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Led nightlamp - Help Needed!

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manjero

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Hi.
I have a background in working with simple circuits i find from the net and modify a bit for my needs.
I'm also an EE student.

I want to build a nightlamp with leds. when turned on, 30 leds are on. every minute, one led turns off until all of them are off and the circuit does not restart.
I looked and found many sequencers that i could extend to 30 but i couldnt find one that keeps that led on.
Any thoughts?
I would prefer not to do this with a pic or other micros cause i never dealt with them and dont own a programmer.

Thanks in advance,
Shahar
 

A microcontroller is the way to attack this project. Programmers are not an issue. Many parts like the Atmel AVR can be programmed with a very simple circuit from the PC parallel port. Cost of doing this in a microcontroller will be X10 smaller than any other solution.
Since your update rate of the LEDs is so slow, I would use a serial to parallel converter chip outside of the microcontroller to hook up the LEDs. There are many, many, many possible chips to choose from. One chip dedicated to this job is:

**broken link removed**

Since this chip handles 16 LEDs, you would use two of them connected in a chain. Then it is just a software exercise to write the code.

If you do not use a microcontroller, then you are dealing with lots and lots of discrete logic chips. You will have to do all the counters and controls in hardware. Any bugs will require you to respin all the hardware. (With the microcontroller approach, most bugs are simple software changes.)

--- Steve
 

If i do go that road, wouldnt a 30 i/o pic would work easier for me?
and if so - i wouldnt even need transistors would i?
I can't get the chip you mention in reasonable time where i live (Israel) so im trying to figure out a way.
 

Hi.
After reading a bit, i got to the conclusion that i should use a micropic.
i went with the 877a which i had lying around.
it has 33 i/o and i made all of them outputs and then wrote the jal software to shutdown led after led with the right delay between them.
just a couple of questions -
1. do i need to use transistors or something like that or the pic can drive all the leds together by itself (using 330 ohm resistors of course)?
2. except for the VCC and a 20 MHZ xtal with 2 caps do i need anything else?

Thanks in advance,
Shahar
 

I think the large number of LEDs is a problem. Assume 10mA per LED to have a good brightness level. Then 30X10mA = 0.3A. This large current would have to exit the PIC via the ground pin. (This assumes that you are sinking current. ) Most microcontrollers can sink larger current than they can source.

There are two solutions. The hardware way is to put a transistor on every LED. The software method is to pulse the LEDs in groups. If the pulses are fast enough, the human eye cannot detect them and the LEDs appear to be continually ON.
 

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