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I'd like to make a beacon system for robots where the beacon will have some IR LEDs around it pulsing at some known frequency. The module that hooks up to the robot will have a few IR photostransistors (are photodiodes better?) to detect direction. I'm thinking I'll have an MSP430 as the brains for each and speak I2C or TTL to indicate direction on the robot side. How do I detect that specific frequency so I don't mistake sunlight, etc. as the signal? How sensitive would this be? Do I need really powerful LEDs? What are all the considerations?

I want something like a 1 way version of this pololu beacon except one way (although two way might be a good idea too).

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Consider these receiver "phototransistors". They automatically decode a 38 kHz signal, which is what most remote controls use. Because it is pulsing it is virtually immune to ambient IR light including sunlight, although there is a limit as to how much ambient light they can reject. There are plenty of code examples and projects on the net which send out 38 kHz signals to control TVs, DVD players, etc. but the idea is applicable to IR beacons; just choose a code/sequence which won't interfere with other products.

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    \$\begingroup\$ And the datasheets show the basic idea of how you would implement this if you were building it yourself. An amplifier followed by a band-pass filter for the frequency you want, demodulator, etc. \$\endgroup\$ – endolith Dec 14 '10 at 20:22
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You could do this cheaply by repurposing several old TV remote controls. Then, you can just use an RC5 decoder on the robot.

Your beacon could be 4 TV remotes each with a different button glued down.

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