# Help with LED lamp circuit please.

#### stringpickin

Feb 25, 2011
3
Hello and good day! Thank you so much for your help and time.

I want to create a string of LED lights using 3v coin battery(s) with 5 or 6 white LED’s. I want to use a miniature on/off switch and make the thing as small as possible..

I have read that leds can become unstable if a direct voltage source is used for leds without drivers or resistors of some kind.

Can someone help me layout the circuit and determine what electronic components I need to do this..?

Thank you

stringpickin

Moderator
Jan 21, 2010
25,505

#### stringpickin

Feb 25, 2011
3
help with led lamp circuit please

Hi Steve, thank you for the point to the article.. It did help me to gain a fair amount of understanding. However, I am not an electronics hobbiest by any means so I am still at a bit of a loss as to what to do to figure out what the requirements are for my needs.. ie. if I use 5 leds wired in series with a resistor before the leds, what voltage of battery will drive how many in the series. It is probably explained so that someone with a basic working knowledge of this info can figure it out, but I am not that person.. Can you give me a quick example..

ie. two 3 volt "lithium coin" batteries with a current rating of ?? will drive how many 3v white leds with the proper resister in front? Or if you could explain to me as to teach me how I need to interpret the information in the article so I can apply it to the needs of my string..

I ask for patience as I say,, talk to me about building a house, a house full of cabinetry, a fine piece of furniture, the theory of relativity or playing guitar and music theory and I could hang with the best of em.. But you have got a patience requiring electronics newbee here.. I really do appreciate any instruction, guidance you can give me.. thanks again.. stringpickin

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
2 x 3V cells will drive *one* white led at 20mA if you place a 220 ohm resistor in series.

Perhaps a better option is 3 cells in series which can power 2 white LEDs with a resistor of 120 ohms.

If you want more LEDs then repeat the resistor and LED(s), placing the string in parallel.

Each string will draw 20mA (approx) so if you had (say) 3 75mAHr cells, and 2 strings of 2 LEDs, the batteries would be exhausted in just under 2 hours.

The LEDs will actually get dimmer and dimmer, and may produce an acceptable amount o light for more or less than this time depending on your definition of "acceptable".

#### stringpickin

Feb 25, 2011
3
Steve, I can't tell you how much I appreciate your time and help.. Couple questions.. So I can assume if I use 230mah cells I will get about 6 hours out of 2 strings of 2 20mA leds while using the same 120 ohm resistor..?

Couple new questions.
1.Is battery life affected if the leds will only be lit for multiple short intervals (ex. 40 seconds) then turned off? Or does the math still basically work as it comes down to raw usage or drain off the battery.
2. I have heard there are new types of leds coming out that require much less current to produce more light, is that something you know about?
3. If one wanted to use 1.5v batteries and correspondingly smaller leds is there any advantage or disadvantage of doing this?
4. It may be the light requirements of this unit may be able to be accomplished with 2 leds.. It seems by your first line in response that 2 x 3v cells will drive one led is energy hogging because you have to use a larger resistor to keep the led in proper range? So does that mean even though you use one less battery and 1 less led that battery life would be cut in half?
5. Is there an easy way to time this circuit to shut off after 40 seconds of on time?

Gonna owe you lunch.. thanks again.. stringpickin aka Barry

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