Page History: Vehicle Reversing Assistant
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Page Revision: 2018/04/12 16:41
The following is a tiny trinket to provide the retro-fit your vehicle with reversing sensors (or fix a crappy Chinese kit) using 40KHz ultra-sound sensor modules available off ebay etc. for about £5. Be careful with buying these - there are two versions that I can find. One has a trigger and output pin so you can get a reading when you want it, the other type has a serial output that is free-running. The code below works with the former. The latter type presents a problem because despite claims of anti-jamming, all these modules suffer from picking up ultrasound pulses from neighbouring modules and will provide false readings. To illustrate, consider that you are some distance from an object and a pulse is initiated. A neighbouring module also sends a pulse shortly after but it "hears" the return from the first module - it can't tell this is not the reflection of its pulse and now calculates a much shorter distance than reality. OK this is fail safe but it makes for a lot of false readings and generally results in a useless system - you have to be able to control the when the pulses are sent and wait for any return. If the modules are free-running, it's mayhem.
There is a wayt ot tell these modules apart... if you apply power and the LED flickers constantly, it is free-running. These are fine for solitary use and give a serial output with the direct reading of the distance. If the LED gives a single flash, it is waiting to be triggered and these are the ones you can use in groups. It is a blunt instrument but these tend to have a blue PCB while the free-running ones have a green PCB. Here are pictures of teh two types - note there is also a differernce int he built-in microcontroller. You will have to source the appropriate modules.
My particular situation has three sensors.