This post covers some of the design elements of an Arduino Laser Range Finder (LRF) shield and provides links to other resources for anyone who wants more details. The design performance of the LRF is:
Update rate: >20 readings per second
The Arduino Laser shield uses a version of the DS00VQ100 timer/controller chip to handle the firing of the laser, gain control of the receiver and timing of the signals. The optical section is a modified DS00/50 module from LightWare.co.za. It has an Osram SPL LL85 laser running at Class 1 power levels and an S238x APD detector from Hamamatsu.
The Arduino Laser board is 4-layer with solid ground and power plains on the inner layers. This provides better high frequency performance than a 2-layer board, something that’s important for an LRF where digital and analog signals are mixed up on the same board. The board material is regular FR4 with 35um copper - nothing fancy. The trick to being able to use this conventional PCB layout is that the highest frequency signals are inside the controller chip, not rushing through the tracks on the PCB.
AL_01 Arduino Laser controller board.
One of the key components in this design is the laser diode. This is an SPL LL85 from Osram which is actually a hybrid made up of a high power, pulsed laser chip, an avalanche FET driver and two capacitors. This compact device produces short flashes of light (about 30ns long) and gives high optical power with very low electrical interference. It is important to understand that whilst the peak optical intensity may be as high as 14W, the average optical power is less than 1mW, making the device safe to use around people.
Osram SPL LL85 hybrid laser.
The hybridization of the SPL LL85 laser means that the high current (10 Amps) and high speed (nanosecond) signals are kept inside the package. This makes the design of the laser driver circuit board much easier and it only needs to be double sided.
Laser driver board.
Another key component in the Arduino Laser design is the optical detector - an avalanche photo diode (APD) from Hamamatsu in Japan http://sales.hamamatsu.com/assets/pdf/parts_S/s2381_etc_kapd1007e09.pdf. I’ve always found these devices to have astonishing sensitivity to the very weak optical signals that the LRF is going to get from distant target surfaces. APDs behave rather like photomultiplier tubes, giving a flood of electrons for every photon that they detect. As a comparison, a regular photo-diode produces about one electron for every two photons that it “sees”, whilst an APD typically produces 100 times this number. You might think that a photo-transistor would do the same thing as an APD but the problem with photo-transistors is that they are far too slow to measure signals that are rising and falling in just a few nanoseconds. The APD looks and works just like a photo-diode but needs to be biased at more than 100V DC to get the “avalanche” effect to work properly. Changing this bias voltage alters the “gain” of the APD making it possible to increase or decrease sensitivity as required.
APD gain as a function of the applied bias voltage.
Laying out a board to carry the APD is a little tricky. The high voltage bias tracks to the APD must be kept well spaced from other components. Also, the track between the APD and the pre-amplifier input mustn’t have any ground plain underneath it, otherwise the additional capacitance slows down the return signals.
In the next post, the boards are assembled and tested…
AL_01.pdf (32 KB)