Let's Make Robots!

Brainless Line Following Robot

Sorry I do not have any pictures of this yet.  I just got batteries for my camera this morning before work.

 This consists of:

2x Fairchild QRD1114 Reflective Object Sensors

2x 10kΩ resistors

1x 180Ω resistor

2x PNP transistors

2x GM2 motors


The idea behind this is pretty simple.  When the QRD1114 sensor sees the reflected IR light it has emitted, the phototransistor closes and sends current through to the PNP transistor which then closes and sends current through to the motor.  When the QRD1114 sensor does not see the reflected IR light it has emitted, the phototransistor opens sending no current through to the PNP transistor.  The 10kΩ pull-up resistor keeps the PNP transistor open, so that no current flows through to the motor causing it to stop.

When the QRD1114 sensor is pointing on the white background, the emitted IR light is reflected back, and the sensor closes.  When the QRD1114 is pointing on the black line, the emitted IR light is NOT reflected back and the sensor opens.

The left QRD1114 sensor is connected to the left motor through the left PNP transistor.  The right QRD1114 sensor is connected to the right motor through the right PNP transistor.

The 180Ω resistor is for the 2 IR LEDs inside the 2 QRD1114 sensors.


Schematic has changed, PNP transistors were apparently upside down.

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the whole schematics looks upside down for me... :D if you're robot is working :) just post it... I'm curious... I don't want to build one if it is not working... anyway black surface will give a value between 0 and 5V... and if white surface is detectected will give you 5V exactly... you'll need an analog digital converter here :D

Looks good. Just be sure to select PNP transistors that can handle the current of your motors.
i agree on your idea done went and got the parts going to finish first one by orginal schematic then going to try one other way but the transistors are backwards on schematic
I love the idea of a line following robot without a MCU, but I never heard of this sesnor. Do you know where to buy one?
They're at several places. Mouser has them for 82 cents each, about the cheapest I've seen. Sparkfun and Digikey have em too.
yes digikey is where i got mine for my maze solving robot
Sure do!  I bought mine on SolarBotics.com (http://www.solarbotics.com/products/qrd1114/), also HVWTech.com (http://www.hvwtech.com/products_view.asp?ProductID=97) has them as well.

I picked up a few of these sensors a while back from Solarbotics myself - I was ordering some other parts and they were just too cheap to say no too =D
With a bit of calibration you can even make them into modulated IR proximity detectors, and although the range is pretty short, they're a lot more compact than building your own from discrete LEDs and phototransistors.

No worries about the mistake Rich, we're here to help after all.

Haha, I did the same thing.  I was ordering up a bunch of gear motors and the shipping was pretty steep for next day so I just added more.

Either your motor+transistor circuit is wired backwards you you wanted NPN transistors instead. As it's shown in the diagram, the PNP transistors won't allow current into the motor. Current always flows through a BJT in the direction of the arrow. You either need to swap the PNPs out for NPNs, or connect the PNP emitter to your +5V and connect the other end of the motor to ground.