Power Point

For home automation projects, a Raspberry Pi offers a simple, low-cost approach to managing and controlling a wide variety of devices. Typically these devices are either digitally wired 0-5V devices such as motion detectors, or wireless Ethernet devices such as smart plugs. It's important to note that a Raspberry Pi can also control USB-powered devices, such as USB fans, lights, and low-end controllers.

In this article, I look at how to monitor, control, and measure USB power in two Raspberry Pi projects. The first project uses Node-RED to create a web dashboard to monitor and control USB lights. The second project turns on USB cooling fans according to the Pi's CPU temperature.

Controlling USB Ports

A number of techniques allow you to control USB ports, and I found that one of easiest approaches is to use the uhubctl [1] utility, which lets you view and control local USB ports and ports on smart USB hubs. To load this utility, enter:

sudo apt-get install libusb-1.0-0-dev
git clone https://github.com/mvp/uhubctl
cd uhubctl
make
sudo make install

Figure 1 shows the output on a Raspberry Pi 4 with no USB devices connected. The Pi 4 has two internal USB hubs: Hub 1 connects to all the USB ports with the USB 2.10 standard, and hub 2 controls all the ports with the USB 3.00 standard and the Ethernet jack.

Figure 1: USB power status with uhubctl.

For the Raspberry Pi 3 and 4, the power on all USB ports is ganged together through port 2, so unfortunately it is not possible to power up and down an individual USB port.

The commands to turn on or off or toggle the USB ports and keep the Ethernet jack powered are:

sudo uhubctl -l 1-1 -p 2 -a on
sudo uhubctl -l 1-1 -p 2 -a off
sudo uhubctl -l 1-1 -p 2 -a toggle

These commands return messages showing the current status, the power requested state, and the new status.

Monitoring USB Power

The uhubctl command lets you check the status of Pi port 2, the ganged power port (Figure 2). With some Bash statements, the power status is parsed to show just the off or power message. The Bash statement

$ sudo uhubctl | grep 'Port 2' | awk '{print $4}'
off

Figure 2: Monitoring port 2 for Pi USB power status.

shows the power status on a Node-RED dashboard.

Node-RED USB Control Dashboard

Node-RED [2] is a visual programming tool included with the full desktop Raspberry Pi install. If Node-RED has not been installed, see the online docs [3].

A number of low-cost USB lighting options can be used with a Raspberry Pi (Figure 3), including LED strips, wire lights, and small USB lamps. Node-RED doesn't have a node to monitor or control USB power, but Bash commands can be used directly in Node-RED.

Figure 3: Wire light string and LED lamp on a Raspberry Pi.

A simple Node-RED dashboard can be created to turn Raspberry Pi USB ports on and off and check the status of power on these ports. The logic (Figure 4) would include two dashboard button nodes, one dashboard text node, and two exec nodes. The uhubctl utility can be used directly in the exec nodes.

Figure 4: Node-RED logic to control and monitor Raspberry Pi USB ports.

The first exec node contains the Bash command to turn the USB ports on or off (Figure 5). The on or off string is sent from the dashboard buttons as a msg.payload message that is appended to the command in the exec node. The output from the first exec node triggers the second exec node to get the latest USB port status.

Figure 5: Use buttons to pass on and off messages to a Bash USB power command.

The USB power status message can be made more presentable by editing the Value format field in the dashboard text node. For this example, I used an <h1> heading and uppercase formatting (Figure 6).

Figure 6: Change formatting on dashboard text.

Once the logic is complete, the Deploy button on the right side of the menubar will make the dashboard available to web clients at: https://raspberry_pi_address:1880/ui. For this project, I added an enhancement to include a countdown or sleep timer (Figure 7).

Figure 7: Node-RED dashboard to control and monitor USB power.