Driving LEDs straight from Relay outputs

[skippy178]

OK, here's the scenario:

We want to do some software testing using a minimalist FRC controller plus an electronics board with some switches, potentiometers and LEDS.

The switches will go to digital inputs

The potentiometers will use the analog inputs

So far, all easy to do

But the LEDS, I would like to drive from the Relay outputs on the controller directly, using the FWD and REV outputs to drive two separate LEDs e.g. green for FWD and red for REV.

Normally the Relay outputs go to a SPIKE, but I would really like to do away with them to make the test rig setup smaller and less expensive.

Is it possible to drive LEDs directly from the Relay FWD / REV outputs ?

Is there any good technical reference that explains the FRC Controller's Relay output signal drive characteristics, i.e. voltage and max current source/sink per output channel ?

Thanks in advance

Jason

[DonRotolo]

First, have a look at the RC specs (http://www.ifirobotics.com/docs/rc-ref-guide-6-13-2006.pdf) especially regarding the relay outputs, then have a look at the spike specs (http://www.ifirobotics.com/docs/spike-blue-guide-sep05.pdf), taking note of the two solenoid example.

Give it a bit of thought and you'll figure out how to drive an LED off the 5 volts that comes out of the FWD and REV pins of the Relay outputs, with the black wire being Ground/Negative/Cathode. Do not forget the dropping resistor.

Alternative would be to use a spike, since then you could drive 20 Amps of lights, but the RC can drive an LED at perhaps 10 mA

Have fun,

Don

[skippy178]

Thanks Don

I did have a look at the RC and Spike specs, but neither gave definitive figures on the amount of current (if I am wrong, please point me to specific page numbers !).

The page on the RC at IFI Robotics site

http://www.ifirobotics.com/rc.shtml

mentions 7mA for the Digital I/O pins, but says nothing about the Relay ones.

In response to your message, the short answer to my question should be that with 5V on the FWD and REV pins of the RC when they are switched on, and assuming a 1.5V drop across an LED (being a typical figure), that leaves 3.5 volts to drop across the series resistor. For a 10mA current, that would require a resistor of 350 ohms (or higher to give a lower current).

The nearest E12 resistor value is 390 ohms - the next lowest, at 330 ohms is too low and would give too high a current.

OK ?

Jason

[Al Skierkiewicz]

Skip,
A hint would be that the input side of a Spike is an LED opto coupler. This in no way means I am telling you to use the RC for this purpose.

[skippy178]

I got an answer "straight from the horses mouth" thanks to the people at IFI Robotics.

See

http://www.ifirobotics.com/forum/viewtopic.php?p=951#951

In a nutshell LEDs can be driven directly. The outputs have 330 ohm resistors internally as current limiters. 8mA or so of drive should be easily accomplished.

Perfect for what I want :-)