface you're trying to perform temperature measurements on.
Why not the usual Maxim DS18B20 one-wire bus temperature sensors? These semiconductor devices are packaged in plastic material for their packages, and not rated for these high temperatures present in some parts of the generator. However, there are other temperatures that might usefully be monitored, such as the outside air temperature and these DS18B20 devices are entirely suitable for that purpose. One of these devices is present on the printed circuit board to monitor the temperature nearby the Raspberry Pi.
This board features a level converter to connect the RS-232C serial signals on the generator's MODBUS accessory connector to the 3.3V TTL signal levels needed by the Raspberry Pi's serial interfaces.
Most significantly, this serial interface is electrically isolated from the Raspberry Pi and its power supply. The RS-232 "side" is powered by the +5V / Ground connections avialable on the MODBUS connector. This power supply is used to run the RS-232 level converter, and half of an isolation device which couples the transmit and receive signals to the "other" side of the board which shares a power supply (and ground) with the Raspberry Pi. This isolation means that you no longer need to worry about ground loops or powering the Raspberry Pi from some other power source not electrically tied to the generator's power supply and ground.
Integration with Home Assistant
(Future) software will sample the thermocouples and 1-wire temperature sensors and publish the values to MQTT for Home Assistant or other applications to access.
A deliberate choice is that there is no in-built policy for over-temperature or under-temperature warnings or actions. The goal is to publish these values to be acted upon or displayed by other applications.
The board is intended to plug into the 40 pin GPIO connection available on all the recent models of Raspberry Pi computers. (The notable exception is the original Raspberry Pi Model A which has a 26 pin GPIO connector.) While it may be possible to install a 26 pin connector on this board for that device, this has not been tested.
Reference this rendering of the board:
The connector mounted in this position plugs into the 40 pin GPIO expansion connector on the Raspberry Pi. When mounted correctly, this board sits directly over the Raspberry Pi.
If you intend to build this board, remember that the GPIO connector is mounted on the back of the board. In fact, this is the only component on the back side of the board.
This connector supplies power to the Raspberry Pi via pins on the GPIO connector. It should be used only if the Raspberry Pi doesn't have a power supply connected to the Micro USB connector on the Raspberry Pi.
If used, you must supply regulated +5V DC power to the connector. Note there is no reverse polarity protection for power fed on these screw terminals.
Power for the board can also be supplied via the Raspberry Pi's expansion connector via the +5V DC signal on pin 2 when the Raspberry Pi is powered via it's Micro USB connector.
When the the PCB is powered, the green PWR LED D5 should be illuminated. This
LED is conneted to the locally regulated 3.3V DC power supply for the devices on
the board (excluding the RS-232 interface; see below.)
These screw terminals are labeled TC1, TC2 and TC3, and are polarized. The Type-K thermocouples connected to each of these terminals are polarized and the positive and negative signals are labeled on the board.
The wires on the thermocouple care are usually marked or color-coded to indicate the positive and negative termainals. Connecting the thermocouple with reverse polarity will not damage the board or thermocouple, but will yield incorrect measurements.
Signals from the Generac generator's controller are connected to J7. This includes the RS-232 data connections and power-supply for the isolated RS-232 interface.
An important aspect of this port, and this part of the circuit, is the electrial isolation of the generator connection from the Raspberry Pi and the rest of the circuit. This interface consists of a RS-232 level converter (+/- 5V) to TTL (0 to 3.3 volts), and an isolation device that literally bridges a gap on the PCB. The power and ground connections of this serial interface are isolated from the power and ground connections of the Raspberry Pi and rest of the circuit.
-
RX in this terminal is for receiving the data transmitted from the generator to genmon. This should be connected to pin 8 of the controller's molex connector.
-
RX out this terminal is the output data transmitted to the generator from genmon. This should be connected to pin 7 of the controller's molex connector.
-
The Ground terminal should be connected to pin 2 of the controller's molex connector.
-
The 5V terminal should be connected to pin 1 of the controller's molex connector, which is a power source intended to power accessories. This circuit requires approximately 50mA (or less) from the power input. While a nominal +5V DC source is expected, there is an internal low-dropout (LDO) regulator on the board that should function with a 4V - 16V DC power input. Do not exceed 16VDC input. This power supply connection includes a reverse-polarity protection diode.
When power is correctly connected, a green LED Gen PWR D20 should illuminate.
External 1-wire bus temperature sensors can be connected to J6. There is regulated +3.3V power on this connector, supplied through a 100mA resettable thermal fuse. This power source is intended to supply only low-power sensor devices.
Note that the usual Maxim DS18B20 sensors are measuring temperatures between -55C to +125C.
This connector is intended to drive up to 6 LEDs to be used to indicate generator status. A selection of the Raspberry Pi CPU's GPIO signals are present: 23, 24,25, 8, 9, 7 as marked on the PCB above the connector location. The corresponding pins below are all connected to ground.
Each active (non-ground) pin on the connector is driven through a 330 ohm current limiting resistor. This is to try to avoid over-current situations which could damage the drivers on the CPU chip.
A 1.8V voltage drop LED should be suitable for use (which may exclude any blue LEDs). Typical current through the LED will be 4.5mA; while not terribly bright, this may be useful as a local status indication.
Of course, these signals could also be used to drive, e.g., a MOSFET device, to drive a higher-current load.
One possibility for using this connector to drive one or more LEDs is the
gengpio.py program, suitably configured.
(See https://github.com/jgyates/genmon/wiki/1----Software-Overview#gengpiopy-optional )
Note that when using these signals, they can be identified by either the GPIO signal number or the pin number on the Raspberry Pi's expansion connector. This document only references the CPU GPIO number.
This connector exposes an I2C interface on the Raspberry Pi's CPU for some future unspecified use. By default, there are pull-up resistors on the SDA and SCL signals on the connector, as well as (fused) +3.3V DC and Ground.
- SDA signal is connected directly to GPIO2 (pin 3 on the Raspberry Pi connector)
- SCL signal is connected directly to GPIO3 (pin 5 on the Raspberry Pi connector)
If pull-ups on the SCL and SDA signals are not desired, then jumper traces s on the back of the PCB can be cut to disconnect the pull-up resistors.
This project was created for my personal use, but it is available under a permissive license to anyone interested.
This software, hardware designs and other information are provided for educational and informational purposes only. Connecting any device to a Raspberry Pi or generator carryies risk, and you should understand those risks before undertaking that action. Interconnection of this design to any other devices provided by any manufacturer may void any warrantee offered by a manufacturer, or cause damage if performed. You are solely responsible for exercising due care and caution when choosing to use these designs and other information provided here.
THIS HARDWARE DESIGN, SOFTWARE AND OTHER MATERIAL IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS “AS IS” AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE OR HARDWARE DESIGNS OR OTHER MATERIALS, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.