Before you ask - I've checked that the LEDs worked before putting them in my circuit. They all work.

I've connected 4 LEDs, shown in the arrangement below, up to my Adruino Lilypad. Pin 9 was used as the + 5V, the "- pin" was used as ground. In the schematic below, the "1" represents pin 9.


simulate this circuit – Schematic created using CircuitLab

enter image description here

When I start my sketch, I set pin 9 to high to check that all the LEDs are working. (this is a PWM pin so I write, analogWrite(9, 255);). At this point, only D5 lights up. If I connect up the two wires connecting R9-D6 and R10-D7, then both D5 and D8 light up... but never D7 or D6?

Doing some basic circuit calcs, the total resistance of the circuit is 100 Ohms (1 / (1 / (100 + 100)) + (1 / (100 + 100))). I'm powering from the usb connection to the computer so power source should be 5 V. Total current draw should therefore be 50 mA? Is that too much? If so, why does only D5 - or D5 and D8 when adjusting the circuit - light up?

  • you have 200 ohms in series with the LEDs .... remove one of the resistors in each leg – jsotola Mar 28 '18 at 20:08

Those LEDs are 3.3V forward voltage. With two in series you need at least 6.6V to light them up.

  • ...omg. So would it work if I put all 4 in parallel? Why did only 1 of 4 light up like that? – Mr. Meeseeks Mar 28 '18 at 16:46
  • Not sure why one lights up. Some oddity maybe. Yes, using them in parallel is what you want to do. – Majenko Mar 28 '18 at 16:47
  • Connecting 4 leds in parallel to a single output isn't advised, as it will exceed the maximum allow current. At most connect 2 leds in parallel to a digital output. – Gerben Mar 28 '18 at 19:02
  • What would happen if I did exceed max current? I mean... I already have connected up in parallel & it's fine and works, but I still wanna be safe...! But no failsafe/fuse has kicked in... – Mr. Meeseeks Mar 28 '18 at 20:17
  • The MOSFET in the GPIO pin will get hot, overheat, and go pop. – Majenko Mar 28 '18 at 20:55

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