Smoothing the output of a 555 timer

Thread Starter

Spence

Joined Apr 23, 2010
49
I need to illuminate a led very slowly and smoothly, as part of a voltage controlled filter. I suppose I'm looking for a sine wave and there are better alternatives than the 555 but wondered what's the closest to a sine wave that could be obtained with a 555.

I can get a very smooth transition with a two transistor flip flop but I can't seem to get a smooth on and a smooth off at the same time, plus there's a dead zone between flip and flop.

Any suggestions appreciated.
 

Wendy

Joined Mar 24, 2008
23,415
You can get a decent triangle wave off a 555. Does that count?





The best way is a standard sine wave oscillator followed by a PWM modulator. This will do what you want, but it needs 9V.

My blog has a lot of links for this kind of stuff.

Bill's Index

LEDs, 555s, Flashers, and Light Chasers Lots of alternate ways to do the same thing. Chapter 5 and chapter 12 especially apply.

My Cookbook

Sine Wave Oscillators A PDF abstract by TI
 
Last edited:

MattQ87

Joined Apr 5, 2011
13
Out of curiosity, how many LEDs are you looking to power up? You could use an LED driver (available from Linear, National, and a number of other suppliers) with PWM input for a current controlled solution (that would probably give you the smoothest output).
 

Wendy

Joined Mar 24, 2008
23,415
Or you could look at the circuits I have posted in Chapter 12. :D It will work down to within a volt of the Vf of the LEDs, or to higher voltage. Nice think about PWM, there are no real LED limits except for the power supply.
 

Thread Starter

Spence

Joined Apr 23, 2010
49
I solved this using a 555 and a dc to dc converter (toshiba k2611).

The output from the 555 pin 2, goes to the gate.
The drain goes to Vcc.
The source goes to ground via a 470 Ohm resistor and the led.
Rail voltage is 12v

I have never seen a smoother transition.

Thanks everyone for the advice
 
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