Hello

I am a mechanical engineering student building a robot for a senior design project. I am using infrared detectors similar to those used on TVs, audio, etc. My problem is that the infrared emitters I have on the robot are not powerful enough to be used in sunlight. I tried using my Cinema 7 remote with the robot and it detects these signals outdoors. Does anyone here know what infrared LEDs the manufacturer uses on the Cinema 7, or possibly have a broken old remote or two they would like to sell? Thanks for any help on this.

Dan Fox

I would suggest that your basic problem derives from one of two possibilities. The first is that the led emmitters you selected were on the lower end of the continuum for power emmission. The second is your supply voltage is too low for the leds to maximize their output. Merely replacing the leds may still not solve your problem if it's the latter case. I'm assuming that your "test" was using the cinema 7 outdoors and seeing if your detectors "saw" the signal.
The packaging your leds came in or the source you obtained your emmiters from should indicate it's maximum voltage use range. Therefore, you may need to redesign your voltage supply circuit so that it's getting at least close to it's maximum voltage allowance. Most remotes operate on either 3 or 6 volts; i.e. 2 or 4 1 1/2 volt batteries. However leds in general may have much broader operating ranges depending on the parameters design in the individual led "model". You may well be using leds that are designed for 12 volt operation but only supplying them with 6 volts.
Some leds are designed to emit low emmission levels; i.e. lower "light power". For use in strong sunlight, you'd need to first select one that emits in the upper range of emmision power, then make sure that you adequately supply enough voltage so that it opertes at it's upper range of emmision "power".
This is akin to trying to operate a 150 Watt , 120 volt light bulb with only 75 volts. You're simply not going to get as brite or as "powerful" a light.
If I was you, I'd first determine what voltage your selected leds were designed to operate at, then make sure that they are getting the maximum voltage for which they were specifically designed. If they are indeed getting the maximum voltage for their design, I'd rexamine the options, select one that has higher output power, and make sure it gets that power, thru circuit redesign if neccessary.
Hope this helps

It's been a long time since I've read such misguided, ill-advised, and technically innacurate gibberish as the jamesgammel response. I can't believe this individual has ever read anything about LED's, let alone use one. LED's are CURRENT devices, not VOLTAGE. And what the heck does ". . . in the lower end of the continuum for power emmission." mean?

There is no such thing as a maximum voltage for an LED. You can run one at 1,000,000 volts if you put a 50,000,000 ohm current limiting resistor in the circuit. Some LED indicator PACKAGES are rated 6V, 12V, etc., but these are usually just an LED in a little tubular package with a resistor, ready for panel mounting. And nobody makes infrared LED's in packages like this.

As for the direct question, the infrared LED sold by Radio Shack is a fairly representative unit, and quite similar to those used in the Cinema 7. Find a good reference on how to select the series resistance in a LED circuit. But here's the bad news. Most LED's are rated for a continuous current of 20 to 60 milliamps, but in nearly all infrared communications they are pulsed at much higher currents for very low duty cycles. The high current gives high output for greater distances, while the low duty cycle gives a low average current that doesn't overheat the LED chip. A typical example might be to drive the LED with 1 amp pulses that are 10 microseconds in duration, and which repeat every 200 microseconds. The duty cycle is 10/200 or 5%, so the average current through the LED is only 50 milliamps.

When you mention using detectors similar to those used in TV's, etc., are you referring to the demodulator detectors that are looking for a carrier frequency in the range of 40 KHz from the LED? (That's the kind of signal the Cinema 7 puts out.)

Tommy Tyler

The receivers I'm using are made by Panasonic, with 3 leads--5V, 0V, and output. Their center frequency seems to be about 38500 Hertz from tests I've run with them. I'm driving the LEDs with a Basic Stamp microcontroller that I'm using to generate the 38.5 kHz. It does not precisely generate this frequency, however; it uses pulse width modulation so that it can simulate a sine wave (if you smooth the signal with capacitors). I'm using the unadulterated signal and sending it to the base of a common emitter transistor, this amplifies the current to around 95 milliamps collector-emitter for the LEDs (and a small series resistor). The maximum these LEDs are rated for is 100mA at 1.7 volts, and their power output is rated at 25 mW/sr at 50 mA.

If I understand you correctly, I should be able to determine my duty cycle precisely with an oscilloscope, then boost my current accordingly, as long as it averages out below 100 mA during the most frequent pulses. Is this correct? Thanks,

Dan Fox

That's pretty much true, but remember the duty cycle concept has its limits. Obviously I can't turn on the LED for 1 hour then turn it off for 100 hours and expect the average current to be 1% of the ON current. That's an exaggerated example, but you see the point. Buried in the specs of infrared LED's designed for pulse applications you'll often find a figure showing pulse handling capability. This is usually a chart of on-current versus on-time, with a bunch of curves for various duty cycles. A typical example allows 1.5 amp pulses at 2 % duty cycle as long as the pulse width is 10 microseconds or less. Increase the pulse width to 1 millisecond and the maximum pulse current is reduced to about half the above. With your PWM signal I don't see how you are going to deal with this. If you want to see what these "Permissable Pulse Handling Capability" curves look like you should be able to find detailed specs at a web site for one of the prominent companies that makes infrared LED's, such as Agilent (formerly HP), Siemens, and many others. Just search for infrared emitters. Hope this helps.

Tommy

Dan,

One other suggestion: If you have the voltage headroom in your power supply, cut back on the resistor value and instead run a pair of IR diodes in series. For the same amount of current you'll get twice the bang for the buck.

Thanks for the help on this everyone. I ended up getting some high output Siemens LEDs that had much better data sheets. The PWM signal from the controller had a high duty cycle, so I have the controller programmed to send a pulse to a 555 timer which generates the 38 kHz signal. I can control the length of this pulse much more precisely so that I can get a lower duty cycle. I'm running two LEDs in series with roughly an amp of current at 0.5% duty cycle, and now it works great outdoors! Now I just need to figure out how to rangefind with this setup.

Dan