Control USB-powered devices with a Raspberry Pi

Rasp Pi Cooling Fan

Raspberry Pis have a number of cooling options that use the GPIO (general purpose input/output) pins to control and power external fans. A similar approach allows you to use USB fans. For this project, I used two littleBits fans [4] that I placed on a littleBits mounting plate (Figure 8).

Figure 8: Pi cooling with littleBits fans.

The first step in this fan cooling project is to get the Pi's CPU temperature, which you can get with the vcgencmd measure_temp command and then a grep to extract just the floating-point value of the temperature:

$ vcgencmd measure_temp
temp=45.7'C
$ # Show just the temperature value
$ vcgencmd measure_temp | grep -Eo '[0-9]+.+[0-9]'
45.7

To check whether one number is greater than another, I use the bc (arbitrary precision calculator) command with the math library (-l) option:

$ # Check number1 > number2. True=1
$ echo "33.4 > 36.1" | bc -l
0
$ echo "38.4 > 36.1" | bc -l
1

Now that all the basics are worked out, a simple script (Listing 1) can check the temperature against a high limit every 10 seconds and turn the USB power on and off as required.

Listing 1

Pi Cooling Script

01 #!/bin/bash
02 #
03 # Check the Pi temperature against a temperature high limit
04 # Turn on/off USB power (to fans) as required
05 #
06 tlim="46.0"
07 while :;
08 do
09   # get the temperature
10   tnow=$(vcgencmd measure_temp | grep -Eo '[0-9]+.+[0-9]'
11   # check the CPU temp vs. the limit
12   if (( $(echo "$tnow > $tlim" | bc -l ) )) ; then
13      # CPU temp is above limit, turn on fan
14      sudo uhubctl -l 1-1 -p 2 -a on 1>&-
15   else
16      # CPU temp is below limit, turn off fan
17      sudo uhubctl -l 1-1 -p 2 -a off 1>&-
18   fi
19   sleep 10
20 done

The uhubctl command outputs status messages after it powers the USB ports on and off. For a quiet command, 1>&- can be added at the end of the line.

Other Controllers

A Raspberry Pi can control the power to other controllers. Figure 9 shows a Pi 4 powering an Arduino Uno, an Arduino Nano (clone), and a BBC micro:bit controller.

Figure 9: Raspberry Pi 4 powering other controllers.

For external modules that don't support WiFi or real-time clocks, a Raspberry Pi could be used as an easy way to power these external controllers up and down.

It's important to realize that a Raspberry Pi is not designed to power devices that have a high power requirement. The Raspberry Pi 3 and 4 have a maximum USB port output of 1200mA for all four ports combined (1200mA is available on a single port if no others are in use). This 1200mA limit assumes that the Pi is getting its required input power, which is 2.5A for the Pi 3 and 3A for the Pi 4.

If you are connecting smart USB devices such as memory sticks or third-party controllers, the device manufacturer has a defined MaxPower rating that can be found once the device is connected. The command lsusb -v outputs a very long list of vendor information for all the connected devices. To get just the maximum power for each device on the Raspberry Pi USB internal bus, enter:

lsusb -v  2>&- | grep -E 'Bus 00|MaxPower'

When this command is run with an Arduino Nano, Arduino Uno, and a BBC micro:bit, the total power requirements can be seen on a per-port basis (Figure 10). In this example, the total USB power used is 796mA (0+100+500+96+100+0), which is within the Raspberry Pi specs.

Figure 10: The maximum power on all USB ports.

A Bash command to total the USB bus power requirements for all connected devices is:

$ lsusb -v 2>&- | grep MaxPower | grep -o -E '[0-9]+' | awk '{ sum += $1} END {print "\nTotal= " sum " mA"}'
Total= 796 mA

Unfortunately, simple USB-powered devices such as USB lights and fans use the USB connection strictly for power, so they do not appear in the lsusb output. To find the power requirements for these kinds of devices, you will have to reference the manufacturers' literature.

Final Comments

For home automation projects I prefer direct-wired GPIO pin connections or WiFi devices over USB-powered devices; however, it's nice to know that you have the USB option if you need it.

For kids' projects that use littleBits or micro:bits, a Raspberry Pi as a power source offers a nice, easy way to control or schedule their use.

The Author

You can investigate more neat projects by Pete Metcalfe and his daughters at https://funprojects.blog.

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