Electronic Circuits and Projects Forum

1. 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.

2. hmm I don't have a schmitt trigger but I will try this on the breadboard and see what I get.

3. 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.

4. How do you get 30mA for the LED with 150R current-limiting resistor???

5. If you just want fade-on fade-off this is all you need:

6. Originally Posted by colin55
How do you get 30mA for the LED with 150R current-limiting resistor???
Try Ohm's Law.

7. Try Ohm's Law.
How did you use Ohm's Law.

8. @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.

9. 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's

10. Is it possible to make it adjustable?
If you mean the post #11 circuit it is already adjustable, using the pot labelled 'Set time'.

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