FreeTrack Forum

I've bought 3 unknown manufacturer IR LED's from my local store (got no nerves to order stuff on web and wait a few days just to find out it's not what I need). I have no idea what is their forward voltage so I assumed that I should start from a singe AA cell and add some more juice if needed. It appears that they light up even at voltage of 1.25V (measured battery voltage from a 1.2V Ni-MH rechargeable battery, A4Tech supplied them with my opto-radio mouse). Here goes my question - should I use any resistors in the circuit, use a 1.5V regular AA non-rechargeable or just use it as it is now? I've got no soldering done yet and I'm not good at it so the less soldering the easier for me.

My testbuild uses a 2 AA, 5 red LED bicycle light housing. I hope to find a different one because this light is useful and working perfectly (when the board with LEDs is in it's place of course).


Bunch of wires with nice LED housings sourced from old IBM PC.


This screen shows my IR's as seen by A4 Tech PK-835, approximately 30cm from the camera. Working pretty well IMO given the small voltage and the fact that IR filter still remains on the camera.


The light is still visible at the proper distance giving nice, white dots.
If 1.2V is not enough I've got some 3.7 Lithium ion Panasonic cells from a laptop battery.

If you intend on using those led's then I would say no resistor require unless they have a resistor in the component they were taken from. If so, just tell me what the color bands on the resistor is and I can tell what the resistance is.

If you are going to use different led's then it is hard to say, because they are all different. Also, if they light up with the rechargeable, don't use 1.5 volt batteries, as they might fry them.

Your setup will work the same with or with out the IR filter since they are not IR leds. but any other light source in view will mess it up.

My system uses 1.5 volts, because I sell them and don't want people getting fried if they don't use rechargeables.

Where are you from? Maybe I can find a location in your area that sells leds for you.

Always use a resistor !!!!

You can't "guess" the current in a led without measuring it, and could overpower your leds , even with such a low voltage as 1.2V (depends of the led model)

If you can gain access to a multimeter, try to measure the led forward voltage, and use the led wizard to get a proper resistor value.

cya

Tristan is right. You should always use a resistor with LED's. Let me give u an example: A Led has a real small internal resistor-value so it could be that u have:
1,0 Volt - too low current
1,1 Volt - current is ok and LED working
1,25 Volt - current 10times too high. LED dying
This is just an example, depends on the type of LED (Forward Voltage).

I bet the easiest way would be:
Just get a 100 Ohm Potentiometer, and start with 100 Ohm, have a look at the Brightness or Heat of the LED or even better measure the current and then decrease the Resistor Value if necessary. You should have a current of 20-50 mA - no more then max. 80-100. And the LEDs should not get very warm.

(And also, normally for 3 LEDs parallel you should use 3 resistors, because if u use 1 resistor and the forward voltages are different then 1 LED gets nearly no current and another gets too much. But maybe it works also with 1, i dont know.)

Erm... short time lurker, first time poster here..

I was thinking of something like the above setup as well.. one (or 2 in parallel for longer lifespan, though I would think it's hardly needed?) NiMH rechargable AA, with IR LEDs I can get from a local store (Here)

The description of them states:

5MM EMITTING DIODELooks like a 5mm LED & has a blue transparent lens.Specifications:- Forward Current (If): 50mA max - Peak forward current (Ip): 1.2A- Forward Voltage (VF): 1.2V @ 20mA - Reverse Voltage (VR): 5V max- Power Dissipation (Pd): 100mW max - Viewing Angle: 30°- Peak Spectral Wavelength(IR): 940nm @ 20mA * Spectral Bandwidth (DI): 50nm@20mA- Material: GaAs

Now, I know the basics, but I'm by no means an Electrical Engineer, but if its forward voltage is 1.2V, and my battery supplies 1.2V, then.... I can hook it straight up, no?

Or do I need to back it off a tiny bit to be safe (any suggestions on what size resistor I ought to use if that's the case?)



Oh, one more thing I was thinking... With it being all IR and non-visible, is the only sign of flat batteries that the software stops working?  Is anyone in the habit of attaching some form of battery meter to the thing?