Hi Alex
I've included a link to a hand drawn diagram its an LZW TIF so its only about 30k in size.
If you have any problems with the file just ask, and if you can't read it etc..
Circuit Diagram
This is a basic circuit if you get this working extra bits can be added to make it more random improve etc.. However I think its best to start more simply. I think building this should help your understanding. Heres a basic description of the circuit.
The first 555 circuit runs at about 0.1Hz (5secs on 5sec off) this makes the clock to the first 4017
The outputs of the 4017 are connected together to make OR gates, when these outputs are on the welder will be on.
You can change these around as you like to change the timings, also if you change the capacitor on the 555 you can change the speed. The one with the capacitor and resistor adds an additional delay (As the Capacitor charges) on one to give a bit more variation. The trasnsitor acts as a NOT gate. You can omit this if you think in reverse and change the diodes around, you will need more diodes for this though.
The second 555 runs at about 0.2Hz you can change this as well, you might want to make it go a bit faster. This makes the clock for the 4017, again the diodes act as an OR gate to stop backfeeding
Then these are connected to the LED via resistors. I have added the forumula to calculate the value. and the approx minimum and maximun values for a blue LED. What you can do here is change the brightness of the arc.
Say you make R1 and R2 47 Ohms you will get a bright flash then off and another bright flash. You might want to then make R3 200 ohms so it will be duller.
More sugtested values, (I'd play around and see what you can get that looks good)
R4 47 Ohhms
R5 470 ohms
R6 270 Ohms
R7 100 Ohms
While it is on now you will get flashing.
Okay this is a start its only basic you could do a lot more with a PIC like a lot of the ciruits use, if I was making it myself I would probalbly use a PIC they are so much fun to program and cheap. However I still enjoy fidlling with circuit like this.
Construction tips, make the 555 circuits and check they work with an LED on pin 3 the output (Don't forget the resistior) Then move onto the 4017s in testing you can use more LEDs to check for signals.
If you want you can connect some of the 4017 LED outputs to a white LED so you get a mixture of blue and white.
Okay have fun, it should work though I haevn't actually built this exact circuit but have made many before like this that do similar things. Any problems don't hesistate to e-mail me.
David
Erata:
On the diagram the resistor above the transistor should be 1k
And the 470uF capacitor on the first 555 should probably be 220uF or maybe 100uF (I made a slight mistake in my initial timing calculations)