How Much Current for Line-Following Sensor QRD1114??

[richardv2]

So I ordered a bunch of QRD1114's for a line-following robot to use as black/white sensors.

Most schematics I've ever seen put a 270 ohm or 330 ohm current limiting resistor in a (regulated) 5-volt setup. So my math would be:

I = E / R = 5v / 270 ohms = 18.5 ma
and
I = E / R = 5v / 330 ohms = 15.1 ma

..and the data sheet for the QED1114 says the max current for this device is 50 ma, so it looks like I'm seriously underpowering the LED.

I = E / R = 5v / 100 = 50 ma
and I verified that a 100 ohm resistor gives me the full 50 milliamps.

The problem: I probably don't want to run these LEDs maxed out. Don't want to waste power - might burn out the LED faster, etc. So I want to reduce the power until I do *NOT* loose sensor sensitivity. I plan to run experiments at 50 ma, and down in increments until I see some change or some degradation of my results. I also see a problem that if I get it "just right", I could be low enough to get blown out by higher than expected ambient lighting if my light shielding isn't perfect.

The question: How would you conduct the experiment I mention above? What am I looking for to tell me I have the right LED current?

[Bigglez]

Greetings (No Name Supplied),

I answered a very similar question on another forum recently.

Perhaps the two of you can get together and work
as a team?

Comments Welcome!

The question: How would you conduct the experiment I mention above? What am I looking for to tell me I have the right LED current?

[rshayes]

There is no need to run the LED in the QRD1114 at 50 mA. That is a maximum rating. The data sheet gives the device characteristics for an LED current of 20 mA and this is probably as good a value as any. At this point, the output curent of the phototransistor is a minimum of 1 mA with the specified target.

A 4.7K load resistor connected to 5 volts will start to saturate with this current. The sensitivity might be much higher. Some calibration adjustment will probably be required and it may be simpler to do this in the phototransistor circuit rather than the LED circuit.

As a start, I would try a 5K pot with one end connected to the 5 volt supply, the wiper connected to the phototransistor collector, and the other end connected to a threshold detector near 2.5 volts. This should allow enough range to use reflective targets in the 45 percent range or greater.