Let's Make Robots!

Resistance and Ohm's law with Leds

Okay, this question is based on OddBot's great build your own ir sensor tutorial.

I noticed that the schematics showed a 150R resistor inline for the 2 IRs, but 82r for the single version.

My understanding of Ohm's law is that if I increase resistance, I decrease current. Therefore, wouldn't I want less resistance for the 2 leds?

Or am I missing an important part of the equation? Is the transistor increasing current, and so more resistance = more voltage?

the schematic:


okay, Go!

Comment viewing options

Select your preferred way to display the comments and click "Save settings" to activate your changes.

Well i guess he's doing that on purpose. If you have 2 LEDs you can have them run with lower currents but if you have a single one then to have the same light you would want to let more current pass through it....i think this is what he wanted to do.

 But yes, you are right about what you said, if you want the same current to pass through the LEDs as the one that passed in the single LED you would get a lower value resistor.


i just checked, he says why did that:

"Because this sensor only has a single phototransistor it isn't quite as sensitive. To compensate I've increased the current to the LED to almost 50mA which is the maximum continuous current allowed."