What I was saying there was that your calculations for power (watts) were wrong.

In total the both LEDs, or "leg" of the circuit to use your parlance, consume around 80mW = 2v * 40mA (P=V*I), not 100mW each like you said before.

You're misquoting my power calculations.

1. This parallel circuit has two legs.

2. Each leg has a resistor and an LED.

- R1 and LED1 = leg1

- R2 and LED2 = leg2

3. Each leg has 20 mA of current flow.

- 20 mA flows through R1 and LED1.

- 20 mA flows through R2 and LED2.

4. The resistors are dropping about 3v and the LEDs are dropping about 2v.

5. There are four components that are dissipating power. (Volts * Amps = Watts)

(leg1)

R1: 3v * 20 mA = 60 mW

LED1: 2v * 20 mA = 40 mW

(leg2)

R2: 3v * 20 mA = 60 mW

LED2: 2v * 20 mA = 40 mW

6. Power totals for the legs:

- 60 mW is dissipating across R1 and 40 mW is dissipating across LED1 for a total of 100 mW dissipating across leg1.

- 60 mW is dissipating across R2 and 40 mW is dissipating across LED2 for a total of 100 mW dissipating across leg2.

7. Power totals by component type:

- The two resistors drop a total of 120 mW. (60 mW + 60 mW)

- The two LEDs drop a total of 80mW. (40 mW + 40 mW)

8. Power total for the whole circuit:

- 100mW (leg1 power) + 100 mW (leg2 power) = 200 mW (total power)

So you don't need 1/2 watt rated resistors, ordinary 1/4 or even 1/8 watt rated resistors should be fine. But hey it doesn't hurt.

Agreed. Each resistor is dissipating 60 mW so the only reason to use 1/4 or 1/2 watt resistors is for the thicker leads -- pretty sure I mentioned that earlier in the thread.

I assume you mention this because of that note on Javeryh's diagram.

- He probably got the idea from smalltownguy's post

here.

Green and blue LEDs drop more voltage (around 3v) than red ones. (around 1.9v)

- Different materials used for the LED's P/N junction forward bias at different voltages and emit different wavelengths of light.

- A 150 ohm current limiting resistor is perfect for your green LED button.

This is not quite correct. Now there are all kinds of LEDs to be exceptions to this, but *in general *your red/orange/green are all around the 1.8-2.2v range, but white and blue are the ones with the higher voltage drops up around 4.2v or so. Now I just wrote that off the top of my head, but you can easily google up some charts of typical forward voltage (voltage drop) and current values that will roughly align with that.

I just transcribed the voltage ratings off a variety pack of LEDs.

- In this pack, green is 3v and lime green is 2.1v -- green volcano buttons look more like regular green than lime green.

As always, it's good to know the ratings for the specific component you're working with -- data sheets are our friends.

You can easily measure the voltage drop over the LED with a multimeter, and/or can use a battery to rig up a test circuit if needed

Yes, you can use a multimeter to find the correct current limiting resistor value.

- Start with a larger-than-estimated resistor that still forward biases the LED and measure the current flow.

- From there, adjust the resistor size to get in the 15 - 20 mA range. A reduction in resistance ==> an increase in current.

Scott