LED help

[SwamperGene]

OK, I tried an experiment with what I had. I set up a test rig that held 1 LED at a set distance above a photoresistor that is hooked to a meter. First I hooked the LED to 4 of the AG3's, just as it is in the flashlight. The 4 AG3's measure 6.1V. Figuring the LED spikes when first turned on, I let it sit for a few minutes then noted the photocell's resistance. Next I hooked up the LED (in series as always) to the rest of the chain which is the remaining three plus a 10 ohm resistor, powered by my new contraption mention earlier. I turned the trimpot on the regulator until I reached the exact same photocell ohm reading I noted under battery power. My measured supply voltage was 14.9.

I put this value as the supply voltage in the LED Wizard, using a 30 mA forward current, and adjusted the Vf til it gave me my 10 ohm resistor. A Vf of 3.66 gave me this:

Solution 0: 4 x 1 array uses 4 LEDs exactly
+----|>|----|>|----|>|----|>|---/\/\/----+ R = 10 ohms

The wizard says:
In solution 0:
each 10 ohm resistor dissipates 9 mW
the wizard thinks ¼W resistors are fine for your application
together, all resistors dissipate 9 mW
together, the diodes dissipate 439.2 mW
total power dissipated by the array is 448.2 mW
the array draws current of 30 mA from the source.

I know I've seen some bright whites rated at or near 4 Vf, given the original 6 volt application is 3.6-3.7 unreasonable?

[dyarker]

Around 3.6V for LED Vf is reasonable.

3.66V x 4 = 14.64V for 4 LEDs in series.
14.9V - 14.64V = 0.26V to be dropped by resistor.
0.26V / 0.03A = 8.666 Ohms, close enough to 10.
0.26V x 0.03A = 0.0078W, 1/8W resistor okay.

To use the 21V wall wart:
21V - 14.64V = 6.36V across resistor.
6.36V / 0.03A = 212 Ohms, use one standard 220 Ohm.
6.36V x 0.03A = 0.1908W, 1/4W resistor okay, 1/2W better.

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Before when you said one LED at end of string was always dimmer even moving LEDs around, you were using 4 photo cells? The LEDs and their wiring were fine, one of the photo cells is "weaker". To compare relative LED brightness use one photo cell moved under each LED at same distance.

To set the trip points for the 4 tracks, use a trim potentiometer wired as variable resistor with each photo cell. How do you have the photo cell circuits wired?

[SwamperGene]

The photocells all tested good, within an ohm or 2 in my test setup. For the cells I used one LED to test, for the LED's I used one cell. I still don't know why that was occuring with the end LED, I wouldn't have believed it either but myself and two other guys that were here saw it, it was a visible light diffence. For 4 bucks I just started fresh with new ones. BTW, since my last post the LED's have been lit and my meter only shows 2 ohms variance, so keeping my fingers crossed I'll be alright with that setup.

The software typically (and simply) polls the LPT port and reads if the pins are above or below threshold. I say typically because there are numerous programs available that will all work with the same setup. Generally though not always the trigger is at around 1k ohms. You have to keep in mind that the reason for this method is not only for programming simplicity but also due to the fact that there are many triggering methods - dead strips and magnetic reeds on the track are two others and all, including optical setups, share pretty equal popularity.

Reeds are technically the most reliable, they are on or off, but there is no magic reed that can handle the full range of cars available. One mixed solution I used for a friend who had phototransistors that would not go low enough was to build a (quad) comparator circuit with each one driving a simple reed relay, his phototransistors on one side and trim pots on the other. You can dial this setup in to read anything from ambient light to an LED setup through a 7/64 hole in the track (at each lane, of course). With this you can run any car, the PC triggers off the reeds.

[dyarker]

Ooooookay! Success is the important thing.

And maybe someone else who wanted to know how to do the calculation, but afraid to ask; will see the example.