LED Single Flash 2 seconds.

Hi all, this is my first post here, and i hope im posting in the right sub.

Me and a friend are making a circuit of a light that pulses once, but we're having some problems, which we hope you guys can help us with.

The stuff we wan't to do is to have, a LED to turn on for approximately 2 seconds, and then stop imediately after, without dimming down.
Thanks in advance

Welcome to the forum!
What turns the LED on?
What is the power source?
Do you want the option of auto-repeat for the flash?

Thanks !
The LED is turned on by a 12V battery and the switch is a cars ignition.
The LED is turned on by the car and then lights up for 2 seconds.
We wan't to immitate this effect on our GoCart.

So i guess that it shouldn't repeat itself, but just be activated for 2 seconds.

Here's a suggested circuit:-

To explain some of the things in Alec's schematic that might be mysterious to you (he's using LTspice to draw this):

• The diode with the big long part # (BZX84C...) is just a 6.2 volt zener diode
• The NPNs can be any general-purpose small NPN transistors (2N2222, 2N3904, BC547, etc.)
• The "22u" is a 22uF electrolytic capacitor

All these parts should be easily available almost anywhere.

If you want to cut it down to a single small chip and a couple resistors/capacitors, you could use a 555 timer wired as a one-shot. Looks something like this:

**broken link removed**

Welcome to ETO!!!
Regards,
Der Strom

Thank you guys for the help.
I'll go with the 555 circuit, as i have a 555 laying around somewhere.
It's been a long time since i've used math and electronics.
How do i calculate the time in seconds?
I remember doing RC circuits with blinking LED's in school, but forgot how it works.
Thanks in advance guys, you've already helped us a lot

gamernixin said:

It's been a long time since i've used math and electronics.
How do i calculate the time in seconds?
I remember doing RC circuits with blinking LED's in school, but forgot how it works.
Thanks in advance guys, you've already helped us a lot

Click to expand...

Here's an online calculator that can help you figure out the timing:

http://www.csgnetwork.com/ne555timer1calc.html

Good luck!
Der Strom

For guidance on using the 555, click the 'Theory' tab at the top of this page.

Hi gamernixin,

supposedly you don't turn off the ignition key after the LED was lit for two seconds which means a non-retriggerable one-shot circuit is needed for your application.

That circuit lets the LED light up for two seconds with the ignition key still on.

Also switching ground for a negative trigger pulse won't apply for your application. Ignition switches turn on the positive rail to switch.

The circuit in the attachment triggers as soon as the ignition switch is closed and makes the LED light up for approximately two seconds.

Even with the ignition switch still closed the LED extinguishes after that period.

You might omit T2 and connect the LED anode to pin3 of the timer IC and via the current limiting resistor to ground.

Check out your LED specification sheet for the maximum allowable LED forward current and change the value accordingly.

Boncuk