to turn on 1to 36 led array

[minu.grg]

i wish to turn ON my led array. it is having 36 led. Switches will be connected to all the 36 led. i wish to turn ON as many LEDs i wish by pressing multiple switches together. There may a possibility to trigger a single LED or many LEDs together. Some times all 36 LEDs need to be turned ON. In that case individual LEDs dont get sufficient current. what can i do for a reliable working?

 

[Nigel Goodwin]

Well each LED has it's own current limiting resistor, so the only limit is the current capability of the battery - are you trying to use a small PP3 type battery?

 

[audioguru]

A little PP3 9V alkaline battery will last for about 10 minutes at 0.5A, dimming the LEDs more and more the entire time. Which gets hotter, the battery or the resistors? A Chinese Sooper Heavy Dooty 9V battery will fall flat on its face.

 

[Pommie]

All LEDs in series with a 20mA constant current source, each one shorted by the nc conacts of a push button.

Mike.

 

[minu.grg]

Dear Mike,
Then voltage across each LED becomes less i think. Atleast 2V we need across LED to turn on right?

 

[Pommie]

You need 20mA through each LED.

Mike.

 

[minu.grg]

ya i am using pp3 9v battery. My problem is actually, i cant say how many switches will be pressed at a time. it may be all 36 switches closed at some time to only one switch closed. if only one switch is closed, that LED need to drive a heavy current. as everything is random, i cannot design the current through each LED

 

[minu.grg]

ya i need 20mA through each LED. that is ok if i connect all LEDs serially. But when we connect LEDs serially, the total voltage of battery will split up across each LED right? so as an example if i need to turn ON all LEDs, each LED will get a very small voltage right? So the LEDs may not lights up

 

[Pommie]

A pp3 battery is a really bad choice.

Mike.

 

[audioguru]

The voltage across an LED depends on its color. A red LED is 1.8V to 2.2V. A blue LED is 2.8V to 3.6V.
A tiny PP3 9V battery is big enough to light only a few LEDs, not 36 of them. Also its voltage keeps dropping as it is used so then one LED will slowly dim but a few LEDs in series will soon stop producing any light.
Then using a lithium rechargeable battery would work better since its voltage does not drop much when discharging. One Lithium rechargeable cell is 3.2V when dead and should be disconnected to the load, 3.7V during most of its discharge time and is 4.2V when fully charged.

 

[Beau Schwabe]

The key is PWM and multiplexing the LED's. Below is a video of a project I did for my daughter for Halloween one year. She went as the night fury from the movie "How to train your Dragon". After Halloween I decided to see just how long it would run on a 9V battery which honestly I was doubtful that the battery would be enough to last the entire night for Halloween. .... From the end of October until the end of March, the lights were still going. In total there were about 16 blue LED's with a 47 Ohm current limiting resistor on each LED.