"Relay Driven LED's", Experiment #7, "Make: Electronics"

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Joined Jun 6, 2014
[Edit: I looked at the resistor again. It looks like I fried it. That doesn't show up in the picture. Maybe it happened shortly after the picture. Anyway, the resistor is where I am pointing with the pencil, so I was partly right. Can you suggest how to resolve this problem?]

I'm new to electronics, but when I have a software problem I usually post to a forum. Now I have an electronics hardware problem, I hope the same works.

I'm doing Experiment #7 in "Make: Electronics", "Relay Driven LED's". I changed some of the connections to soldered connections, because doing it all with alligator clips is extremely difficult. I'm using a hacked AC to DC converter as a power supply. It's set for 12V.

I saw a light go on and I thought it was working. Now, when I connect the items I'm pointing to in the picture below it gets very hot and I smelled a little smoke. Can you help me to fix this problem by looking at the pictures below, or otherwise offer any advice.

Slide Show

Other postings I found online agreed that doing it all with alligator clips is not a good idea. One guy converted it to a breadboard project, but that's beyond me.

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Joined Apr 11, 2010
I don't see any resistors. They are needed to limit the current flowing through the LEDs. Without them, the magic smoke will be let out of the LEDs and they will no longer work.

If they fail as a short, then that would explain why everything is getting hot.

You need to know the forward voltage (Vf) of the LEDs and the current required (10-20mA is typical. Call it Ia). Also, you need to know the supply voltage (Vs). Then you can calculate the resistance required.

R = (Vs-Vf)/Ia

Thread Starter


Joined Jun 6, 2014
I made some remarks and questions inside one of your pictures:
Thank you Bertus for your helpful response.

Resistor: This may have been my mistake.
The resistor is 1 ohm. The directions call for 680 ohm, or 1 K ohm. My mistake? Substituting 1 ohm for 1k ohm?
Now I need to get new LED's, the correct resistor, and try again.

Power Supply Voltage: set to 12 V

Anodes of the LED. Are pointing to the + of the power supply. It's a bit confusing, but I tried them pointing pointing both ways. It's a bit confusing trying to figure out the direction of electricity flow.

The Green clamp: You may be right. I'll have to see if that's the problem.
Is that what you understand from this image?



Joined Apr 5, 2008

With a 1 Ohm resistor, the leds will burn up in a flash.

When you hold a led against the light, you will see an arm and a cup inside:

The arm is the anode of the led and the cup the cathode.



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Joined Jun 6, 2014
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