How to Build an Optical Rangefinder

Mount the diode laser to the platform so that the output beam is normal to the plane of the mounting surface. That is, if the mounting platform is placed flat on a table, the laser should point straight up in the air.

Mount the bandpass filter to the front of the photodetector. The bandpass filter will allow the reflected laser light to pass through while eliminating much of the background. This will extend the range of your system.

Attach the photodetector to the telescope. The telescope is there to increase the range of the system by collecting more of the reflected laser light. You can choose something as fancy as an 8-inch diameter astronomical telescope to something as simple as a 1-inch diameter, 4-inch focal length lens followed 5 inches later by a 1/4-inch diameter, 1-inch focal length lens.

Mount the photodetector assembly to the platform adjacent to the laser and pointing in the same direction as the laser. Ideally, the reflected laser spot will appear in the center of the photodetector. You can check this by removing the photodetector from the telescope, shining the laser at a reflective target and observing the position of the laser spot coming out of the final telescope lens.

Connect the laser and photodetector to the timing circuit. The timing circuit will initiate a laser pulse and start a clock, then stop the clock when the photodetector signal rises above a threshold value.

Calculate the distance by multiplying the clock reading by the speed of light and dividing by 2. That is, distance = 3 x 10^8 meters/second x clocktime (sec) / 2. The factor of 2 accounts for the fact that the laser pulse travels to and from the target in the measured time.