LED blinking array using LC-circuit

Question

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.)

schematic

^{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...

• Spehro 'speff' Pefhany: 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?
• maximus_de: 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.
• Spehro 'speff' Pefhany: 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.
• bitsmack: @SpehroPefhany Yep, I'm looking for a good 555 image :)
• Stephen Collings: Hmm... 500 Henries... You'd need a very large, permeable core. Perhaps made out of mu metal. All the mu metal. Everywhere.

Answer 1

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: -

enter image description here

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: -

enter image description here

• bitsmack: Well, @AndyAka beat me to it, but here is a tutorial on exactly what you are looking for :)
• maximus_de: 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.
• Andy aka: You could use that but I'd recommend using a 555 astable multivibrator
• maximus_de: 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?
• Andy aka: dunno pls show a circuit

Answer 2

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.enter image description here CMOS can drive small LEDs.