Rangefinding methods

Methods of finding the range of something:


Timed Pulse

This is as simple as sending out a pulse of energy and waiting on the response. Issues:

  • Some signals, light light, travel very fast and need a very fast clock to detect. Using analog methods (Charge Time Measurement Unit on some Microchip parts) can alleviate the response time.
  • Gain: as this is a pulse, the shorter it is, the less jitter there is in the signal. Also, for the absolute best timing on the fastest signals, you cannot time off the firing trigger. Instead, you need a secondary transducer that senses the fired energy. If using a laser, they often have a feedback photodiode built into the laser itself for power control. This could be used for local field triggering

Phase Comparison

In this case, we use more energy at lower levels to sense the target. This is far less timing intensive than the timed pulse, but takes longer.

The analog traditional way was to modulate the signal (via crystal) and downmix both the sensed signal and the recieved signal and compare the phase difference (See TI Analog book).

Today, this can be done digitally. Two oscillators are started at the start of measurement. They have a very tiny frequency difference. At the same time, a counter counts one of the oscilators. The counter is set to stop counting when it recieves a signal. This signal is provided by a S-R flip flop. One oscillator drives the light source and one side of the SR latch. The other oscillator drives the other side of the S-R latch. As each pulse happens, the starting edge of the oscillators drift away from each other until the latch trips. At this point, after removing a fixed delay value, the number in the counter times whatever the difference in the two oscillators period (say 50ps) provides the time of flight of the signal.

Passive Binocular triangulation

This is how most creatures determine distance. For an automated solution, this requires a high end computer and dual cameras with a known distance between them. By determining the centroid of the object under observation, the PC can then run a triangulation algorythm knowing the base angles and spacing of the cameras to determine the distance to target. This is the most robust but possibly least accurate means of determining distance. It is very accurate at close ranges but this drops off quickly.

Active triangulation

Active triangulation is popular with the DIY community. By using either a camera, linear CCD, or 1D PSD with the correct optics, a spot made by a focused light source (LED, Laser) can be determined based on the angle and known separation of camera and lght source. This is highly suceptible to lighting conditions (as it is effecively a low DC signal) but can also provide very fast update rates without a lot of processing or signal bandwidth.

Active Oscillation

I originally heard of this on Sam's Laser FAQ. A laser driver is tied to the inverted output of a sensor. This creates an electro-optical oscillator who's frequency is based on the 4 x (distance plus fixed electronics delay). This creates an odd hybrid. You get a continuous sensor that will quickly respond to changes in distance. However, the frequency increases exponentially as it gets close, with one foot being around 300MHz from my memory. (I have a spreadsheet somewhere where I did these calculations). Other than the high frequency, though, they've been able to make low cost frequency counters/meters that can measure this range, so it should not be an issue determining range with minimal math and at high speed.



Optical systems will need at least an near inifinite focused optic around the reciever to allow for maximum detection of the signal. Beam spreaders for laser sources are optional at low power but become critical at higher powers to keep the systems safe.

All receivers should have an optical pass filter (even just a colored piece of cellophane!) to prevent non-target light from triggering or at least increasing the noise floor of the sensor

Critical Componnets


Photo Detectors

These are normally divided into two groups. PIN diodes and APDs. PIN diodes are normal high speed photodiodes. APDs are Avalanche photodiodes. Much more expensive (minimum list $100, Ebay $16), but have a natural amplifier built in, making detection of weak pulses far easier.

Linear CCDs


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