Let's Make Robots!

Getting the most from an LED

Hi guys. I have recently written the walkthorugh on how to transmit serial data with an IR led and it works fine. Now to the next step: i want to achieve the highest range possible with my LED, without using external aids such as lenses. (that is: only with electronics).

I have a TSAL 6100 from Vishay ( http://www.farnell.com/datasheets/93358.pdf ). If I am right, to achieve the most out of it, i have to get a current similar to the "peak current" value from the manual in the LED. But as for the voltage i guess i just need to give it at least a value equal to its forward voltage. 

Problem is, i am not sure about the things i have just written... :) Is it really the right way to do it?

Suppose it is, i cannot give enough current to the LED with an output pin, so i think i'll be using either a mosfet or a standard NPN. But again, how do i exactly know what current i will get with it? As far as i know i have to measure the "amplification value"( hFe?) with my multimeter and multiply that to the current entering the base. Am I right? 

Comment viewing options

Select your preferred way to display the comments and click "Save settings" to activate your changes.

back on the topic!

i need to get a mosfet for the IR LED circuit. The above circuit suggests this one: http://www.irf.com/product-info/datasheets/data/irld110pbf.pdf

Could you just give me a brief explanation of its features?
For example : 

Drain to Source Voltage (Vdss)100V
Current - Continuous Drain (Id) @ 25° C1A

what are these two things ( i suppose the first stand for the maximum voltage the drain can pass to the source, but don't have many clues on the continuous drain )? Is there an internet site that explains these kind of things so that i don't have to bother you every time? :=) 

2 different things are shown, first voltage, with a max of 100v, then the max continuous current (Id) of 1 Amp when the device temp is 25deg C. At a higher temp (100 deg C) only 0.70 A (700 mA) should be allowed

it is strange though....look at the resistance values at the top left, they're really low ( i guess they are the ones to be used with the IR LED), considering we came up with a max value of 200mA at 38kHz and 50% duty cycle and he has a min of 347mA and 56kHz (without considering the use of power mosfets). 

These things make me wonder... 

Remember that the IR LED datasheet had a max of 200 mA at a rate of 5000 Hz, 50% duty, repeated ad infinitum. The next parameter down is a non-repetitive surge current of up to 1.5 A for a similar 100 uS as before, but not repeated. The LED won't just immediately fry if going above the 200 mA, but this is a point in a curve of what it can take for an extended period of time. The other circuit shown is probably estimating some cool-down time for the LED to rest when it is not being driven, and tried different values to get even more current through it, without burning it up. And the FETs were probably used to get the currents greater than 350 or 400 mA, about the limit of many basic BJTs. I think 2N2222 BJTs can go up to 500 mA pretty easy, but getting much higher would require the FETs or stronger BJTs in a different package (TO220).

THe kHz is probably to match the detectors they are using, different TSOPs.

yeah, no worries about the KHz. But no, it is not expecting any cooldown time. As it says in the protocol page, the system can "shoot" up to 29 times per second, and that is sending 16 data bits continuously. But then he clearly states "[...]as the limits of the Infrared LED are often stressed in Laser Tag designs to gain maximum power and range."

I guess that in these cases you just have to use a bit of common sense. You won't be keeping your finger on the trigger for more than a sec anyways, so at most you'll just stress the LED a little more than you should and substitute it in case it burns out.

No matter how many times a second something is being sent, the LED will always be on half the time, off half the time at the rate of 38 kHz, so the IR receiver can see it. The spec gives 100 us that the LED can take 1.5 A, but at 38 kHz it is only on for 13 us, not fully being heated to failure, but only given another 13 us to cool. If the data packets are all ones, all high all the time, then you would probably need to be closer to the 200 mA spec than the 1.5 A. But the packets are going to have zeros as well as ones, in which the LED can rest further and so can be driven a bit harder.

i've found this schematic. It was taken from lasertagparts.com, and wants to do a similar thing, well, actually it is exactly the same, as we are trying to do.

here it is:

aa.jpg 

question is: why is he using Power Mosfets, or well, let's say field-effect transistors in general, instead of standar bipolar transistors like i am using? Is there something to gain?

The FETs generally will have less of a voltage loss across them than the BJTs will. And will carry more current if that is needed. BJTs have a standard "diode drop" across them typically being 0.3 to 0.8 volts when fully on. Just like calculating the current across a diode, to size a current limiting resistor, the transistor CE junction can be considered too. Note Darlington trnaistors will have 2 "diode drops" across their output. With FETs, the Rds-on value is figured as a resistor when the FET is turned on, so current times the Rds-on would give the voltage lost in a FET.

It works! The problem i had was related to..well...misreading of the datasheet, i thought the collector was on pin 2 whereas it was on pin 1...

Anyways, i've used the schematic of the AND gate with transistor and placed my resistor and IR led on its output (on the output of the AND gate) and it works. As for the coding I used T (true) reception on the receiver and it works (notice that i kept N on the sender).

how cool :) 

Glad you got it working, pretty cool idea of what it's going into.