just wondering if you can suggest a good LED forum for complete beginners to ask very basic questions. I am trying to build a toy figure out of beach driftwood and it has two eye sockets drilled in where I would like to place red LEDs. I don't know much about how to power these LEDs and I would like to go buy the parts I need. A neighbour suggested using two resistors, one for each LED, and tried to draw a schematic for me but he wasn't exactly sure either.

Could anyone tell me how to do this? I believe I will need to use 2 x AAA batteries and a resistor for each LED but I don't know what to ask for when I go to the electronics shop.

Very much appreciated!

 

Well, your neighbour is right. 2x AAA batt's are ~3V. An red led is typically 2V. 3V-2V = 1V to drop by the resistor at 20mA, so 50 ohms.

Two 50 ohm resistors would do the job. Since 50 doesnt exist, take 56 ohms

 

I would be a little more conservative in that the batteries may be a little greater than 3V and the maximum current of the LED's may be 20mA, pluse the LED forward voltage may be 1.8V. I would go with a standard value of resistor of 68 ohms 1/4W. I can rarely see the difference in intensity of LED's that are operated a 1/2 their normal current, so a little less is better.

 

I would be a little more conservative in that the batteries may be a little greater than 3V and the maximum current of the LED's may be 20mA, pluse the LED forward voltage may be 1.8V. I would go with a standard value of resistor of 68 ohms 1/4W. I can rarely see the difference in intensity of LED's that are operated a 1/2 their normal current, so a little less is better.

So, to power two red LEDs I can use two AAA batteries but I need a resistor for each LED. You are recommending a 68 ohms 1/4W for each LED, correct? Again, I am very new to this, but in essence I could use a double AAA battery housing, run separate wires from the positive head to two 68 ohms 1/4W resistors, then to the LEDs, connect those wires at a switch and finally return the connection to the negative head? Does that sound correct?

Thank you both for your help!

D

 

Double AAA battery holder.
Connect the battery holder + lead to one terminal of a SPST switch.
Connect one end of the two resistors to the other terminal of the SPST switch.
Connect the other ends of the two resistors to the anodes of the two LEDs.
Connect the cathodes of the LEDs to the - lead of the battery holder.

LEDs have two leads; one is the anode, the other is the cathode. The cathode's lead is generally a bit shorter than the anode. The cathode gets connected to the more negative side of the power supply.

 

Thank you! OK, so I've been reading a bit more and now I'm trying to sort out how electrical diagrams work. Does this diagram look like what you were just telling me?

Thanks again,

D

 

Yes, that will work.

Note that when you are drawing schematics, it is typical to have the more positive voltages shown towards the top of the schematic, and the more negative voltages towards the bottom. Basically, the way you've drawn it is inverted.

I took your schematic and flipped it around. See the attached.

 

To identify the polarity of the leds, look at the base of the lens near where the leads are. There will be a small flat area near one of the leads, that will be the negative.

 

Fantastic, thank you all. I will post photos of the toy when it is finished!