1
\$\begingroup\$

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...

\$\endgroup\$
6
  • 1
    \$\begingroup\$ 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? \$\endgroup\$ Apr 18 '14 at 19:05
  • \$\begingroup\$ 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. \$\endgroup\$
    – maximus_de
    Apr 18 '14 at 19:08
  • 1
    \$\begingroup\$ 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. \$\endgroup\$ Apr 18 '14 at 19:15
  • \$\begingroup\$ @SpehroPefhany Yep, I'm looking for a good 555 image :) \$\endgroup\$
    – bitsmack
    Apr 18 '14 at 19:17
  • 1
    \$\begingroup\$ Hmm... 500 Henries... You'd need a very large, permeable core. Perhaps made out of mu metal. All the mu metal. Everywhere. \$\endgroup\$ Apr 18 '14 at 19:41
2
\$\begingroup\$

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

\$\endgroup\$
14
  • \$\begingroup\$ Well, @AndyAka beat me to it, but here is a tutorial on exactly what you are looking for :) \$\endgroup\$
    – bitsmack
    Apr 18 '14 at 19:19
  • \$\begingroup\$ 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. \$\endgroup\$
    – maximus_de
    Apr 18 '14 at 19:33
  • \$\begingroup\$ You could use that but I'd recommend using a 555 astable multivibrator \$\endgroup\$
    – Andy aka
    Apr 18 '14 at 20:01
  • \$\begingroup\$ 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? \$\endgroup\$
    – maximus_de
    Apr 18 '14 at 22:46
  • \$\begingroup\$ dunno pls show a circuit \$\endgroup\$
    – Andy aka
    Apr 18 '14 at 23:31
0
\$\begingroup\$

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.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.