Basic LED circuit

[Matt__H]

Hi guys,

I'm making a toy cooker for my partners niece.

I've got half way through and now need to start looking at the LEDs for the oven lighting etc.

My intention is to have 6 white LEDs in parallel with a POT to adjust the brightness.

I've got the following items already:

- 50 x white LEDs (FORWARD VOLTAGE 3.0V MAX CURRENT 20 mA) (EBay item number:400816590565)
- 1 x 1k pot (a 3l3??)
- A ton of 180Ohm resistors
- A breadboard with wires for learning on
- 1 x 9v battery

I've managed to sucessfully blow up lots of LEDs whilst learning to her them connected correctly and and managed to get them to be in parallel and working through the pot. The problem I have is that at the pots lowest setting the LEDs are half lit. I want a pot that when I fully trust to the left it'll actually turn the lights off, and when twisted to the right the get brighter.

How do I know what size pot to get? I need a pot that I can stock knob onto as it'll be a child that will be playing with the cooker. 4 out of 5 dials will do nothing lol.

Is there also a way I can work it out so I can learn for the future please?

Many thanks
Matt

 

[Taylortwocities]

You'll have a problem just using a variable resistor to vary the brightness of a LED. The "strike" voltage is quite low and won't be linear.

You may need to use a Pulse Width Modulation technique to do what you need. This hand article may give you a steer in the right direction.
http://www.reuk.co.uk/LED-Dimmer-Circuit.htm

 

[JohnW2]

My intention is to have 6 white LEDs in parallel with a POT to adjust the brightness.

Because of the variation in exact forward voltage of the LEDs, you're likely to have problems trying to run bare LEDs in parallel - if you just do as you say, it's likely that the current would be enough to blow up some LEDs at pot settings at which others were not very bright. You need to have current limiting for each of the LEDs (or maybe each pair of LEDs). You could try something like:

(1) put one of your 180Ω resistors in series with two LEDs. This will limit the current through the LEDs to a maximum of about 17 mA. Make three of these resistor+2xLED combinations.

(2) connect these three resistor+2xLED combinations in parallel and put a pot in series with that 'combination of combinations'. The value of the pot is not crucial. Your 1kΩ one may well be too large a resistance to give reasonable control, but you will have to discover that by trying. With the 180Ω resistors in series with each pair of LEDs, you will not damage the LEDs, no matter what potr you use, or what you do with it.

Unless you use so large a value of pot that you have very poor control, you'll probably not be able to turn the LEDs off completely with the pot, unless it has a switch - you certainly will not be able to reduce the current through the LEDs to zero.

Kind Regards, John

 

[Matt__H]

Hi gents,

Thanks for the reply. John - that has been my intention to work in pairs with the resistors.

Do you know of there is a way I can work out the 'math/science' to workbout the pot size needed?

Secondly I've been using ebay to get my gear so far does anyone know a local shop that I can buy from? I've looked at maplins but their stock seems pretty small in the shops.

Thanks again for the advice thus far!!

Matt

 

[JohnW2]

Thanks for the reply. John - that has been my intention to work in pairs with the resistors. .... Do you know of there is a way I can work out the 'math/science' to workbout the pot size needed?

To do it really scientifically, you would need to knowthe current/light-output characteristics of the LEDs and how low you wanted to be able to turn down the light output (when the pot was in its 'closest to off' position).

With a 180Ω resistor for each pair of LEDs, with the pot (any pot) in its 'zero resistance' setting, there would be roughly 17mA going through each LEDs (close to their maximum - hence pretty bright). If you used your 1kΩ pot, when it was in its maximum resistance position, the current through each LED would fall to just under 1mA. Without knowing the characteristics of the LEDs, I couldn't say how much (if any) light output that would be represent.

Since you already have it, I suggest you try with the 1k pot and see what happens (and report back to us!). If it transpires that you need a higher (than 1kΩ ) pot to achieve the sort of minimum brightness you want, if the dimming is then 'all near one end of the pots travel', it might be worth considering trying a 'log'/'logarithmic' (rather than 'linear') one, which might improve the control.

Secondly I've been using ebay to get my gear so far does anyone know a local shop that I can buy from? I've looked at maplins but their stock seems pretty small in the shops.

There are not that many high street shops left which sell electronic components. However, I would have thought that any branch of Maplins ought to have the very basic sort of components we're talking about.

Kind Regards, John