Let's Make Robots!

IR Homing Beacon?

I'm looking to build an Beacon, IR or otherwise, that a robot can see and simply drive to. Pretty much like a docking station for the robot to be recharged or a robot that follows you. (unless that's something different...)

I did a few searches, but I've only come up with robots that use a beacon and not how to build the circuit and write the code.

Any help? 


Edit: 8.31.09


I don't know if this helps, but I'm pretty much looking to build what CtC did here: http://letsmakerobots.com/node/6499

He talks a bit how he does it, but I don't really understand, so I'm just going to have to do more research on how works in general. Like how different frequency signals are encoded, sent, received, and decoded. 

Am I on the right track?

Does this have anything to do with IR serial communication?  

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An IR flashlight? How dare you, belittle me! :P

I've stumbled upon that, but isn't there some frequency it should be oscillating at? What about the receiver? Coding?


Did you ever use a 555 to modulate your beacon, or is it just constantly on? What is the range on that thing?
There are some ideas but no part numbers or code in the old Pololu IR Beacon guidesheet.  Their new transceiver beacon is a bit pricy, but probably not overly so based on the parts that appear on it.

"If detectors are responding above an appropriate threshold, the most responsive detector direction is determined"

That's what is said in the Pololu IR beacon datasheet, but how does it do that? I mean, since light bounces all over the place it might happen more than once that several sensors get hit.... If we understand how it works we could make a similar setup and spend less.  

I'd use my ir flashlight (continuously on) and 2 ir photodiodes on the 'bot to home in on it. just check which one sees more IR and have it turn until both see equal ir...

possibly you could make it pulse with a cap. to reduce IR noise.

by the way, does anyone know how to tell a white LED from a photodiode short of hooking it up?

that's the point...how do you check which one sees more modulated IR?
these little buggers had resistance that dropped as it saw less IR. so all you gotta do is check the resistance on them and cross your fingers...?

Photodiodes don't work the same way as photoresistors; they're much more like NPN transistors but instead of feeding current into the base to up the CC current you feed in IR light to increase the current.
If you put one of these in series with a resistor tied to V+ or Gnd you can read the voltage from the resistor and calculate back to the through current, which is itself related back to the amount of IR landing on the photodiode receiver.

The issue here is that ALL incoming IR will affect the photodiode, not just modulated. Strong heat sources and the Sun both output significant 'DC' or 0Hz IR, and household lighting typically produces a fair amount of 50/60Hz IR interference too. These ambient IR sources swamp the signal from any modulated IR source that isn't powerful enough to overcome them, which makes reading the signal nearly impossible. Typically you'll need either a very sensitive ADC to read the signal, which you then filter electronically, or you'll need an analog filter to attenuate any signals significantly below your intended broadcast frequency. If you're broadcasting at a frequency below the standard ~38kHz then it can also be useful to filter out signal above ~36kHz so your system is not affected by IR remotes, etc.

Filters...that sounds like the thing we needed! I'll search for them on the web and tell you what i come up with. Thanks for the information!