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Mimimalist Light Meter

    

User manual

The user manual is here.

Overview

This is a simple ambient light meter using a Liteon LTR-303ALS light sensor and an EFM32 Tiny Gecko ARM microcontroller. It is powered by a single CR2032 coin cell. The firmware is written in C and the PCB is designed using KiCad. Readings are displayed using a circle of 24 LEDs, with two additional LEDs indicating ±⅓ stop adjustments. The user interface consists of three touch sensitive pads.

The goal of the project is to make available a simple, compact and inexpensive light meter suitable for analogue photography.

Dimensions: 87×56x7.5mm

The bottom side of the PCB (the side with no components mounted) is intended to be used as the top panel of the case. This works best with a standard 1.6mm PCB thickness.

The case is designed for 3D printing. It's best to use multi-jet fusion, as PLA and ABS are usually not very light tight. (You could use PLA/ABS and then paint the inside of the case black.) The STL file and Fusion 360 design for the case are in the case directory. The PCB attaches to the case using five M1.6 4mm countersunk screws. The dimples under the LEDs should be painted white and then given a gloss varnish. A 10×10×2mm piece of transparent acrylic should be glued in place over the sensor window. (If you don't add this, adjust WINDOW_ATTENUATION_STOPS in config.h.)

The device functions as a reflective light meter. The same design could be adapted for a spot meter or incident light meter by placing a lens or diffuser over the sensor.

The LTR-303ALS integrates over a period of 50-400ms, which is far too long for effective flash metering. A different design would therefore be required for this purpose (probably using a discrete photodiode).

Build notes

See firmware/Readme.md for notes on the dev toolchain.

Note that a recent (as of June 2020) development version of KiCad will be required to open the board and schema files. Version 5.99.0-3928-g3c8396cd0 is known to work. Each component in the schematic has a MAN (manufacturer) and MPN (manufacturer part number) property.

I recommend using the interactive BOM plugin for KiCad to aid manual placement of components.

Known issue

  • LEDs a little dim in bright daylight.

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