# Calculate number of LEDs, and resistor values, when some is turned on/off from time to time?

The LEDs that I have available is 3.2V ~ 3.4V and 20mA (exLED 5mm LED). (i'm planning to use the clear ones).
The resistors is 1/4W metal film resistors (1/4W Resistance Metal Film Resistors).
And this powersupply - that says 14.8V 700mA.

I want to use this power supply in particular because of the plug in the other end. That way I don't have to "destroy" any other wall warts, and this one, to connect it...

What I basically want to know is how many white LEDs I could have connected, and turned on all the time as a light, in addition to 5 green ones that is turned on and off from time to time?

I'm planning to only use the clear ones. Those light up in different colors.

So I put the data (14.8V, 3.2V, 20mA, 10) into this online calculator, and it returned a couple of variations to choose from. I put in ten LEDs because of 5 green LEDs (it has to be exactly 5), and 5 white ones (can be whatever, but minimum 4).

I've learned that there's a bit different in voltage from various colors?
I'm not sure exactly what voltage each of the LEDs are since I don't have any datasheet to look into...

The real life scenario goes like this:
The white LEDs should turn on, and stay on, all the time when the power supply is connected. Just like a light (it's a lightbridge for a slot car race track).
In addition to this light. There's a start light with 5 additional green LEDs (clear, but light up in green) that is turned on from time to time (when a race starts, and is on until its over/paused) by a mechanical relay (Keyes 5V Relay).

I was hoping to be able to use the same power supply for both actions... The green starting part is controlled by a microcontroller - if that matters...

My concern is when the green LEDs is not ON, there's going to be more voltage in the circuit. By using higher resistors on the white LEDs would prevent them from being overloaded, but would make them very dim when the green LED's is on - which is when they are needed the most...

Any suggestion would be highly appreciated...
I'm a beginner to electronics, and don't know a lot of terms, and solutions, and available components, and such yet...

• How many white LEDs do you really want? At 15mA (it's wise to run them at less than maximum current) you could run something like 140 with triplets in series. Also, when the green LEDs are switched (so that the total number changes) the white ones might change brightness slightly since that power supply is unregulated. – Spehro Pefhany Nov 21 '14 at 15:13
• I just need enought white LEDs to make a couple of phototransistors to work. From the transistors to the lights is about 10-12 cm. And each transistor is about 5,5cm apart from each other, on the same level... – ThomasK Nov 21 '14 at 15:47
• With one resistor per LED, and operating at 15mA per LED you can run 46 LEDs total (700/15). If you're using much less than 40 or so LEDs you can use 1K otherwise try 820R. – Spehro Pefhany Nov 21 '14 at 15:52
• I was planning about 4-5 white LEDs... – ThomasK Nov 22 '14 at 7:19