Decoding USB PD with Color-Coded LEDs

The USB Power Delivery (PD) specification has turned USB ports into a fast-charging powerhouse capable of delivering up to 240W of power. This capability is rapidly turning USB PD into a universal charging standard for everything from smartphones to high-performance laptops. This sounds perfect on paper, but in reality, USB PD power supplies aren’t always everything they're cracked up to be.

One of the nice features of the standard is that the charger and device transparently communicate with one another to determine the best voltage to operate at. We just plug the device in and everything magically works—usually. However, some power supplies do not negotiate the correct voltage level with a device. Or more commonly, they do correctly determine the best setting, but fail to deliver the voltage that was agreed upon.

This can not only lead to slow charging sessions, but can also cause damage to the devices being charged. With this problem in mind, Meticulous Technologies has developed a monitoring device called USB VSense. It connects between the charger and device, and provides an easy-to-read display that shows the negotiated voltage level. It also has indicators that let you know if that voltage is being faithfully supplied, or if there is a meaningful deviation.

Rather than relying on a numeric display or a companion app, USB VSense uses a simple but effective array of color-coded LEDs. Each LED corresponds to a fixed USB PD voltage level, ranging from the standard 5V all the way up to the latest 48V Extended Power Range mode. Subtle blinking patterns indicate whether the measured voltage is within 5% or 10% of the expected value, making it possible to spot undervoltage or overvoltage conditions at a glance.

Drawing less than 6 mA under normal operation, the device can be left plugged into a hub or charger indefinitely without affecting performance. This also makes it useful as an early warning system: if a hub or power supply becomes overloaded or unstable, the LEDs will immediately reflect the change. The device supports input voltages up to 55 V, allowing users to safely verify potentially hazardous USB PD sources before connecting expensive hardware.

Internally, USB VSense is built around an ATtiny406 microcontroller, using its 10-bit ADC and internal voltage reference to achieve millivolt-level resolution across a wide measurement range. It passes through USB 2.0 data, configuration channel, and sideband signals, ensuring compatibility with normal USB operation. The compact, stacked-PCB construction results in a rugged tool that measures just 56 mm long and requires no enclosure beyond the circuit boards themselves.

As an open source project, USB VSense also appeals to developers and hardware hackers. Both the hardware design and firmware will be available for modification, allowing users to customize voltage thresholds, LED behavior, or even repurpose the device for other USB power experiments. USB VSense sells for $65. If you’d like your own, check out the full details over at Crowd Supply.