Boop

Boop

A small board that lets ESPHome press the power and reset buttons on a PC, and tell whether the PC is actually on.

It carries a Seeed XIAO ESP32-C3 on sockets. Two relays sit in parallel with the front-panel power and reset buttons, so the real buttons keep working. A third input watches the power LED header so Home Assistant knows the state instead of guessing.

Status: designed and checked (ERC clean, DRC has no errors, JLCPCB's pre-flight passes). The first board has been built and soldered, and it works.

How it works

D2 (GPIO4) -> 150R -> opto -> relay K1 -> in parallel with the PWR button
D3 (GPIO5) -> 150R -> opto -> relay K2 -> in parallel with the RESET button
D1 (GPIO3) <- opto <- motherboard PWR LED header (low = PC is on)

• Each relay coil is switched by an LTV-817S opto (rank D, so there is enough gain to drive a 23 mA coil) with a 1N4148W flyback diode.
• The LED sense side is an opto with a 470R series resistor and a 10k pull-up to 3V3. A small diode across its LED protects it if the wires are swapped.
• The board is fed 5 V either from the motherboard USB header (through a Schottky, so nothing flows back into the PC) or from the XIAO's own USB-C. If both are connected the higher one wins. One thing to avoid: the XIAO's 5 V pin is wired straight to USB VBUS, so if the header feeds the board while the USB-C is plugged into a computer that is switched off, the board pushes 5 V into that computer's port. Unplug the USB-C when you are done flashing.

Heads up: for the PC to be startable from off, the motherboard USB 5 V has to stay alive in shutdown. Many boards turn it off unless you enable something like "USB standby power" or disable ErP in the BIOS. If yours cuts it, power the board from the USB-C instead.

Connectors

All the 2-pin headers are 2.54 mm. The left column is always the motherboard side and the right column is always the case side, so it is the same on every pair.

The relay contacts are volt-free, so polarity on the button headers does not matter. The LED pair does have a polarity, follow the +.

Four M3 holes in the corners for standoffs.

ESPHome

The full config is in boop-esphome.yaml. Pins are the XIAO ESP32-C3 D-numbers: D1 = GPIO3, D2 = GPIO4, D3 = GPIO5.

It exposes three buttons to Home Assistant (power tap, power hold for force off, reset) and a "PC is on" sensor. The relays are internal and always start off, so a reboot can't hold a button down.

Create a secrets.yaml next to it with wifi_ssid, wifi_password, boop_api_key (openssl rand -base64 32), boop_ota_password and boop_ap_password, then flash the first time over USB:

esphome run boop-esphome.yaml

If the board isn't detected, hold BOOT, tap RESET, release BOOT. Unplug J1 (the motherboard 5 V) while flashing over USB-C. Later updates go over Wi-Fi.

Building it

Everything you need for JLCPCB is in fab/.

Things worth knowing

• The XIAO ESP32-C3 uses an external antenna. A metal PC case eats Wi-Fi, so route the antenna out of the case.
• The relays are the 4.5 V coil version (G6K-2F-Y DC4.5), on purpose. The coil only sees the 5 V rail minus the Schottky drop and the opto's saturation voltage, which is about 4.0 to 4.3 V at the USB minimum of 4.75 V (the opto's saturation voltage at 20-odd mA is not in the datasheet, I assumed 0.2 to 0.4 V from the curves). A 5 V coil needs 4.0 V to pull in, which is too thin; the 4.5 V one pulls in at 3.6 V. Worth measuring the coil voltage on the first board.

Licence

Copyright 2026 Dejan Noveski. Released under CC BY-SA 4.0, the full text is in LICENSE. In plain words: build it, sell it, change it, as long as you credit me and share your changes under the same licence. That covers the schematic, the board, the Gerbers, the script and the ESPHome example.

The exception is lib/, which holds trimmed copies of KiCad's stock libraries. Those keep their own licence, see lib/NOTICE.md.