# Flash Led

Discussion in 'General Electronics Chat' started by FroceMaster, Sep 16, 2014.

1. ### FroceMaster Thread Starter Member

Jan 28, 2012
402
4
Hi
Need a simpel Circuit to make a LED flash,(4 in parellel)
i know i will need 4 resistors on 330 Ohm,
i have a 5v aoutput, 20 mA
Have can i make that, blink 2 times a second, 2 times off and 2 times on each second, aproxx.
Simpel as possible,

Correct, have 5v 20 mA as control and 5 v 1A total.(allways on)

Last edited: Sep 16, 2014
2. ### wayneh Expert

Sep 9, 2010
12,368
3,224
So you have a 5V supply and want to flash 4 LEDs. In parallel, they will all flash together. Is that OK?

I don't understand your blink pattern. Are you saying you want it to flash at 2Hz, on and off twice each second? Roughly equal on and off times?

When you say your supply is 20mA, is that the total output capacity? That allows only 5mA for each LED. They will light with that, but it's not much.

Do you have any parts already? You could use a 555 timer or an op-amp to build your flasher. A transistor may also be needed.

3. ### FroceMaster Thread Starter Member

Jan 28, 2012
402
4
Transistor is ok to use, but not a 555, yes 20 mA it total output from mu MCU. have access to 1A.
use only resistor , cap and transistors., and yes all blink together, common ground. on all leds.
1 Hz or 2 or 3 is all ok.

4. ### Bernard AAC Fanatic!

Aug 7, 2008
4,231
414
Does your MCU supply a 2 Hz signal? 330 ohms seems large, @ 2 V - Vf - & 330, would give only 9 mA. Do you really want the LEDs cathodes @ ground or just connected together? For consideration: MCU is supplying a 5V P-P signal thru a 510 ohm resistor to the base of a NPN transistor, 2N3904 etc., emitter grounded, collector to 4 LED cathodes, LED anodes each to a 330 ohm resistor, other end to 5V, 1A supply.

5. ### FroceMaster Thread Starter Member

Jan 28, 2012
402
4
have designet this
this is how the leds will be connected, and i will use the MCU as "flashing".
but i can only Draw 20-25 mA from MCU, Thats why i need this.
what value for the resistor to base ?

eason for use BC337 : have 10 of them,

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6. ### wayneh Expert

Sep 9, 2010
12,368
3,224
The base resistor should be sized to allow up to 10% of the load current to pass thru the base. This ensures it is turned on hard and will have minimal series resistance.

So R =10 x ∆V/I load. Assuming your 4 LEDs are running at 15mA each, R = 10 x (5-0.6)/0.06 = 733Ω. I'd probably choose a 820Ω or maybe even a 1K.

You can use a larger base resistor if you are indeed running the LEDs at just 9mA each.

7. ### FroceMaster Thread Starter Member

Jan 28, 2012
402
4
Thanx 1k i have.

8. ### Bernard AAC Fanatic!

Aug 7, 2008
4,231
414
1k should be fine.
Wrote yesterday, forgot to hit Post Reply?? Another marble down the drain. They are getting in short supply.