Jakub Jirůtka | InstallFest 2026
Electronic door signs
at FEL CVUT
System engineer and developer
Works at FEE CTU in Prague
FOSS developer & contributor
Alpine Linux developer
$~ whoami
Source: https://lezebre.lu/en/sticker-simons-cat-claws/
@jirutka
@jakub@jirutka.cz
jakub@jirutka.cz
Source: ChatGPT 5 (DALL-E 3)
Source: ChatGPT 5 (DALL-E 3)
Cat by: ChatGPT 5 (DALL-E 3)
- expensive updates (paper, ink, and human labour)
- doesn’t display non-periodic events
- block teaching
- exams
- reservations
- “what’s happening now” lookups are not O(1)
Limitations
- display complete, up-to-date information
- lessons, exams, reservations, study room
- display what’s happening now / in <15 min + today
- remote control
- two power options:
- over Ethernet (available in newly renovated rooms)
- battery → LCD display is not an option
- sufficiently large and legible display
- frame printable on a standard 3D printer (e.g. Prusa MK3)
What we wanted
E-ink display
- electronic ink / electronic paper
- mimics paper, doesn’t emit light
- microcapsules with B&W particles move under an electric field
- consumes power only when
the image changes - slow refresh rate (seconds)
- limited colours
First prototypes
First lessons learned
- 7.5” displej is too small
- standard PoE (Power-over-Ethernet) is problematic:
- PoE converters are quite large
- RJ45 is quite large
- PoE emits a lot of heat (48 V → 5 V)
- Ethernet cable is firm
- standard electrical installation box is small
- gesture control doesn’t work well with e-ink
- long delay is confusing for users
Source: ChatGPT 5 (DALL-E 3)
Cat by: ChatGPT 5 (DALL-E 3)
- 10.2” e-ink, 2-bit grayscale
- ESPink v2 (ESP32 board)
- (10 Ah LiPol battery)
- custom firmware
- custom 3D printed frame
Final version
-
ESP32-WROOM-32 16 MiB
- Xtensa dual-core 32-bit LX6 240 MHz
- 448 kiB ROM / 520 kiB SRAM / 16 MiB SPI flash
- 802.11b/g/n (WiFi), Bluetooth V4.2 & BLE
- EPD connector (SPI)
- USB-UART converter
- LDO regulator (3.7–8 V)
- battery charger
- USB-C for programming and power
-
open hardware
- https://github.com/LaskaKit/ESPink
ESPink v2.5
Power Supply
Battery
- 10 Ah LiPol
- last for ~12 months
- can be installed anywhere
Power over Ethernet
- passive PoE 7 V
- last “forever”
- requires Ethernet
Frame
Frame
Inspired by Vlad Waas’ 7.5” Bauhaus frame for Živý obraz.
- reverse-engineered from STLs in FreeCAD
- scaled for 10.2” display
- two snap fits replaced with screw clips
- adapted for wall mounting
- added spacer corners
- added side reset switch
- many small adjustments
Frame (assembled)
Top part (bottom view)
Bottom part (top view)
Middle part
Architecture
“Timetable API”
Door Signs Controller
Grafana
Prometheus
MQTT broker
HTTP server
NTP server
HTTPS
MQTT
HTTPS
NTP
MQTT
firmware → (OTA update)
HTTPS
PNG, settings →
← telemetry, logs
metrics
Energy saving
Controller sends the image to be displayed, along with a timestamp indicating when the next update is expected.
ESP wakes up, renders the image, sets the timer and goes into deep sleep.
- in C/C++
- based on Arduino and ESP-IDF
- used libraries:
- jirutka/GxEPD2_4G
- Arduino display lib for SPI e-inks with 2-bit grayscale
- fork with support for GDEM102T91
- pngle – stream-based portable PNG decoder
- PubSubClient – not so good MQTT client for Arduino
- ArduinoJson – JSON (de)serializer for Arduino/C++
- jirutka/GxEPD2_4G
Firmware
Monitoring
Source: ChatGPT 5 (DALL-E 3)
{
"ts": 1761483972,
"wakeTs": 1761483960,
"rstTs": 1760803498,
"rstRsn": "SW",
"nextWakeTs": 1761494760,
"awake": 2336,
"chip": "ESP32-D0WD-V3",
"fw": { "ver": "0.2.0", "size": 1018608, "freeSpace": 1310720 },
"disp": { "w": 480, "h": 800, "clr": "4g" },
"heap": { "total": 273172, "free": 180408, "minFree": 168848, "maxAlloc": 110580 },
"net": {
"mac": "D4:8C:49:CA:FF:EE",
"ipv4": { "addr": "192.168.123.42", "mask": 24, "gw": "192.168.123.1" },
"dns": [ "192.168.123.1" ],
"iface": "wlan",
"ssid": "fel-XXX",
"bssid": "08:96:AD:CA:FF:EE",
"chan": 1,
"rssi": -72
},
"battVolt": 4.22,
"battPct": 100
}Telemetry from door signs
Grafana
Grafana
Grafana
Grafana
Grafana
Fuckups Challenges
- ESP brownout
- RTC on ESP32 is inaccurate
- 5 V over PoE wasn’t enough → 7 V
- WiFi fast scan
- PubSubClient doesn’t support QoS ≥1
Why not “Živý obraz”?
We have a specialised use case with different requirements.
Also…
- firmware code-quality 🤢
- custom image format 🤨
- no support for observability (telemetry, remote logging)
- server is closed-source(?)
What’s next?
- replace PubSubClient (QoS >0)
- optimise WiFi – full scan only on first connection
- cache logs in Flash memory
- management UI
- deploy on more faculties, universities…
Q&A
Source: ChatGPT 4 (DALL-E 2) trained on art stolen from the Internet
Electronic door signs (IF 2026)
By Jakub J.
Electronic door signs (IF 2026)
At the Faculty of Electrical Engineering (FEL) of the Czech Technical University (ČVUT), we have started replacing paper classroom schedules with electronic paper. The new door signs display the daily timetable as well as what is currently taking place in the classroom. They are built on the open-hardware ESPink board from a well-known Czech manufacturer, feature a 10.2” e-ink display, a battery (or passive PoE), and a 3D-printed frame. We designed the firmware, the frame, the control software, and monitoring ourselves to suit our specific needs. The door signs communicate via Wi-Fi and MQTT – they receive images to display from the control server and send back telemetry data.
