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Topic: stumped by this LED array (Read 1 time) previous topic - next topic


Mar 21, 2013, 04:45 pm Last Edit: Mar 21, 2013, 04:54 pm by treebykooba Reason: 1
I am controlling 8 LEDs in an array (these: https://www.sparkfun.com/products/8860) through a TIP120 and an arduino...doing PWM on them. They are powered by a seperate 9V wall wort. There are two for each resistor, 33ohm 1/2W...so 4 resistors in all. I used an LED calculator to make the array. (this one: http://led.linear1.org/led.wiz)

it goes: 9V to positive end of LED, negative end of same LED to positive end of next LED, negative end of this LED through resistor, resistor goes into a ground controlled by TIP120. repeat 4 times.

It's working fine except for two of the eight are not working and I'm not getting any multimeter reading on their wires. However, when I mix the wires, use the power from one and gnd from other, I get light. But I don't get any power the other way around. I redid the whole thing and still getting the same.  this is so weird and I'm very stumped. Why would just 2 not be working even though they are wired exactly the same as the other 6? any ideas? thanks!

also: these two lights in the array WILL work if I wire them up like this:
both + sides are in the 9V rail, both - sides are in the resistor that goes to the GND controlled by tip120

could I do this or this wouldn't be good in the long run, right?


No, I don't answer questions sent in private messages (but I do accept thank-you notes...)


no broken wire as the wires work fine when configured differently...also i tested the wire connections with a multimeter...everything seems normal.


One of the transistors is faulty?

Try swapping transistors between the faulty string and a working string of LEDs.
[ I won't respond to messages, use the forum please ]


Mar 21, 2013, 06:01 pm Last Edit: Mar 21, 2013, 06:42 pm by treebykooba Reason: 1
i'm just using one transistor...it works fine for the other 6 LEDs. currently everything is working it's just I have those two weird LEDs wired up wrong. Both + sides are in 9v rail and both - sides are in the 33ohm resistor, into the transistor's GND rail. Does anyone know why/if this is bad? it's been working fine for the past 30 minutes, pulsing away. thanks for the help!

edit: both LEDs just fried. what is going on????????

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