Sliding Window Opener

Retrofit, not a replacement

Nothing structural changes

The panel it was built for slides horizontally, hangs entirely from a sealed ceiling track with no access to the carriage inside, and has no bottom track or guide. That constraint shaped the whole design: nothing can be added inside the track, so all the hardware hangs off the jambs at either end and the panel is pulled by a GT2 belt clamped to its vertical stile.

The sealed track stays sealed, the panel is never removed or modified, the glass is not touched, and no part of the window is cut, drilled or replaced. The entire intervention is two brackets on the jambs and a clamp on the panel's stile. Both brackets can be bonded with VHB tape rather than screwed, if you'd rather not drill at all.

It also stays a normal window. The driver is left disabled when idle, so the panel still slides by hand with the same effort as before — assist mode picks up the nudge and takes over. Nothing is captive, nothing is geared, and there is no clutch to disengage. Take it all off again and you're left with an ordinary sliding window, which is rather the point: this is a weekend addition to a window you already have, not a renovation.

How it works

Belt, two idlers, and a clamp

The motor mounts vertically to one jamb with its shaft pointing down. A 20T drive pulley sits under the housing; a 16T guide idler and a 20T tensioning idler set the belt path. At the far end, a printed bracket carries a 20T idler on a sliding carrier that provides the main tension adjustment.

The belt is a single open length. Both cut ends terminate in an aluminium GT2 clamp bolted to the panel's stile, so the clamp both joins the belt and is the thing that gets dragged. The loop lies in a horizontal plane with the belt's 6 mm width vertical, which keeps the whole mechanism inside a shallow band just above the panel.

Auto-calibration

Total travel is not hardcoded. On boot the device seeks the closed limit, backs off until the switch releases, creeps back on slowly and zeroes there, then repeats the same two-stage approach at the open end while counting steps. This sidesteps the microstepping question entirely — whatever mode the driver comes up in, the measured count is correct in those units.

The two-stage approach matters: a single fast pass bounces off the end stop, so splitting it into a fast approach, a backoff, then a slow creep at 200 steps/s means the panel is barely moving at contact — it stops on the switch instead of overshooting, and the trigger point becomes repeatable since it no longer depends on arrival momentum.

Drift correction

On every normal arrival at the open limit, actual position is compared against stored travel. Under 10% difference, the stored value is silently updated; over 10%, it logs a warning and leaves it alone. Belt skips therefore heal themselves on the next full traverse.

Bill of materials

What you need

Electronics

Mechanical

Printed parts

PA6 if you're screwing the brackets to the wall — printed at a 70 °C chamber temperature with no warping. ABS or PETG is fine if you're mounting with VHB tape instead, since the load path is then adhesive rather than a screw pulling through the part. The VHB standoff block exists specifically so tape has a flat, properly sized face to bond to — without it, screws are sufficient but VHB is not.

Wiring

Four sub-circuits

Logic (ESP32 ↔ TMC2209 over UART), power distribution (PSU → driver and buck → ESP32), the motor coils, and the two limit switches.

Coil A = green + black, coil B = red + blue; verify ~1–5 Ω per pair. MS1, MS2 and CLK are deliberately left unconnected — on the v1.3 board RX and TX are broken out separately, so no external 1 kΩ resistor is needed. All connections are hardwired.

Driver current. The BTT TMC2209 v1.3 uses 110 mΩ RSENSE. Vref is set to 1.27 V, about 1.4 A RMS. Turning the pot clockwise reduces current.

Assembly

Three sub-assemblies

Build the motor housing, the idler carrier, and the belt clamp separately, then install brackets, route the belt, and fit the tensioning idler last (see How it works).

*Fitted here so you can see where it lands — actually the last thing installed. See Fit the tensioning idler last. Shown as M5 in the CAD; it's an M3 through a 3 mm bore bearing.

Installation

Onto the window

With the three sub-assemblies built, this is where the mechanism actually goes onto the wall and the window. Motor unit on one jamb, idler bracket on the other, belt clamp on the panel's leading stile.

Mounting

Both ends screw to the jambs with 4× M3 countersunk screws each, or bond with VHB using the standoff block. Fix the motor end first, then stretch the belt across and position the idler bracket so the carrier still has adjustment left in both directions.

Belt routing and length

Cut roughly 2.5× the window width. The belt leaves the drive pulley, runs the length of the opening, wraps the far idler, and comes back. Both cut ends go into the aluminium clamp, teeth meshed, and the clamp bolts to the panel stile.

Tensioning

Target 20–30 N. In practice: plucked, the belt should give a dull thud rather than a ring, and mid-span it should deflect 5–10 mm under light finger pressure. Adjust with the two M3 screws on the idler carrier. If that's not enough range, fit the tensioning idler at the motor end (motor unit step 7, "fit last" — see Assembly) and take up the rest there.

Limit switches

Two MC-38 enclosed reed switches, tape-mounted, one near each end of travel. A single magnet on the stile triggers both. Full positioning procedure in Commissioning below.

Commissioning

Power up and calibrate

If it runs the wrong way, flip the Reverse direction switch in Home Assistant. It inverts the sign used throughout and triggers a recalibration, so nothing downstream needs changing.

Limit switches

Two MC-38 enclosed reed switches, tape-mounted, one near each end of travel, with a single magnet on the stile triggering both.

Tuning

Home Assistant entities

cover.window with open / close / stop / position. Sliders for run speed and acceleration. Switches for obstruction stop, assist mode and direction reversal. Buttons to recalibrate and re-home. Diagnostics for position, measured travel and WiFi signal.

Known issues

What's not finished

• Belt skipping at higher speed. Partly tension. Acceleration is also suspected — torque demand peaks at breakaway, which is the start of every move. Both are sliders for live tuning.
• Microstepping mode is unknown. The motor ran about 40 rpm at 750 steps/s, implying roughly 1/8. Auto-calibration makes this moot for correctness, but it matters if absolute millimetres are ever needed.
• Nudge-to-assist is limited. It only fires near the closed end, keying off the MC-38 releasing while the motor is idle; it can't detect a push mid-travel. The proper fix is polling the TMC2209's MSCNT register over UART, which tracks shaft rotation even when the driver is disabled.
• Buck converter whine. The LM2596 pulse-skips under light load. Output is a clean 5.0 V and it's expected to be inaudible once enclosed; if not, add a small load resistor on the 5 V rail or more output capacitance.

Gotchas

• A 3 MΩ reading between coil pairs is open, not a short — that's leakage through the driver chip when probing at its pins.
• The bulk capacitor makes VS↔GND beep on a continuity test while it charges. Wait for it to settle before calling it a short.
• ESPHome rejects initial_value and restore_value together on a template number — the compile fails. Values reset on reboot as a result; once settled, promote them into substitutions.
• The driver must stay disabled when idle (EN high) or the panel resists being pushed by hand, which defeats assist mode. Holding torque also means back-driving dumps energy into the driver.

Ideas not yet pursued

• Weather-triggered auto-close on rain forecast or humidity
• PM2.5-aware opening (relevant in Bangkok)
• A physical button on a spare GPIO as a WiFi-independent fallback

Downloads

Everything, to build your own

Designed by Pawin Jirasuntornsiri. Firmware MIT licence, hardware and documentation CC BY 4.0. Share it, remix it, sell it if you like — just say where it came from.