Bluetooth lightmeter

A darkroom incident-light meter you can build yourself, read by the Photo Paper Profiler app.

Guide in preparation. The step-by-step guide will be published after the second prototype has been tested.

How it is made

A digital light sensor VEML7700 (the same physics as the 2022 Arduino instrument of this project), a Seeed XIAO nRF52840 module that talks Bluetooth LE to the iPhone, a LiPo battery charged from the USB-C port, a slide switch and a 3D-printed case.

There is no display and no button: the instrument is controlled by the app, and a slide switch in series with the battery turns it on and off. No LED should light in the darkroom, so take care with the charging LED of the board while the USB-C cable is connected.

What the app does with it

The Lightmeter page offers the ASA, LUX and Tf modes, single and continuous (Live) readings, sending the EV200 to the points of the calculation, the battery voltage and charge, and the firmware version of the instrument. In dim light the instrument waits as long as needed for a reliable reading. See chapter 6 of the manual.

What the guide will contain

• the list of parts;
• the wiring and the assembly of the case;
• the 3D files of the case;
• the firmware and how to load it;
• bench checks of voltages and currents, with the expected values;
• calibration against a reference lightmeter.

Safety first