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QRD1114 Resistor Values

Hi all,

I'm using a QRD1114 sensor for a line-follower. Datasheet: http://www.fairchildsemi.com/ds/QR/QRD1114.pdf

Im hooking it up with this schematic:

However, I'm unsure how the value for the 10K resistor was calculated? It's acting as a potential divider, right?

For the LED im guessing that the value of 220 was taken from doing:

R=(Input Voltage - Max Forward Voltage)/(Optimum forward current)

so: R = (5-1.7)/0.020 = 165

and 220 is closeish to this?

I'd appreciate any help!

Thanks in advance!



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sweet. I'll test it and let you know if it works ok. :)


With a 5V LED a 220Ω resistor is common practise.

The 10k resistor acts as a pull up resistor so the microcontroller definetely detects only when the phototransistor pulls the µC pin low. When doing this type of level detection (especially using interrupts) it is a good idea to place a pull up or pull down resistor on that pin.

The 220 Ohm resistor gives 4.3V / 220Ohms = 19.5 mA. They have probably chosen this one because 165 Ohms is a resistor of the E96 series and 220 Ohms is the closest E12 series resistor because the E12 series is easier available, not every supplier stocks resistors from the E96 series.