Best practice to run multiple LEDs

[Many__Of__Horror]

I am looking to build an LED desk lamp for my daughter with an on/off switch and dimmer just like the one the make at Coppersound Pedals



I am wondering what the best practice is to run 9 x LEDs from a 9V battery. I am also a little unsure where to put the dimmer pot in the circuit.
So far I have come up with:



My concerns with this circuit are mainly the fact that a 9V battery will only last about 5 hours on it (i think) Are there more power efficient ways to deal with this?

I would also likely change the resistors to include a 25% ceiling at the cost of max brightness to increase LED life and battery life, and also measure the actual Vr of each LED to calculate the exact resistor needed to account for manufacturer variance.

Anything tips or info is always appreciated

 

[giovanni]

Is the dimmer supposed to dim all LEDs? I think you want it in series with the parallel of LEDs otherwise each of the tiny 120ohm resistors will absorb most of the current. 10K may be too low to limit the current and you may want higher resistor values than 120. I also think you want each LED to have its own resistor, assuming each one can be turned off?
I would measure the largest resistor you need for maximum brightness (you don’t want it to be too low otherwise it will unnecessarily drain the battery as you mentioned), call it R1. Then the largest resistor for minimal brightness, call it R2. If the LEDs are individually switchable, you need 9X R1s in series with the pot. The value of the pot will have to be 9 times the value of R2 and it should be in series with the parallel of the LEDs+R1s. Does that make sense?

 

[giovanni]

Not sure why my phone uploaded that upside down, but hopefully it gives you the idea.

 

[giovanni]

Ops replace 0.7V with the forward voltage you have (0.7 is by habit!).

 

[fig]

Not sure why my phone uploaded that upside down,

I know right? They are amazing but sometimes mysterious little devices.

 

[zgrav]

Is it likely to be turned on at night and left on for a long time (either on purpose or inadvertently)? That would likely go through a lot of batteries. Might be better to build this to use rechargeable AA batteries, or to run from a wall adapter, perhaps through a USB cable instead of "wasting" a 9-volt adapter. : ^ )

You could also consider using 2 9-volt batteries in the enclosure to give you longer usage between changes although it makes the replacements twice as expensive. you could also look saving battery drain by putting in a 555 timer circuit to power it down after some period of time.

 

[Many__Of__Horror]

Thanks for the suggestions all. You are right @zgrav in that it is likely to be left on for a few hours as she falls asleep etc so yeah I need to look at alternatives to make it more efficient.
In regards to the dimmer @giovanni all LEDs should dim at the same time with the dimmer pot, max brightness when fully clockwise & really dull when CCC

 

[giovanni]

My schematic should work then!

 

[giovanni]

And by the way I think you can have just one resistor instead of 9 in series with the LEDs if you don’t intend to switch them off individually.