# led and potentiometer questions

Discussion in 'LEDs and Optoelectronics' started by swampy1978uk, Dec 16, 2012.

1. ### swampy1978uk

12
0
Dec 16, 2012
Hello
I am looking for some help
I am building a toy for my daughter
For it I need some leds to be work from a potentiometer
The leds have a forward voltage of 1.8 to 2.2 with a 20ma draw
They will be running from a 6v lantern battery.
I need 4 sets of 36 leds in a set so I was thinking wiring 3 leds together in series and 12 of them in parallel.
My main question is what resistance potentiometers would I need.
I also need a 2 sets of 3 to run of another potentiometers all run off the same battery.
Ian

2. ### duke37

5,364
772
Jan 9, 2011
You cannot run three leds of 2.2V from a 6V battery, two will be enough, a suitable series resistance will be needed in each string.
You will have 72 strings each taking 20mA so the total current will be 1.44A.

You do not say what the potentiometer is supposed to do. If it is to control the brightness, then a chopped supply would be best. The potentiometer would then control an integrated circuit such as a 555 and a bigish transitor (perhaps a fet) would be used switch the current.

3. ### swampy1978uk

12
0
Dec 16, 2012
Hi
Yes sorry would need the brightness to go from off to full with a 1 turn pot
Would I need to post in the circuit section for help on that?
Im looking for the cheapest And easiest way to do this.
Ian

4. ### swampy1978uk

12
0
Dec 16, 2012
just to confirm would they all dim and brighten at the same time as this is what i need
would you be able to say what the transistor value is i cant work it out.
and the resistors inline with the leds what resistance would they be
i need 32 to work of the one potentiometer would i just keep adding leds to the circuit
i am ordering the bits from rapid electronics
thanks

Last edited: Dec 21, 2012
5. ### swampy1978uk

12
0
Dec 16, 2012
once again thanks for you continuing help
the leds i have have a forward of 1.8 to 2.2v so i hope 3 will be ok i think i will need a resistor of 33r
thanks

Last edited: Dec 21, 2012
6. ### Relayer

39
0
Dec 15, 2012
You're welcome

Because you may border on the LEDs being Vf = 2.0 volts they may not light up. Start with two LEDs per string until you know the dimming circuit works and you haven't done any wiring mistakes, then add the extra LED/s to see if three will work ok.
Regards,
Relayer

7. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,505
2,852
Jan 21, 2010
It would be far better to only put 2 in each string. I could give you a heap of reasons why (and I will if you're interested).

8. ### swampy1978uk

12
0
Dec 16, 2012
ok cool thanks once again i will give it a go

9. ### swampy1978uk

12
0
Dec 16, 2012
hi me again
i have wired the circuit as shown and the leds start of very dim then gradually get brighter the potentiometer has no effect.
or if anyone know where i can buy some pre made ones or anyone willing to make me some i will pay! let me know.
thanks again.

Last edited: Dec 23, 2012
10. ### Relayer

39
0
Dec 15, 2012
What did you build it on; A veroboard, Breadboard, or did you solder each lead and leg to each other?
Either way, you need to recheck your work.
I've made this exact circuit on Breadboard several times and a prototype on a PCB I etched, all worked as intended.
Make sure the two diodes are opposed to each other as shown in the schematic.
I'll try and reproduce the fault later today using a Breadboard setup.
Regards,
Relayer

11. ### swampy1978uk

12
0
Dec 16, 2012
made it on stripboard coper strip on 1 face and might be veloboard.
im not sure if i have the potentiometer wired correctly is the wiper the one thats connected to pin 6 on the ic?
might it be because i only have 1 set of leds hooked up at the moment?

Last edited: Dec 24, 2012
12. ### swampy1978uk

12
0
Dec 16, 2012
hello back on it this morning
just want to say thanks for all your help i've been a complete moron
moral of the story make sure you know which pin is 1 on the IC not just what you think you remember from school 20 years ago.
thanks again
ian

Last edited: Dec 24, 2012
13. ### Relayer

39
0
Dec 15, 2012
Don't worry, I still make those kinds of mistakes.
I've made a small rushed tutorial on how to make the circuit using veroboard. Here's the link:
https://www.electronicspoint.com/led-dimming-circuit-t255549.html

You're welcome.
Regards,
Relayer

14. ### swampy1978uk

12
0
Dec 16, 2012
just go back to it after a few days off over christmas
here is the first one of the circuits i made
attached pic
and here is a set of 5 fully working.
attatched pic
my next question is can i make it dim to off
thanks
ian

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Last edited: Jan 5, 2013
15. ### Relayer

39
0
Dec 15, 2012
No unfortunately. It operates within a duty cycle of 5% to 95%, but replacing the two 1N4148 diodes with germanium types (eg. 1N60) you can get a duty cycle of 1% to 99%.
Regards,
Relayer

16. ### swampy1978uk

12
0
Dec 16, 2012
ok i'll see if can find some.
at 1 point i did have 1 unit that dimmed to off then i removed my test wires and couldnt get it to do it again when i went back to it not sure what i was shorting to get that to happen.
thanks again for all your help

17. ### KrisBlueNZSadly passed away in 2015

8,393
1,272
Nov 28, 2011
Using germanium diodes won't affect the maximum and minimum duty cycle. Also, germanium diodes have fairly low current ratings and could be damaged when the duty cycle potentiometer is at either end.

You can widen the duty cycle range to maximum by disconnecting the diodes from pin 7 and connecting them to pin 3 instead. You can also remove the resistor from pin 7 to the positive supply rail.

18. ### swampy1978uk

12
0
Dec 16, 2012
thanks i'll give that a go
now if there was anyway to get it to go off not too worried if it go all the way on it would be perfect.
i think i will have to add a timer so it only works for a limited time
did find this but its a bit over the top for me http://www.opensourcepartners.nl/~costar/leddimmer/
unless someone want to make me some pcb's for it

Last edited: Jan 6, 2013
19. ### KrisBlueNZSadly passed away in 2015

8,393
1,272
Nov 28, 2011
How long do you want it to run for?
If it's only a few minutes, you can do that with another 555 connected as a monostable (google it). In fact you can use a 556, which is a 14-pin chip that has two 555s inside it. There are various ways to gate the LED-driving 555 on and off. But if you're hoping to save your battery, it won't do that. The 555 draws quite a bit of current at all times. You would need a different way to control it.

That design you linked to is interesting but it's not what you want. It's optimised for efficiency and contains a lot of components, including an inductor. You don't really need to go that far.

Post a full description of how you want the time control to work, and I will suggest something.

Last edited: Jan 6, 2013
20. ### swampy1978uk

12
0
Dec 16, 2012
I was thinking of putting a momentary push switch in that will turn the power on for say 5 mins or so exact time isn't essential. Just so they don't stay on all the time just when its not being played with. I have found a timer from rapid electronics for about £3 but it needs 12v so would need a step up and have found one for £4 from eBay but f there is a way I can make it myself. I have 5 dimmer circuits as shown above each driving 36 led's
Any help would be super.