# LED blinking array using LC-circuit

Basically I want to create a blinking unit to drive my LED's. The LED's are used to be driven at a frequency of ~1.5Hz (that made me choose the values of L and C).

So here is my schematic. (I am physicist so there might be some issues with current direction...)

(The PNP is used to block the supply-current if the capacitor is charged.) simulate this circuit – Schematic created using CircuitLab

After a while of thinking about it I got to the point that this wont work because the resistance in the LC-circuit will cause that there wont be any oscillation. Even if in D3 is only a single LED...

And now I have no idea how to get my 12V DC current into a 12V AC current with a frequency of 1.5Hz. I searched a long time how to do this but I don't get it how to get it working.

I found i.e. circuits of power inverters but all of them (I found) are at 50Hz and 230V AC output.

How could I get it working to transform my 12V DC to 12V AC (f=1.5Hz) without using an IC. Well I thought of using a quartz but no one I found has a sufficient low frequency...

• 500H is a VERY large inductor. Is there some reason why you don't want to use an IC and want to use LC as opposed to RC? Apr 18, 2014 at 19:05
• I know it is very large, that is from some easy calculation. I already decided that I am going to change that. Well i might us an IC as long as I dont need to program it and it is more more efficient than not using one. Apr 18, 2014 at 19:08
• Okay, the science of blinky-LEDs is fairly advanced. Someone should be along with a 555 circuit momentarily, as well as a suggestion on using avalanching BJTs or a PIC. Apr 18, 2014 at 19:15
• @SpehroPefhany Yep, I'm looking for a good 555 image :) Apr 18, 2014 at 19:17
• Hmm... 500 Henries... You'd need a very large, permeable core. Perhaps made out of mu metal. All the mu metal. Everywhere. Apr 18, 2014 at 19:41

Get a 555 timer chip. No programming necessary and there are a gazillion circuit ideas on the net. Google "astable oscillator". It should be able to directly drive a LED directly but if you need more power use a mosfet amplifier.

EDIT to show a calculator for the 555

This website has calculators for various 555 timer circuits. Here's a picture: - Note the values of R1, R2 and C - these give a frequency of basically 1.5Hz and a duty cycle of about 51%. This is how easy it is but take note that power supply voltage variations will "tweak" the results slightly.

Here's another one: - • Well, @AndyAka beat me to it, but here is a tutorial on exactly what you are looking for :) Apr 18, 2014 at 19:19
• So if I would take this one: commons.wikimedia.org/wiki/File:Astable_multivibrator.png The 555 timer chip is? I just see two NPN's. Apr 18, 2014 at 19:33
• You could use that but I'd recommend using a 555 astable multivibrator Apr 18, 2014 at 20:01
• For my use I calculated the following: a) R_1=20 Ohm, R_2=470 Ohm and C=1 µF or b) R_1=20 kOhm, R_2=470 kOhm and C=1 mF. Which one would be the better choice? And what kind of capacitor should I use? Apr 18, 2014 at 22:46
• dunno pls show a circuit Apr 18, 2014 at 23:31

Using 1uF capacitor and a few MOhm feedback will get you a square wave. Symmetry is skewed if R is too high compared to capacitor leakage R or too low R as in below example compared to old CMOS drive impedance. CMOS can drive small LEDs.