Monday, November 29, 2010

Fast project time!

I need to swith lights (led strip) on and off on defined time of day. For example turn on at 8:00 and turn off at 21:00. I need to make a device till this weekend.

What I have is attiny25 mcu, ds1302 rtc chip, some transistors, mosfet for supplying power to led strip, 1 meter led strip (30 5050 white leds I guess). It would be great if I can make a pwm so lights can slowly turn on to full brightness and then slowly turn off till fully off, by slowly I mean 5-15 minutes ore even more.

For power I have a 14.4V power supply. I need 12V for led strip, 5V for mcu and rtc chip, 3.3V battery backup for rtc, I'm going to use 3V lithium battery for that with holder taken from dead pc motherboard. So for power I should use 7805 regulator for 5V, LM317T ajusted for 12V.

For switching I could use some 5-6V rated relay, but I guess it's not good for pwm, so I chose F5305L mosfet (actually I have no relays anyway). This is maybe an overhead for 1 meter led strip which takes 0.6A at 12V but it's first I found in my parts bin.

Here is a first circuit:

Firmware day tomorrow.