Need advice for a LED circuit

[mustafa56]

Good Day to All

I'm a plastic aircraft modeler with limited knowledge in electronics. I am illuminating a model with 3 micro LED and one 3mm white LED lights. Of these one has a simple flasher from Radio Shack, (yeah, the one you buy for $5 bucks that comes with one LED in a small printed board and hooked up to a 3vdc battery pack that serves for any proyect). All these LED's and the flashing board are hooked to the same battery pack.

I'm feeding the circuit with a 3.0 vdc battery pack. The problem is that when the circuit is on, the flasher goes to work and every time it flashes it dims the rest of the LED's in the circuit. I know, from my limited knowledge, that there is something I can include in the circuit to prevent that from happening and to maintain a constant voltage across the rest of the LED's. Will anyone please advice what can I do and use to prevent that effect? See the simple drawing included of the circuit.

Many thanks for your time and advice.

George

 

[HarveyH42]

Try a 100 uf electolytic capacitor parallel to the battery. Wath your polarity on the capacitor...

 

[mustafa56]

Thanks

Thanks for the input. I'll try that.

George

 

[justDIY]

you need some resistors on your non-flashing LEDs ... the different colors (red versus green/white) have different forward voltages ... so your RED led is going to hog a lot of current compared to the green and white leds.

get some resistors for each led, and that should also help resolve your dimming problem.

how to calculate the resistor:

Resistor = (Vbat - Vf) / If

so If is the current for your led, I'd go with 0.015 = 15ma ... Vf is the voltage of your led, for red and possibly the green led, I'd go with 2v, for the white, go with 3v. Now one catch is, if the green is a super bright type, it will also need 3v ... a lot of the small green are 2v however... start low and work up

here's a wizard to help you if you prefer:
http://linear1.org/ckts/led.php

 

[audioguru]

The little 3V battery cannot provide enough current for 4 LEDs without its voltage dropping. A huge capacitor in parallel with the battery will help but not cure the problem. Using a separate battery for the flashing LED will fix it.

 

[justDIY]

I agree and disagree ...

Agreed, a capacitor will not solve the problem.

I disagree however, I think that a small battery pack (as the poster mentions) is more than enough current for 4 leds including the flasher ... the issue at hand is, there is no current limiting going on, so the three leds wired in parallel are heavy load on the battery... the batteries don't have enough current to instantly kill the leds, so instead the voltage is pulled down to where the leds are happy to operate. Limiting the current to 10 or 15mA per LED will resolve this problem.

 

[audioguru]

I agree and disagree ...

Hee, hee. I disagree with your disagreement.
The red LED across the battery is a perfect shunt 2V regulator. The green and white LEDs shouldn't light with the resulting 2V supply and the flashing LED shouldn't affect its brightness.
Which gets hotter without any current limiting resistors? The LED or the battery?

 

[ljcox]

I agree with jusyDIY and Audio.
You need resistors in series with the LEDs.