Hi all,

I'm after a circuit diagram for a flashing LED circuit. I've done a search and come across this:

I've got a few questions about the circuit. Firstly, as you can see this circuit diagram is intended for 6v use, however I want to power it from 12V DC. What values do I need to change and to what? Next, I would like it to flash from one LED to the other, is this the correct configuration? And next, what (if any) values to I need to change to have the flash rate at one per second, and what to?

Any help is very much appreciated,

Cheers,

Heinz

 

The 555 is pretty flexible, so the LED resistors are the only thing you have to change. What current do you want to go through them, and what are their colors (color partly determines how much voltage they drop)?

Have you read this?

Bill's Index

LEDs, 555s, Flashers, and Light Chasers

The 555 Projects

The 555 projects have been published in the AAC book shown on the top of this page, volume 6, as part of the experiments.

 

The 555 is pretty flexible, so the LED resistors are the only thing you have to change. What current do you want to go through them, and what are their colors (color partly determines how much voltage they drop)?

I'm going to be using standard orange LEDs. The current will be 20 - 25mA

Thanks,

Heinz

 

Hmmm, don't know the voltage drop for orange, so I'll assume 2.5V (which could be way off).

For the bottom LED 12V - 1.2V (internal transistor drop of the 555) - 2.5V (Vf) = 8.3V.

R = 8.3V ÷ 0.02A = 415Ω ≈ 430Ω


For the top LED (the 555 can go to ground) 12V - 2.5V = 9.5V

R = 9.5V ÷ 0.02A = 475Ω ≈ 470Ω

 

I think its 2.5v drop.

Are all the other component values correct for 12v operation?

Thanks for your help, much appreciated.

Heinz

 

Actually, the voltage for cap C1 should be upped. That would work, since the 2/3 rating for 12V is 8V, but it is never a good idea to run a component too close to its upper rating. A 16V cap would work.

 

Ok thanks,

I've just been on Rapid Online to have a look at the components. They don't seem to have a 10uf capped at 16v. Theres one capped at 25v, I take it this will be ok?

Cheers,

Heinz

 

It's almost always OK to use a component rated for a higher working voltage.

Rule of thumb for capacitors is 2x the voltage it will see across it's terminals, or higher.

The 25v cap will be just fine. Any cap rated 16v or higher will be fine in that circuit.

 

Thanks, now I've got the component values and schematic sorted, I've made a vero-drawing of the design:

Is this drawing correct?

Thanks,

Heinz

 

I simply don't have the time to verify your veroboard layout.

Try Pebble:
http://www.4dsystems.com.au/prod.php?id=93
It makes layouts much easier to do, and very readable.

 

Just one comment, you do notice this is a top view? You need to flip it to show the bottom view. I've made this mistake, it can be embarrassing.

 

Also, that 100k resistor that is in line with the 1k resistor should be a 100k pot. That will allow you to adjust the timer to vary the flashing rate.

 

Thanks for the help, much appreciated!

What do you mean by a 100k pot?

Cheers,

Heinz

 

A pot is short for potentiometer. It is basically a variable resistor. In this case, you want a linear pot, as opposed to an audio pot which has a logarithmic adjustment.

This will allow you to adjust the frequency of the 555 timer to dial in how fast you want the lights to flash.

 

Ah I see.

Interesting, thanks!