Low cost LIDAR experiment. Part 1


Introduction

LIDAR (Light Detection And Ranging) is a technology similar to Radar, used to measure distance, etc using light. Generally the principle of measuring is same of basic radar, using time of flight technique. (Measuring time taken to light for travel). https://en.wikipedia.org/wiki/Lidar

I wanted to build or find a low cost setup a few years back, but the search was quite useless 😛 . Building one need sensitive components and finding these specialized components in Sri Lanka is almost impossible, so I gave up.

At the present, there is one or two low cost products like LIDAR lite ($150) approx. with  max range of 40m. There was another kickstarter project as well, similar pricing.

Objectives

For the first stage, I’ll simply experiment with available research, low cost components. Later on, optics and range will be improved.

  • Will use PIN photodiodes instead of commonly used APD (Avalanche Photo Diode) – cost and requirement of high voltage circuit for APD makes it unattractive.
  • Attempt to use phase shift method and time of flight method.

Implementation

  1. Light emitting part
  2. Light receiver and amplification circuit. This is probably the most crucial stage for a low cost setup due to the less sensitive PIN photodiodes.
  3. Timing circuit
  4. Final processing circuit.

Trial 1

My first trial was using Osram SFH 4545 IR emitting diode and Everlight PD333-3C photodiode.

IR emitter

I used a TI Tiva C launchpad to generate the needed pulse output and a BC337 transistor to drive the LED. This setup is not much ideal and I need to use a MOSFET to get good rise/ fall time and current output. (This LED can handle up to 1A pulse).

IR Reciever

I started with available components I had. Judging on the literature available, the needed circuit is called “trans impedance amplifier” – this simply mean the amplifier convert a current to a voltage. The reason was that photo diodes actually generate current than a voltage on different lights. After referring articles mainly by Texas Instruments, I constructed the following circuit. I did not do any formal calculation or analysis, this was just a trial and error setup.

The Op amp I had at hand was TLC25L4A. This is a low power op amp with decent gain but the bandwidth is not that impressive. Nevertheless, the circuit was the foundation of the next iteration.

amp_circuit

Op amp : TLC25L4A, R2 = 3Mohm, CR1 = PD333-3C. Image from SBOA060 – TI application note.

Results and limitations

The circuit amplified signals picked up from the photodiode quite okay. But the range was severely limited. Measurements were taken from my 5 year old DSO Quad oscilloscope.

The main issues I faced was the lack of optical filter on the photo diode, therefore it picked up 50Hz light came from the lights in my room, etc. Attempts for filters were rather futile (I tried RC filters only, didn’t have inductors at hand).

16388423_10210955954912213_4228562809503519770_n

At maximum range, approx 60cm!. The amplifier did not have a big gain.

In the next installment, I’ll explore the next set of circuits and other changes such as IR pulse width, etc.

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