# I thought this would be simple!

#### Rob.Richards

Joined Sep 3, 2011
9
So, I foolishly thought being given a soldering iron and some other bits to get me started would mean I could do the project that I want to do.

Basically I want to build a strip of 50 flashing colour changing LED's connected to a couple of AA batteries, but all the tutorials I can find show you how to wire up just one LED or a matrix! Am I being to ambitious? Should I just start with 1 LED?!?

I just wanted to start off with this for a photo project so would love some assistance. I will be honest this is my first post and I know very little about electronics but am looking to learn as i go!

#### MrChips

Joined Oct 2, 2009
29,809
An LED requires about 2V at 20mA. One AA battery is 1.5V. That is why you need at least two AA batteries. This would give you 3V which is more that 2V. So now you have to prevent burning out the LED. To do this you have to limit the current to less than 20mA and for this you need a resistor in series with the batteries and LED.

For two LEDs you will need at least 4V. The current draw is still 20mA.
So now you will need at least three AA batteries. Now the resistor you will need to limit the current to 20mA is going to be different.

You need to do some math.

For 50 LEDS, the math is the same.

#### Pencil

Joined Dec 8, 2009
272
First, what LED's do you have?

Describe the effect you are looking for. "Flashing"

50 LED's is quite a lot to power from AA batteries.
You may need to consider another power supply,

Let's see what you have in mind.

#### Rob.Richards

Joined Sep 3, 2011
9
Well I can reduce the number of LED's! Basically I want a long line of them to flash maybe once a second or if it was at all possible for it to be adjustable but I want them all to flash at the same time for this project!

PS so 20 LED's require 14 batteries is that right?!?!

PPS how come ones in shops only need 2 or 3 batteries?

#### Pencil

Joined Dec 8, 2009
272
Show us the LED's you have in mind, or tell us
the effect (brightness etc.) you want. If you
have the LED's already show us make/model/color
or where you got them. LED specifications vary
widely.

#### #12

Joined Nov 30, 2010
18,224
Parallel circuits also exist. If you put about 39 ohms in series with a 2.2 volt LED with two alkaline AA cells, you will give .0205 amps to the LED. If you put 50 of these circuits in parallel, the batteries will need to provide 1.025 amps and should last 1.4 hours of "on" time. If your switching circuit has the LEDs on for 10% of the time, the batteries will last 14 hours, if I did the math right.

You will also need a switcher that can run on less than 3 volts...not likely.

There is a start on what you need to do. A 555 timer chip can do the timing and it will need a power transistor to throw the amp required to fire the lights.

#### Rob.Richards

Joined Sep 3, 2011
9
I haven't chosen the led's yet, I had no idea they varied so much. I don't need them to stay on for long maybe a minute or so at a time.

#### debjit625

Joined Apr 17, 2010
790
Why dont you use a DC adapter like used in cell phone chargers, typically the cell phone chargers provide you with 5V DC and 1 - 2 Amperes ,I think thats enough to power 50 LEDs at 20mA.Anyway decide which way you want to go and let us know so that we may help you to complete your first circuit.

Good luck

#### Rob.Richards

Joined Sep 3, 2011
9
I think I should have been clearer. I need it to be portable so it can be used outside at night! It is probably cheaper to buy something pre made I just fancied making my own!

#### Rob.Richards

Joined Sep 3, 2011
9
I should clarify - I am not giving up just probably going to simplify my original idea to a 10 led line of lights powered by aa batteries and with a simple on/off switch

#### JingleJoe

Joined Jul 23, 2011
186
Well all LEDs need resistors in series with them, but you can put many LEDs in parallel with just one resistor in series with them (look up series and parallel circuits if you dont know what i'm talking about, it's simple) this means you can power lots from just 2 or 3 volts.

You don't need to be super accurate about getting the right resistor if you just use standard LEDs, I tend to whack in any resistor between 100 and 1k ohms, lower resistance = brighter LEDs because more current is allowed through, and vice versa.
Sure this might mean they get a tad too little or too much current which can lead to shortened life span of the LEDs, but it works!

#### Rob.Richards

Joined Sep 3, 2011
9
Cheers Joe, so I think I need to research a paralell circuit for what I am looking for. I am guessing the 100ohms offers less resistance than 1k ohms so will see what I can find! Thank you - I will keep you posted on how I get on!

#### Wendy

Joined Mar 24, 2008
23,317
LEDs, 555s, Flashers, and Light Chasers

You will need to break those 10 LEDs into chains of whatever (depending on the battery voltage). For example, you can have one or two LEDs per chain, and 5 chains.

Tell us about your pile of parts and we can help. One of the things you need to know (if you don't know it you can measure it and find out) is the dropping voltage of the LED (AKA Vf).

#### Rob.Richards

Joined Sep 3, 2011
9
I haven't got any parts yet I was going to try and work out what i needed first! doh! In my mind, I thought I could put a nice row of led in straight line with maybe 1 resistor (if thats the right thing!) for each one and then some how wire in a switch (going basic now so not flashing) and a power supply. Maybe a 9v battery is a good thing?

Talking about this has made me realise what a novice I am i dont even know what sort of board to put it all on!!!! I know I should read through all the theory but I learn better through practice!

#### Potato Pudding

Joined Jun 11, 2010
688
So this is an LED Camera flash.

Instead of 100 in one module - design something that has 10 Leds and a slave control line. It can work from a press of the camera's shutter

Make 10 of these and slave them together. That will give you more flexibility for placing them and some redundancy for reliability. Instead of one huge module that dies and spoils a shoot you lose one small module and make due with nine other small modules.

More ideas.

Camera Flash modules generally work by charging a capacitor.

Make a very basic switching supply that charges a capacitor and discharge it to pulse your series string of LEDs. You can do this without a dropping resistor if you have the wattage dropped from the storage in your capacitor worked out right to be safely less than 10x the pulse energy allowed for each of the 10 LEDs.

There was some other stuff I was thinking, but I am trying to get some other chores done and when I sat back down and they were gone from my mind.

#### Rob.Richards

Joined Sep 3, 2011
9

#### Potato Pudding

Joined Jun 11, 2010
688
A bar graph! Very interesting idea.