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Topic: Speedometer newbie guidance (Read 5008 times) previous topic - next topic


Feb 21, 2011, 01:55 am Last Edit: Feb 21, 2011, 01:59 am by Graynomad Reason: 1
The schematics have disappeared.

Forward voltage: 1.8v

This has changed.

Supply Voltage: 18v

Why? I can see no reason to drive the LEDs with 18v unless there are several LEDs in series. It's hard to say without knowing the display being used but if you're MUXing columns of 8 LEDs then they are not in parallel, they are 8 individual LEDs and the current resistors should be set accordingly.

because 8  * 1.8 = 14.4v

Are there 8 LEDs in series? You only ever turn on one column at a time and drive the eight LEDs of that column, even if they are all on they are in parallel not series so it's no different to just having normal 8 LEDs hooked up to you outputs.

Yes my gadget will still run. If I turn my ignition to its 'On' state it will turn on the electronics of my bike, then I can turn on the engine with the start button after the warm-up regulator did its job.

That's true of many vehicles but has nothing to do with a gadget that flashes some LEDs. Anyway I'll stop asking about this signal now, if you need it you need it.

Another thing to think about is the sinking ability of an AVR pin, you can drive (source current to) a single LED from a pin but cannot sink the current from 8 LEDs, so the columns need a high-current device or some transistors. By using a special LED-driving shift register you solve this problem and also the lack of IO pins.

Rob Gray aka the GRAYnomad www.robgray.com


Sorry, my internet was busted and my server went down with it. The schematics should be back now.
But I really should stop trying to make up with some random calculations. Makes me look more like a noob. haha.  :D

Thanks for the shift register tip. I will definitely look into this option and also again thank you for your time and advice Rob!

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