1
\$\begingroup\$

I am using a perfboard for my first attempt at putting some LEDs together. Only one of the LEDs is working, and I am very confused.

What is wrong?

One LED working:
enter image description here

Bottom:
enter image description here

\$\endgroup\$
4
  • 4
    \$\begingroup\$ From what I can see in the photo it looks like the unlit LED is shorted out \$\endgroup\$
    – Finbarr
    Feb 23 at 11:26
  • 2
    \$\begingroup\$ That said, lighting two yellow LEDs in series using 3V is asking a lot. \$\endgroup\$
    – Finbarr
    Feb 23 at 11:28
  • \$\begingroup\$ I understand now, thank you @Finbarr. \$\endgroup\$
    – shushi
    Feb 23 at 22:35
  • \$\begingroup\$ But if you ask something like "How can i light two yellow LEDs at 10mA total using a CR2032?" we can show you how to put the two LEDs in parallel, each with its own current-limiting resistor. \$\endgroup\$ Feb 24 at 0:58

1 Answer 1

4
\$\begingroup\$

It appears that you have shorted the LED that isn't lighting up:

enter image description here

It looks like you have both pins of that LED connected together.

It is unlikely that your circuit will ever light up both LEDs. The forward voltage of both LEDs together is higher than the voltage from your coin cell. A yellow LED has a typical forward voltage of 2V. Two in series means that you will need 4V to light them.


This is the voltage/current trace I made of a yellow LED when I built a simple curve tracer a couple of years ago.

enter image description here

It takes a little above 1.5V to make current flow through a yellow LED and light up. The voltage of coin cell is a little above 3V. Some sites say 3.6V, others say 3.2V. It is barely possible that you can get two yellow LEDs in series to glow (dimly) using a single (brand new, fresh) coin cell.

\$\endgroup\$
3
  • \$\begingroup\$ I see, I think I understand. But I'm also a little confused, because I eventually want to build this LED Matrix Badge, and it also uses the same coin cell battery yet has 20 LEDs? How come it has enough voltage to supply? hackster.io/amalmathewtech/… \$\endgroup\$
    – shushi
    Feb 23 at 22:38
  • \$\begingroup\$ The LEDs in that circuit aren't connected in series. They are connected in a matrix that allows the microcontroller to switch every LED individually. \$\endgroup\$
    – JRE
    Feb 23 at 23:26
  • \$\begingroup\$ @shushi -- #1. All of those LEDs are not in series, but one pin per LED, or some form of multiplexing, as JRE says. #2. Of all the leds, the red LED has the lowest forward voltage. I know the web page says to pick any led, but any other LED than red is going to work at a lower voltage with acceptable brightness. #3. LEDs can look much more bright than they really are when being recorded with any camera, which automatically adjusts for brightness. (and where you can artificially give the impression of great brightness by adjusting the exposure). \$\endgroup\$ Feb 24 at 1:15

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.