hmm I don't have a schmitt trigger but I will try this on the breadboard and see what I get.
LED fade in/fade out
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1st April 2012 03:08 PM #11alec_t
Ok. This should do it.
U1b/R2/C3 form a ~23kHz oscillator whose output is integrated by R3/C4 to give an approximation of a triangle wave. When the switch is closed C5 charges via the pot to generate a rising voltage ramp. The wave and ramp are summed, at the input of U1c, by R5/R6, to give a PWM output which switches Q1 and Q2 to drive NMOS FET M1 and hence the load (your string of LEDs plus current-limiting resistors). The PWM signal has a duty cycle which increases over the ramp period (settable with the pot) to fade-in the LEDs.
When the switch is opened the ramp voltage falls and hence the PWM duty cycle decreases over the same set period to fade-out the LEDs.
N.b C5 should be 220μF in practice, but to make the simulation run faster it is 22nF.0My circuit designs should be regarded as experimental. Although they work in simulation, their component values may need altering or additional components may occasionally be necessary when the circuit is built. Due safety precautions should be taken with any circuit involving mains voltage or electrostatic-sensitive components.
Alec's First Law:-
Every problem has a solution (given the right information and resources).
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1st April 2012 11:16 PM Thread Starter #121-3-2-4
0
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2nd April 2012 08:41 AM #13ljcox
Here is a circuit that I designed for someone else some time ago.
It does not do exactly what you want but it could be adapted.0Len
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2nd April 2012 08:55 AM #14colin55
How do you get 30mA for the LED with 150R current-limiting resistor???
0
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2nd April 2012 09:24 AM #15colin55
If you just want fade-on fade-off this is all you need:
UpDownFadingLED-2.gif0
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2nd April 2012 09:30 AM #16ljcox
Try Ohm's Law.
Originally Posted by colin55
0Len
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2nd April 2012 09:35 AM #17colin55
How did you use Ohm's Law.Try Ohm's Law.0
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2nd April 2012 11:56 AM #18alec_t
@colin55
The circuit in post #15 will need adapting to get a 30 sec fade period. It currently gives a 3-4 sec period.
Re Ohm's law, take a look at the stated voltage at point X in the schematic and work things from there.0My circuit designs should be regarded as experimental. Although they work in simulation, their component values may need altering or additional components may occasionally be necessary when the circuit is built. Due safety precautions should be taken with any circuit involving mains voltage or electrostatic-sensitive components.
Alec's First Law:-
Every problem has a solution (given the right information and resources).
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2nd April 2012 02:42 PM Thread Starter #191-3-2-4
Is it possible to make it adjustable? not a huge deal if it requires a whole change.
also for post #15 mine is a LED strip so I don't know how much that has in common with the luxeon LED's0Last edited by 1-3-2-4; 2nd April 2012 at 02:48 PM.
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2nd April 2012 06:33 PM #20alec_t
If you mean the post #11 circuit it is already adjustable, using the pot labelled 'Set time'.Is it possible to make it adjustable?0My circuit designs should be regarded as experimental. Although they work in simulation, their component values may need altering or additional components may occasionally be necessary when the circuit is built. Due safety precautions should be taken with any circuit involving mains voltage or electrostatic-sensitive components.
Alec's First Law:-
Every problem has a solution (given the right information and resources).
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