Geekworm Smart Fan and Power Expansion Board

Note: Your fan HAT may or may not be a “Geekworm” as they seem to be made under different names.

I read many articles that state that the Raspberry Pi does not require a cooling fan as the board is designed to handle temperatures in the 70ºC range and will throttle the CPU/GPU to reduce the heat if required. But, personally, I still like to use heat sinks and fans and keep the temperature down even if it won’t do any actual damage. Besides, it was a good project to refresh my math skills.

I will assume that at this point you should have some basic knowledge about heatsinks and cases. There are many different types of heatsinks available to you should you decide to use one. Not only do they come made of different materials (copper, aluminum, etc) but they come in different sizes. The number, size and type is up to you to research and also depends on the model of your Raspberry Pi. Keep in mind that if you are going to install a heatsink then the placement of your fan will also be important and the available space for these components must be considered. If you are using a case then it may or may not have slots or mounting mounts for a fan. You may have to make some modifications to the case yourself. Some people claim that the method you use to attach the heatsink to the chip can actually increase the temperature. I will let you do your own research, but the short version is that you should use a proper thermal adhesive if you use a heatsink.

aluminum case.jpg I have a number of Raspberry Pi and I like to keep things consistent. I’m a bit OCD that way. I chose a simple case design that provides lots of basic ventilation rather than a fixed fan mounting point. I like the sturdiness of the aluminum cases over the ABS plastic too. The grill effect of this case provides some visibility of the LEDs inside the case as well as providing lots of air flow. On its own this is likely a good solution for many users. I found too that it was easy to mount a fan to the top of the case using the existing holes and I could place the fan where I wanted rather than a position determined by a manufactured mounting point. I don’t have many components to install so this was an ideal part of my overall solution. Ultimately, I also chose to use the Geekworm HAT (hence the title of this article) which also fits nicely into this case.

I spent many hours reading all about fans and various ways to control them, or not. Some solutions are quite simple and are literally a 5V 0.2A fan plugged into the GPIO pins for +5V and GND. There are even water cooled and mineral oil cooled solutions. I won’t go into those.

Basic 5V fan. This option has some obvious benefits, and obvious drawbacks. Benefit: can’t get much simpler. Drawbacks: No control. The fan runs at maximum speed all the time. These little fans can be quite noisy at high speed so this is not ideal if you plan or being anywhere near it. You could splice an on/off switch into the wiring and mount it in your case to provide some basic control. Some people suggest that using a 12V fan and supplying only 5V will result in a quieter solution as the fan will not run at full speed. However, the fan may not run at all either, depending on your power supply and the fan.

fan with transistor.png

The Raspberry Pi can provide something called Pulse Width Modulation, or PWM. Essentially this is a way to send a signal to your device (fan, LED, etc) that quickly pulses the power on and off to allow you to control the “strength” in a sense. This can be used to control the brightness of an LED or the speed of a fan. This diagram shows the use of a transistor and resistor to safely provide speed control to the fan. There are many other examples using various fans, transistors, resistors and other components but the basic principal is the same. By adding a couple of basic components, you end up with a simple and inexpensive hardware solution. But, that is only half of the overall solution. You need a software component (script) to set / control the PWM value to turn the fan on, off, or to a specific speed. More on that later.

By adding a temperature sensor such as the DS1820, DHT11, DHT22 or one of many others, you can monitor the temperature of a specific component or generally within the case. With a script you can then set an appropriate fan speed to balance noise levels with the temperature control.

Note that the Raspberry Pi does allow you to monitor the CPU and GPU temperatures WITHOUT adding any additional sensors.

There are many HATs available to choose from that essentially allow you to plug a daughter board onto the GPIO pins. Installation can be quite simple and you may get other components for “free” by following this solution. In addition to a PWM fan controller, some include additional temperature sensors, power control, relays, real time clocks, etc. Keep in mind that your case must accommodate the HAT used and again may require some modifications so that you can access ports, switches, LEDs, etc. Some really fancy (and expensive) Fan HATs include code in firmware to control the fan. While this has some advantages, it also may provide less flexibility. I settled for the Geekworm (also available under a few other names) HAT pictured here. At the moment, I am focusing only on the PWM fan control although there is an onboard temperature sensor, 6V-14V power input and some other components on the board.

This HAT comes as a simple kit and all that is really required is to mount the fan using the bolts. All the parts shown here came with the kit I purchased. Because I have some relatively tall heatsinks that I like to use, I chose to install the fan on the opposite side of the HAT than it is typically mounted. To facilitate the wiring I cut a small slot into the unused portion of the PCB on the side of the opening for fan near the fan power connector. I also had to trim some of the power switch plastic slider as it was too long by about 1mm to fit in my case.

full kit and case.jpg

The full version of this kit includes a “case”, which is really just a couple plastic plates and brass standoffs. This image shows the “standard” mounting of the fan and placement on the GPIO pins.

More images and sample code can be found on this web site:

http://www.raspberrypiwiki.com/index.php/Smart_Fan_and_Power_Expansion_Board

The last bit that I will discuss briefly in relation to this HAT compared to some others is that it does NOT have a real time clock (RTC) module or battery. At one point there was a product called the PiCoolFan which seemed ideal. It was a small board (see image) that included a temperature sensor, PWM fan control and RTC. I have as yet been unable to determine its suitability for use with the RPi3B+ and it seems it may have been replaced by another product which does not include a RTC. In any case, one thing I like about the Pi Power Hat is that it includes access to the GPIO pins so I could add my own RTC quite easily.

Enough about the hardware, let’s get scripting!

Almost all the PWM fan control scripts I could find were pretty basic. At best, most seemed to provide an incremental speed control based on temperature ranges and fixed increments. I was after something better than 0, 25, 50 75 or 100% fan speed control and something more flexible and dynamic. As an example, most scripts would use the logic equivalent to: If the temperature is above 40 but lower than 50 then set the fan speed to 25%.

Using these scripts as a starting point I wrote my own script that should be easily configurable to match most installations.

The script uses a couple different methods to query the Raspberry Pi internal CPU temperature sensor. The modular design of the script allows for the easy addition of other methods such as querying I2C sensors (such as the one included on this HAT) although I consider the onboard CPU temperature to be the one I am most interested in. In the configuration settings within the script you can set the minimum temperature at which you want the fan to start thus allowing the fan to be off, if desired, at lower temperatures so that there is no noise. You also configure a temperature above which the fan will run at its full capacity to provide maximum cooling.

A calibration script should be run on each Raspberry Pi with each specific fan to determine the minimum value for the PWM at which the fan will actually begin to turn (although you could configure a reasonable “guess” as this is only used as a base starting point). A maximum PWM value can also be configured so that the fan will max out at 100% or whatever value you set thus enabling you set only run at a speed at which the fan noise is appropriate for you.

The script will use the configured temperature range and the configured PWM values to automatically determine the fan speed. Actual real-time percentages are used rather than a fixed scale. For example, if it is determined that the actual temperature is 15% of the way between the minimum and maximum values, then the fan will be set to run at 15% of the maximum rate configured in the script.

While this may be overkill, it was relatively simple to calculate a dynamic fan speed based on the temperature.

The time between each sampling of the current temperature can be configured as well. As the script runs under Python, which carries some overhead, you don’t want to sample too often or the script itself will cause the CPU temperature to rise. Generally speaking, sampling the temperature once every 3-5 seconds is often enough without putting additional load on the CPU. In many cases sampling every 10 or 30 seconds would even be sufficient.

The script is designed to run as a background task but there are 2 diagnostic options that can be enabled. A real-time output can be displayed to the console (telnet or SSH shell) and optionally be redirect through a standard pipe to a file for logging purposes. Also, the last loop iteration can be enabled to be logged to a file making it easy to see at a glance though shell access, a web site or file share what the last temperature and fan settings were. These optional diagnostics can be individually disabled or enabled.

Update September 28^th, 2018

Ok, so one thing that I didn’t like was the issue that I have always liked having a Real Time Clock (RTC) installed. In the past I’ve tried them all, DS1302, DS1307, etc and made lots of little add-on boards with batteries. I ended buying a bunch of the DS3231 modules as they were small, simple to install and used a super-cap to maintain the time.

The issue then became installation. The fan HAT obviously blocked the GPIO header. Idea! Remove the header from the DS3231 module and use the blank header on the fan HAT to install the module.

I’ve also been playing with the temperature sensor on the HAT. It turns out it is a LM75 chip (8-pin chip just above the fan power connector). This is great as both the LM75 and the DS3231 use the I2C bus but default to different addresses. Also, a quick mention that the on-off switch does work, if you use the power connector on the HAT. Slide it towards the GPIO header to turn on the power, and slide it towards the middle of the HAT to turn it off. I couldn’t find this posted anywhere and although fairly obvious, I wanted to make mention of it. You may also notice that I installed the fan on the opposite side than intended. I did this to make more room for the heatsinks and RTC module. There is just enough space in the case I used for a slim fan.

More details and examples of how to use the LM75 will be posted in the sample scripts.