Why is ShiftX brighter on AnalogOut than GPIO?

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GTIspirit
Posts: 233
Joined: Wed Jan 09, 2013 11:20 am
Location: SE Michigan

Why is ShiftX brighter on AnalogOut than GPIO?

Post by GTIspirit »

I wrote some scripts for ShiftX to be staged, variable blinky. 8)
http://www.autosportlabs.org/viewtopic.php?t=4328

I noticed that the ShiftX lights are appreciably brighter running on the AnalogOut than on the GPIO. Why is that?

I'm using the same 5V supply for both, and the unit works by low side switching. So when the control pin is high the lights are off, and when the control pin is low the lights are on. So if the high isn't a full 5V the lights will be very dimly lit. And if the low is 0V they should be fully lit.

GPIO is either off or on, high or low, should be either 5V or 0V. Since the AnalogOut pins are brighter when they are low they must be more zero than when the GPIO is low. Only thing I can think of is the GPIO doesn't switch between 5V and 0V, but maybe 5V and say 1V?

Anyhow, the advice to anyone using the ShiftX is the lights will be brighter on the AnalogOut pins than on the GPIO pins!

Oh, and to prove my point I measured the current since current equals brightness.

GPIO1 - Green - 0.073mA
GPIO2 - Yellow - 0.247mA
GPIO3 - Red - 0.229mA
Total draw on 5V supply - 0.529mA

AnaOut1 - Green - 4.3mA
AnaOut2 - Yellow - 14.1mA
AnaOut3 - Red - 3.4mA
Total draw on 5V supply - 21.7mA

P.S. so I see part of trick in designing the ShiftX was to get all LED's at the same visible brightness when driven with the same signal.

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