flourescent effect

vla7

I built·a robot that included a ir emitter and sensor.· The robot worked perfect at my house but when I took it to work it behaved very sporadic.· I determined that it must be either the light sensors (the kind that when you walk into a room and the light automatically turns on) or the flourescent lighting or both.· Has anyone ever had this problem and if so can you explain what is going on and how to fix the problem, (ie, is there a type of sensor out there that is not effected by flourescent light).

Mike Green

If they are compact fluorescents, then the frequency they operate at is very close to the frequency used by the IR emitter and sensor (around 38KHz). You can replace the IR sensors with some that use a very different frequency and adjust the program to use a different IR emitter frequency that matches.

MovieMaker

How would you do that, Mike? If you send out a different frequency and recieve it, it will also work? I did not know that. Am I understanding right? This could be the answer to all of my IR problems.

erco

Pololu has a 56 kHz module at http://www.pololu.com/catalog/product/837 that might provide better noise immunity than the usual 38-40 kHz units. This unit claims to be "continuous signal" compatible, which is nice if you want to use it to receive a simple·unmodulated 56 kHz carrier signal, such as from a 555 IR beacon.

I have not tried this 56 kHz module myself, but it looks just like the Waitrony 38 kHz modules I use regularly and are easy to connect to a Stamp.·However, I can't say for certain if the BS2 can send a 56 kHz pulse. I know the BS2's ability to output a 38 kHz pulse only works due to harmonics.·I'll defer to·tech gurus·Mike, Allan, Chris,·& PhiPi for comment here.



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·"If you build it, they will come."

Mike Green

The BS2 can't put out a frequency of 38KHz normally and actually uses the 2nd harmonic of 19KHz to produce the modulated IR. For 56KHz, you'd probably use the 3rd harmonic of 18.66KHz or 4th harmonic of 14KHz.

erco

Another option to consider is the Sharp IS471F for $2 at http://www.junun.org/MarkIII/Info.jsp?item=46·. This 4-pin module has an output to drive an IR LED at the proper frequency (I make it to be ~7.7 kHz). So it drives its own LED, freeing up the Stamp to do other things. Looks interesting, but I have not used it myself. At that price, I'll order a few and investigate.

There is a similar item, the Hamamatsu S6986 for $4 at http://www.junun.org/MarkIII/Info.jsp?item=79·. Twice as good for twice the price? Only time will tell.

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·"If you build it, they will come."

erco

And yet another option is to build your own sensor that you can adjust the frequency to avoid the specific interference you are encountering. I really like the circuit that MondoTronics uses on their Octobot Survivor robot. The instructions are online at http://www.robotstore.com/download/OctoBot_Instructions_1.2.pdf·and if you have a look at the schematic on page 9, it shows two nearly identical circuits built around a 567 tone decoder. The only differences are the RC values to determine center frequency, so you can see how to adjust that. These use 909 and 1300 Hz, considerably lower than 38 kHz. More work to build, but useful and adjustable.

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·"If you build it, they will come."

vla7

So if use the pololu sensor, would I then just change my freqout command to "FREQOUT 8, 1, 56000" instead of "FREQOUT 8, 1, 38500" since this sensor is tuned for 56khz·frequency?·

erco

It MIGHT be just that easy, but I don't have the sensor to verify. I was hoping one of the experts·could say.

Worst case, assume the Stamp can't generate a 56 kHz signal. There is a very simple LM555 circuit on page 11/12 at·mil.ufl.edu/imdl/papers/IMDL_Report_Summer_04/siddiqui_salman/bob.pdf· that only uses a few parts. BTW, the part marked DEVICE is your IR LED. You could drive several IR LEDs (each with its own series resistor, start with ~ 100 ohms each) from one such 555 circuit. Just leave this circuit running continuously (low power draw) and now you've freed up a Stamp output pin to do something else.



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·"If you build it, they will come."

Beau Schwabe

You can combine an IR transmitter/receiver break beam style detector using the tried and true LM567 tone decoder. You can adjust it to virtually any frequency you want with the proper Resistor/Capacitor selection for the LM567.

f = 1 / ( 1.1 x R x C )

www.datasheetcatalog.org/datasheet/nationalsemiconductor/DS006975.PDF

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Post Edited (Beau Schwabe (Parallax)) : 11/3/2008 5:02:28 AM GMT