Quote:
Originally Posted by Al Skierkiewicz
Eric,
The LED outputs on the OI are the 5 volt power supply through a resistor. They were designed so that you don't need to add a resistor you just connect the LED straight to the OI output. (The OI REf Guide on the IFI site explains this) The internal resistor value was chosen to provide an LED current of 10 ma. If you add external resistors you will lower the LED current even more, perhaps making them so dim you can't see them. We have used LEDs on the operator station for many years to do exactly what you are asking. We have software that helps us calibrate everything on the robot and select a variety of auto mode solutions. We do not use the LED inside a switch but rather mount them individually so that we can identify them with labels. PM me and I will give you my email if you need anything further. Time is short, good luck.
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Al,
On Jan 29th, I communicated with IFI regarding their documentation being in error with regard to this, and to their credit the OI documentation was updated the next day.
See page 5 of the current documentation here:
http://www.ifirobotics.com/docs/oi-r...de-1-30-07.pdf
In addition to their explanation here are some additional comments and observations...
The outputs are
not derived off a 5V supply, the voltage varys as a function of the input voltage and appears to be a fairly consistent voltage drop of about 2.5 to 2.6V (as measured between the tether input and the open circuit LED output). Given that the tether power input could be about 12.5V, the LED outputs could see as much as 10V.
At competition, the OI power input via the competition connector would be about 9.9V at its lowest (confirmed with IFI), so the open circuit LED voltage would be on the order of 7.4V minimum.
When using the wall wart, depending upon the load that open circuit voltage will probably be around 8.4V (per IFI).
The resistor inside the OI is 470 ohms (per IFI). I calculate an effective resistance closer to 500 ohms, and IFI's new documentation puts it closer to 550 ohms.
Assuming 470 ohms, with a RED LED that has a forward voltage drop of 1.7V, a minimum of about 10-11mA would be available. When tethered, it could be as much as 16mA.
At the other end of the extreme, a BLUE LED with a forward drop of 4.5V would only have about 5-6mA available at competition.
Perhaps previous generations of OI's did power the LED's from 5V and the user manual never got edited.
The bottom line is that we do agree that when using the LED outputs to power LED's, no additional resistor is needed and I think IFI's new wording helps to clarify this.