An IR transmitter for testing

These are brief notes on an IR test transmitter designed for use in my To the Birdhouse geocache¹.

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Desiderata

• This emitter has to flash an infra-red LED at 40kHz modulated with both a 800Hz tone, and the dit-dah patterns of Morse Code.

• We need a push button to select different Morse messages.

• It will typically run on someone’s desk, so USB power is convenient.

• Beyond this, it should be cheap and simple for me to build.

Hardware

At the time I designed it, it was cheaper to buy Blue Pill STM32 dev boards than the bare processors, so I designed a daughter board with a couple of LEDs (one IR, one visible), a current limiting resistor, and a push button.

The daughter board fits over the progamming header on the Blue Pill. A little finesse is needed to avoid blocking the header.

Schematic

As you'll see the schematic is almost trivial:

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PCB

The PCB is simple too. The two unplated holes allow the user to see the status LEDs on the Blue Pill.

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Firmware

The STM32F103 on the Blue Pill is perfectly fast enough to drive the LED with simple bit-banging. There is nothing clever about the code.

• All of the action happens during the sys_tick interrupt which is called every 12.5μs.

• The data for the Morse messages is defined in morseout.c which is generated by morsetool⁵. Morsetool lets you defined multiple messages, and the push button cycles through them.

• libopencm3⁶ is used to initialize some of the hardware; the source is laid out in the libopencm3 way.

Design Files

The firmware and PCB design can be downloaded from GitHub⁷. The electronics was designed with KiCad, the firmware is written in C and uses libopencm3⁸. Random bits of Haskell and python were used too.

Written by Martin Oldfield on 23 Feb 2022.
Contact: Martin Oldfield, ex-atelier@mjoldfield.com
This work is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.