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

It is essentially a clever Vishay detector chip which receives a coded signal embedded on a 40kHz carrier, then amplifies and low-pass filters the output. All of the audio information is encoded on the transmitted signal.

[The Receiver] 2

Desiderata

As discussed above3 the detector will be built by people who might have very little experience of building electronics. Accordingly the components in the design all look different to reduce the chances of someone e.g. confusing resistors of different value.

Schematic

[Schematic] 4

There is little say about the circuit.

Sensor

The sensor has significant output impedance, and R1 to 100kΩ was chosen to feed about 1V into the op-amp.

Op-amp

The op-amp isn’t critical: indeed although the schematic calls for an OP-07, I managed to find a cheap source of NE5534s and used them instead. Either way, it is configured as a unity-gain Sallen-Key5 low-pass filter with a cut-off at about 1.45kHz and a Q of 0.5.

The cut-off frequency probably isn’t ideal, but I wanted to use only one resistor value, so the only freedom was in the choice of capacitor. Happily not all capacitors look the same, so it was easy to choose a new dielectric and thus a new value.

PCB

Most of the components are mounted on the top of the board. By mounting the battery below it can be used as a convenient stand or handle. The sensor is on the underside to shield it from the sun: I have no idea if this is necessary or not.

Assembly manual

I used Jaroslav Malec’s pcbdraw6 plugin for KiCad to generate an instruction manual. I wrote some hacky code around pcbdraw, which isn’t really fit for public consumption. Do contact me if you’re interested though.

[Schematic] 7

The reference to ‘The Bloomsbury Group’ is related the geocache puzzle: please ignore it.

Design Files

All the design files can be downloaded from GitHub8. The electronics was designed in KiCad. Random bits of Haskell and python were used too.