I have a 12V battery as my power supply. I want a light to be on for about 15 min, then I want it to slowly dim over about 5 min before being fully off.

 

Interesting problem, and not as simple as it might first appear. Need more spec, namely: The wattage of the bulb, the accuracy of the timing, and the allowable heat dissipation of the circuit. Simpler linear controls will run hot, while more efficient solutions are more complex. More info please joco, and then I'm sure the circuits will start flowing, it's a neat challenge.

 

The light is for a pigeon shed so they can see to feed when dark and light dims slowly to allow them to perch before dark again.
Joco

 

What turns the light on?

 

thatoneguy:

What turns the light on?

Pigeons! I recall them being used as missile guidance systems - they're trained to peck at pictures of battleships for food rewards, and then they're stuck into the transparent nose of a special missile that is guided by their pecking at the sight of a real battleship below. They're single-use pigeons, sadly.

joco, we still need to know the bulb wattage. Knowing the application I guess the timing accuracy isn't too critical. I'm guessing too that you don't want to burn down your pigeon shed, so the power budget is important, and this will be a function of the bulb wattage.

OK, maybe it isn't pigeon-operated. Hopefully a switch can be assumed.

 

This should work:

The light is PWM controlled so the circuit should run cool and be pretty efficient.

The circuit around IC1a is an astable multivibrator, with the capacitor voltage cheekily tapped off to provide a crude triangle wave with a bit of DC bias to feed into IC1b, which is the PWM generator. When the switch is on, C3 is fully charged (R8 is quite small, just an inrush limiter to protect the switch), and the output of IC1b is high, as V1 is much greater than V2 (graph), and this turns TR1 on fully, illuminating the pigeons. When the switch is turned off at t=0, V1 starts dropping linearly as the voltage across C3 is drained by the current source I1, and when V1 reaches the upper peak of the triangle wave of V2 then TR1 starts to pulse-width modulate.

The bulb gets progressively dimmer over the course of 5 minutes as V1 sweeps down through the triangle wave of V2, and finally when V1<V2, the bulb turns off fully, and the pigeons can sleep.

joco, if this looks like what you need I can explain in more detail and work out some values.

 

Unfortunately the pigeons cannot operate the switch or they would put it on, eat, and then take turns..last one to roost puts out light!!!
The owner puts on the light when he puts out the feed. All he wants is for to be able to head off to work knowing the light would dim and then go out to allow birds to roost safely. The light would just have to be bright enough to light loft approx. garage size. appreciate your efforts.
joco

 

My query is for a friend BUT neither of us speaks electronicese and have not got a clue what the units are in the circuit. I would simply be taking your idea/circuit diagram and try ing to get someone to make it up.
How much approx. would the bits and pieces cost to make it up?

 

Probably about $10 for the components built on a bit of free board. A box, battery, cabling, switch and bulb would be the expensive bits.

You'll need some values on that circuit - that's the bit that takes the time, which alas I have little of right now. I could rustle up some numbers in a couple of weeks, while coming up with the circuit took about 20 minutes. If you're keen to build it I can work out all the values, just not quickly right now. Let me know and I'll sort something out.

 

Hi. Thanks for your time so far. I would appreciate any assistance you can give. Regards, Colin

 

put copper traces on the floor use that with a pull up to trigger an lm555 pin 2 as one shot 15 min. and light not lamps but LEDS put a limiting resistor in series with the leds to a big cap on the output since leds are diodes the cap will stay charged and slowly discharge due to leakage want a faster time just add a big resistor across the leds. just one lm555 and some passive parts