Missing Link Research Notes – Updated 29/3/11

This is a working document and a sketchpad for information and links that will be updated as I go along. The various sections will be tidied up and made into separate documents.

The Missing Link is an interface black box that will allow the communication and synchronization of multiple digital still cameras, enabled with the CHDK software, such that they can simultaneously capture time-lapse photographs.

Technical Brief

The Missing Link (MLink) will be a discrete electronic device that can run on batteries or low-voltage DC from an external power supply.

The MLink will have USB in/out, RS232 serial, Infra-red / IrDA interfaces and possibly I2C, 1-Wire and CAT5 Ethernet.

It is intended as a multi-protocol interface box that can be used to synchronize the capture of stills on multiples Canon PowerShot still cameras, enabled with the Canon Hack Development Kit (CHDK) software. CHDK allows the triggering of some Canon PowerShot cameras via their USB bus.

Interface Technologies



SPI – Serial Peripheral Interface


I managed to find a link to a proof-of-concept design to link a PICAXE to Ethernet using a serial-to-USB adaptor and an Asus WL-520gU router. Somone also is using a Linksys WRT54G access point / router.

This uses OpenWrt. Problem is, I have no idea whatsoever what all this means. This is the biggest hole in my knowledge, the ethernet TCP/IP business.

Although this looks like it could be asolution, it looks as though it would require too much of a hardware / cost overhead for what I have in mind, which is a very simple trigger device.

Also found a reference to “SimpleLAN”

Industrial Serial-to-Ethernet

At first glance, it looks as though it might be most efficient to use ready-built, industrial Ethernet-to-serial units, although I still need to research how to address them.

There is a net server module available for the PICAXE platform but it is a bit of an overkill and would make the unit cost too high. Also, what I am initially trying to achieve is only an on/off trigger at the far end. Maybe the sensible thing to do would be to create a Missing Link box with an optional serial-to-ethernet add-on, rather than including it in the basic design.


Serial Ethernet Proto Board

Ethernet Connector Board

Microchip ENC28J60 Ethernet controller with SPI interface



RS232 / RS422 Serial

Infra Red (IR)


In 2005 I did a did a lot of research on PIC microcontroller technologies.  The project I was working on did not happen and I moved onto other work in the meantime. Six years later, when I come to look at microcontrollers again, I find the world has changed quite a lot. There are many excellent, freely available resources and several hardware platforms to choose from.

Resistor Colour Codes

Voltage regulation

Solar Charging


For this project I will probably be basing its physical design on the PICAXE system. This is an education oriented hardware platform using Microchip PIC microcontrollers that can be bought pre-loaded with loader software that allows them to be re-programmed in-situ. Historically, flash ROM microcontrollers had to be re-programmed in a dedicated hardware programmer. This innovation allows the custom software to be updated after the chip has been installed in the circuit.

There are a number of other excellent development platforms but I favour the PICAXE system because it has a very low unit cost and can be easily programmed in-situ. The hardware overhead for my project should be very low and I would like to make the resulting design as small and power-efficient as possible.

A comparison of the different PICAXE chip is here:

The tiny 8-pin chips will make extremely compact and low-power units but probably do not have enough in/out pins to support all the interfaces I want to include. However, it might be desirable to create more than one model of the MLink to enable an extremely low-cost receive-only version that will support USB in (1 output pin + ground), USB out (1 output pin + ground), IR (1 input pin + ground + positive) and serial (1 input pin for Rx, 1 output pin for Tx + ground). This scenario require 5 in/out pins and leaves none spare. An indicator LED is desirable for some visual feedback.

At this stage I am still inclined to implement as small as possible an it might be possible to make a unit that includes 2 x 8-pin PICAXE chips to gain more in / out pins and take advantage of the convenience of using ready-made circuit boards.

There is also a low-profile surface-mount version of the PICAXE 08 Proto Board which is extremely compact and could be used as the basis for a mass-produced version.


I am also interested in the Arduino platform. This is an open-source design that can be copied. For this R&D project I am not interested in designing all the PCBs from scratch so am planning to use the pre-designed boards. The smaller ones interest me as I want to make a small, battery-powered device.

Cool Components supply a box for the Arduino with the Ethernet Shield add-on.

Advertised dimensions are 87x64x28mm which may well be small enough. I’m torn between the apparent convenience of the Arduino platform and the lower cost and smaller projected footprint of PICAXE-based modules. However, if I calculate minimum costs, the basic components are going to be a minimum of £50+ per unit.

Arduino Uno £20.58

Ethernet Shield £35

Case £8.10


Adafruit Industries make a range of Arduino clones.

Sparkfun Electronics



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