Google Wall Calendar

March 28, 2025

I have been working on a new wall calendar that stays in sync with my family's Google Calendars.

If you are thinking that looks like a regular wall calendar: you are correct! That is how it looks 99% of the time but it comes alive when the wall calendar is out of sync with our Google calendars. When there has been a change on a date, a red square is projected onto the wall calendar date to show me that something might need to be written there.

I check my phone to see the notification and then update the wall calendar, or clear the notification if doesn't need to be seen all the time. Then the red square automatically goes away and the wall calendar is back to being a normal calendar.

I like the wall calendar because it gives me things Google Calendar does not:

• Passively helps me see events coming up in the next couple weeks
• Is handwritten by my family. We can draw pictures, draw deadlines in big text or write minor things small.
• Aesthetically it's just a wall calendar. I find the “tablet with calendar app open mounted to the wall” too cold to have in my general living space.

My family needs to use Google Calendar to manage events on the go and get reminders and this system is a way to keep doing that while also having an accurate wall calendar.

Hardware

The projection and sensing comes from a Raspberry Pi + RPi Camera 3 + Projector on top of a nearby bookshelf. The best part is the projector: a tiny MEMS Laser projector (Ultimems HD305D1-C1) that is completely silent. This whole package is small and hidden enough that no one knows there is a tech involved with the wall calendar.

Software

• Script that checks for changed calendar events every 5 minutes
• Web app to see and clear suggested events to write on the wall calendar.
• Mini Web Apps for doing a manual four point perspective calibration for the Projector and Camera. This was a simple approach because all objects are fixed in place so I calibrated everything a month ago and haven't thought about it since.
• Script running on Pi to projection map the red highlight squares on the wall calendar.

Computer Vision Challenges

I really wanted to be able to get prompted to add a Google Calendar event when I wrote something on the wall calendar, but I have yet to get that to stable enough to leave running all the time. The vision system works well for short periods spans of hours, but leaving it on for a day always results in a couple errors in detection and meaningless notifications.

Challenges include:

• Lighting changes throughout the day and night.
• Camera set to use auto focus and auto exposure (generally keeps lighting levels consistent but can momentarily be out of focus or badly exposed)
• Low camera is low resolution
• handwritten marks are often low contrast
• People walk regularly walk or stand in from of the camera
• Bumping the bookshelf that the camera+projector are on can subtly move the system enough to calibration inaccurate
• Sometimes light reflections are visible from the whiteboard's surface.

I have been building up my library of sample images that I validation vision pipeline changes on but need to make more progress here.

Link to the code on Github: github.com/jhaip/calendar-sync

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