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I would like to be able to output 75 digital out signals from the Rasberry Pi. However, the Rasberry Pi only has 26 GPIO pins. Precise timing is not critical as long as it is within 1 second. I need to be able to turn all the signals on or off at the same time, so charlie-plexing won't work.

I'd imagine such a thing would be possible with another module connected by USB but I was wondering if there would be an easier way to do this using the existing hardware?

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3 Answers

up vote 4 down vote accepted

The best solution to this is probably to go for a 'port expander'.

There is an expander kit available from HobbyTronics in the UK: http://www.hobbytronics.co.uk/mcp23017-port-expander-board

or a more do-it-yourself solution here: http://dbyz.co.uk/?p=98

For a general briefing on port expanders, here's a page on Wikipedia: http://en.wikipedia.org/wiki/Port_expander

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It is possible to expand up to 64 additional GPIO ports using some MCP23008 chips connected via I2C interface. Each adds 8 GPIOs and up to eight can be connected through I2C. As you need 75, I suppose you go with the big brother of MCP23008 the MCP23017 which has 16 GPIO ports. It's not possible to mix both and get over 128 as the limit of 8 on the I2C-Bus applys to both together because the addresses overlap.

For ease access to the I2C-Bus you could use the I2C-Python-API from Quick2Wire. Quick2Wire has also a post on the chips.

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Take a look at this answer where MAX7219, MAX7221, MAX6950, MAX6951 and TLC5940 were discussed as an option for similar problem.

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