Laser Break Beam Sensor
Laser break beam sensors are usually kind of cumbersome. They work by detecting an object breaking a laser beam - but you usually have to set up a laser on one end and a sensor on the other. Unless your sensor is big, it can easily get out of alignment.
...but not this break beam sensor. It has a light sensor+lens next to the laser to detect the reflected dot when the beam is broken, so it can sense when someone breaks the beam from the starting location. There's no need for a matching end point sensor! It forms a good strong laser with a sensing distance of up to 1 meter. We found that it can go a little further depending on the ambient light and the color of the thing distrupting it. You'll want something that is good at having the dot show up soft textures may not work as well. We used a hand, paper, or cardboard box with great success!
Set up is pretty easy, connect 5VDC to the red wire (you really do need 5V, 3V wont work!), Ground to the black wire and a pullup resistor on the output pin. You can have the pullup go to any voltage really. When the beam is broken, the output blue wire will pull down to ground. If it's being pulled up, the voltage will be 'high' when there's nothing in the way of the beam. There's a potentiometer which can be used to adjust sensitivity, we didn't find it to make much difference.
Emitter: 1mW red laser diode
Sensing Distance: Up to 1m (depending on light levels, contrasting material)
Operating Voltage: DC 4.5 VDC - 5.5VDC
Operating Current: 25mA
Output Current (NPN transistor output): 100mA max
Sensing Angle: <10 degrees
Response Time: 2ms
Working Environment: Indoor (not waterproof!), 0-70 degrees C
Dimensions w/components: 20mm x 18mm x 10mm / 0.8" x 0.7" x 0.4"
Cable Length: 250mm / 9.8"