Thread: Help with simple circuit needed, ....

... any electronics geeks in the house? I need a simple circuit, to generate a biased sine wave, or dampened square wave, at low frequencies, sub 1Hz, pref in the range of 0.1Hz to 1Hz, to drive an LED and make it slowly rise and fall in brightness.. Most circuits deal with higher frequencies, and it's been an age since I did any electronics, and although I recall using a capacitor and resistor to set the frequencies, can't remember what the formula was.

I can't answer that off the top my head but I'm sure the people at electro-tech could sort it out.
f=1/2piRCsqrt6 is the RC formula, I don't know how to format it properly :?

You might be able to use the age old 555 timer chip with the right size cap to dampen the rise and fall. If you limit the current charging the cap with a resistor it should charge up at a somewhat sine. At the very least you'll get a triangle wave, might not be all so bad. If you set it up right you could use a transistor to amplify the wave in order to drive a heavier load. Keep in mind LED's don't turn on until around 0.7 volts. The 555 has the ability to do variable duty cycle, in this case you would want to shoot for 50%. There are ways to do all this with basic CMOS logic chips as well. You could become very sophisticated and use an 8 bit D/A converter and cycle the correct series through. That of course would need a CPU to get it to make a true sine. Well it's not the only way.

I'm not sure if a PLL would produce a sine, I think they do. It's been a while. I'm sure an op-amp or two in the right config would make a nice sine.

Originally Posted by (In)Sanity

You might be able to use the age old 555 timer chip with the right size cap to dampen the rise and fall.

That's kinda the way I've been thinking, as it's been 19years since I studied elctronics, and 555's solved a lot of my problems back then!

And yes, the trigger voltage of the LED hadn't escaped me, I was probably just going to use a couple of resistors to hold an appropriate PD, and feed the output of the 555 in on top, rather than trying to bias the 555 output direct.

Originally Posted by Phlogistician

Originally Posted by (In)Sanity

You might be able to use the age old 555 timer chip with the right size cap to dampen the rise and fall.

That's kinda the way I've been thinking, as it's been 19years since I studied elctronics, and 555's solved a lot of my problems back then!

And yes, the trigger voltage of the LED hadn't escaped me, I was probably just going to use a couple of resistors to hold an appropriate PD, and feed the output of the 555 in on top, rather than trying to bias the 555 output direct.

Ahhh that's a good idea. Bit of a voltage divider. A Mosfet comes to mind if you want to drive a heavier load and not put a draw on your charge, discharge section to allow you to get a closer sine. I just fear that if you bias the LED with a divider it may play havoc with the charge discharge cycle in such a way that you end up more and more square, or some version thereof.

Good luck, sounds like fun.