user28663: 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

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?

user4143: Those LEDs are 3.3V forward voltage. With two in series you need at least 6.6V to light them up.

28663: <p>Before you ask - I've checked that the LEDs worked before putting them in my circuit. They all work.</p>

<p>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.</p>



<p><img src="https://i.stack.imgur.com/W8qNq.png" alt="schematic"></p>



<p><sup><a href="/plugins/schematics?image=http%3a%2f%2fi.stack.imgur.com%2fW8qNq.png">simulate this circuit</a> &ndash; Schematic created using <a href="https://www.circuitlab.com/" rel="nofollow">CircuitLab</a></sup></p>

<p><a href="https://i.stack.imgur.com/MLyli.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/MLyli.jpg" alt="enter image description here"></a></p>

<p>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, <code>analogWrite(9, 255);</code>). 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?</p>

<p>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?</p>
 4143: <p>Those LEDs are 3.3V forward voltage. With two in series you need at least 6.6V to light them up.</p>
