USB-PD sprinkler solenoid control has moved from curiosity to genuine engineering exercise, with the team at OpenSprinkler putting four distinct methods through their paces to find out how well USB power delivery can stand in for the 24 VAC supplies that automatic irrigation systems have relied on for decades.
It is easy to forget how deeply 24 VAC is baked into the infrastructure of older sprinkler systems. The solenoids that open and close irrigation valves were designed around alternating current at that voltage, and that assumption has sat largely unchallenged since the technology became widespread. USB, of course, was designed for something else entirely, but USB Implementers Forum standards have expanded its power delivery capabilities substantially, and the OpenSprinkler project is now exploring whether that expanded capability can bridge the gap to legacy hardware.
The Four Methods of USB-PD Sprinkler Solenoid Control
The project examines four approaches to energising the solenoids from a USB-PD source. The first is unipolar PWM, which uses pulse-width modulation on a single polarity to approximate the energy delivery of an AC signal. The second is a dual voltage approach, switching between two DC levels to produce the variation the solenoid expects. Third is synthesising an AC wave directly from a DC supply, which attempts to reconstruct a proper alternating signal from scratch. Fourth, and arguably the most interesting of the group, is the bipolar square wave.
That bipolar square wave method produced results the project describes as getting pretty close to the behaviour of a true sine wave, while keeping demands on the power electronics relatively modest. For anyone who has worked with solenoid drivers, that is a useful combination: sine-wave-like behaviour without the complexity and component cost of a full sine synthesis stage. The other three methods each carry their own trade-offs in circuit complexity, component count, and fidelity to the original AC waveform, and the project works through those trade-offs methodically.
Why USB Makes Sense for Ageing Irrigation Hardware
The appeal of reaching for USB-PD in this context is straightforward. The connector and the supply rails it provides are now genuinely ubiquitous in a way that purpose-built 24 VAC wall warts simply are not, and the cost of USB-PD hardware has fallen to the point where it is a reasonable choice for a hobbyist or small-scale maintenance project. Adapting it to a sprinkler system that was never designed with USB in mind is exactly the kind of pragmatic reuse that keeps older installations running without a full rip-and-replace.
OpenSprinkler is not alone in approaching this problem. The report also points to a separate project that takes a different route entirely, replacing the AC source with a DC one rather than synthesising AC from USB. That DC-direct approach comes with its own caveats, and the two methods represent genuinely different philosophies: one tries to give the old solenoids the AC environment they were designed for, the other asks whether the solenoids can be persuaded to work without it at all.
For anyone maintaining older irrigation infrastructure, or indeed any legacy system still dependent on 24 VAC solenoid valves, the OpenSprinkler work is worth a careful read. The four-method comparison gives a practical framework for deciding which approach suits a given installation, and the bipolar square wave result in particular suggests that close-to-sine behaviour is achievable without heavy power electronics. USB-PD sprinkler solenoid control, it turns out, is not just a novelty: it is a credible, low-cost path to keeping old hardware in service. The full technical write-up, including waveform analysis for each method, is available at OpenSprinkler.
