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How to fade a led in over 10 minutes, using cheap ic's? Answered

I want to fade in 4 leds over a period of 10 minutes then have then turn off after another 50 minutes or so, voltages of leds dont matter. Any ideas would be great! Thanks Cybot



Best Answer 9 years ago

First thing that comes to mind is a fairly large capacitor, that charges from source when you apply power - it slowly discharges through a huge resistor...and a transistor 'reads' its voltage and powers the leds from the power supply. Tweaking the resistor would adjust how long it takes to fade.

can You give a estimate of the values or a link to somewhere with a schematic / equation?

Off top of my head no...I hadn't put that much time into thinking about it :( google 'bowdens hobby circuits' - you can learn a lot by studying how those work.

i have built the circut "eyes" but cant seem to adjust the rate of fading, any ideas?

You could do this quite easily with a Picaxe 08m chip which would cost around 2 dollars. It's a programmable microcontroller which is programmed in a BASIC like language straight from your PC, and will drive up to 75mA if you tripled up the outputs (or as much as you like if via a transistor). The program would consist of nested loops, using a PWM (pulse width modulation) method to increase the brightness.
Total component count would be 3 resistors, one LED, one (8 pin) chip.

Is there any pre programmed chips comercially avalible, as i want it to be so that when they are purchased i just connect then to the circut and they start, as this a project designed to be mas produced... Thanks so far :)

Nothing 'off the shelf' which I know of for something that specific, but for that sort of delay a microcontroller is the way to go. The PicAxe 08 would come down to around 1 dollar 60 for quantities of 250 or more. If you went for a standard PIC solution, it would be cheaper but harder to program.

What sort of quantities are you thinking of? For very large quantities the most cost-effective way would be a custom die chip (ASIC). I've no idea what the breakpoint would be, but the initial tooling cost of that would be very high.

Ive made some code - I have a Arduino- so how would i make the arduino sketch into a format for flashing onto PicAxe?