CMOY Led Question

[ezzieyguywuf]

I am building a CMOY, and have completed the power section. When I test it as recommended on the Tangent tutorial, I get the proper voltage readings. The only problem is that my LED doesn't light up! I have some hookup wire connected at the same place that Tangent does. I didn't want do solder the LED yet so I used alligator clips on the hookup wire to connect to the LED. I did initially have the LED on backwards but corrected that. I tested for continuity all the way through from one side of the hookup wire, through the alligator clips and LED, and back down the other hookup wire and it looked good. My continuity test was actually just testing the resistance, which I now realize wasn't probably the best way to do that... Hm. Besides a continuity issue can you guys offer other suggestions of where the issue may be? I connected the diode straight to my battery and it lit up (albeit the wrong color)

Thanks!

 

[Tomo]

Hey,

You can check LEDs by checking voltage when it is on.

LED generally runs on a specified current. This just means the voltage across it should be a known fixed value. For example, red LED should be running at 1-2 Volts.

Resistor you see on Tangent schematics is designed to stabilize this voltage. (So you shouldn't directly connect LED to batteries.)

Have fun, T

 

[r_d]

Quote:

Originally Posted by ezzieyguywuf /img/forum/go_quote.gif I connected the diode straight to my battery and it lit up (albeit the wrong color)

It lit up the wrong color because you were frying it. Diodes need a current-limiting resistor in series with them. If you connect them without the resistor, the battery will dump a ton of current through it and destroy the LED.

You probably need a new one.

Current through the diode can be calculated with this formula:
I = (V_source - V_forward)/R

I is the current in amps.
V_source is the voltage of the battery or power supply. A shiny new 9V battery will be well over 9V, so do the calculations assuming 12V to have a reasonable factor of safety.
V_forward is the voltage drop across the LED (usually about 2V)
R is the value of the resistor in ohms.

Keep I below the maximum current of the LED. For most small LEDs, it should be less than 20mA, or 0.02A.

 

[ezzieyguywuf]

Well my battery is as 9.43 volts right now. As for your V_Foward, do you mean the rated voltage drop for the LED? If so, the V_forward for my LED is 2.1 Volts. I'm using the 10k-ohm resistor per Tangents BOM. I calculate tha 7.3 mA are flowing through my diode, which is well under the rated 20mA. Is that too low?

 

[ezzieyguywuf]

Oh hey I just tried it off the board and it works! I tested with a brand new one (bought ten b/c I anticipated frying some). Thanks guys!

 

[Tomo]

Oh hey!

Geez. You got LED to work in one day? It took me two weeks on and off. Smart people.

T

 

[ezzieyguywuf]

Quote:

Originally Posted by Tomo /img/forum/go_quote.gif Oh hey! Geez. You got LED to work in one day? It took me two weeks on and off. Smart people. T

Lol well like I said it works off the board, so its not actually part of the circuit. All I did was attach the resistor to one leg then gave it some juice from my battery. I think the problem originally may have been that in order to get my 18-gauge wire through the board, I had to cut some of the threads. This probably caused it to not transmit the power, and so my LED wasn't getting any. I'm hoping to get some 22-gauge at radioshack today. When I went last week they didn't have any, but checking online it is something that they have in their catalog, and since I'm in a different zipcode I'm hoping that this location will have it.

Wolfgang

 

[ezzieyguywuf]

OMG I got sound!!! Awesome!!! Now to enclosure it...

 

[nullstring]

Quote:

Originally Posted by ezzieyguywuf /img/forum/go_quote.gif OMG I got sound!!! Awesome!!! Now to enclosure it...

enclosing it is the hardest part, I've found.