Ultrasonic HC-SR04 distance sensor with waterproof sensors?

[Haku]

I wanted a waterproof distance sensor and figured I could make my own by replacing the 40khz sensors on one of the HC-SR04 modules I have with 40khz waterproof sensors destined for the rear bumper of a car.

I got a cheap car reversing kit with 40khz sensors, tested that and it worked fine, then hooked up a couple of waterproof sensors in place of the non-waterproof ones on a HC-SC04, programmed a 20m2 with tested code that works fine on an unmodified HC-SR04, then tried the modified module.

Nothing, just 0's returned to the terminal window from the pulsin command.

I can hear the sensor sending out the pings when held up to my ear but the other one doesn't seem to be receiving them. If I hold the two sensors face-to-face just a few mm apart then I get some low numbers returning in the terminal window.

Anyone have any ideas why it might not be working as hoped? Could it be that the car reversing kit sends out a much louder ping?

 

[erco]

I'm guessing 12V vs 5V makes a difference?

 

[Haku]

Good question, had a look at the control board for the car sensor setup and it all appears to run from a 78L05, the 12v line doesn't go near the sensor sockets or control ICs.

edit: Another option would be to figure out the serial signal that the sensor box sends to the LED+speaker module, but I fear that's beyond my diagnostic capabilities.
The car sensor setup is indentical to this one http://www.ebay.co.uk/itm/130882114117

 

[BeanieBots]

Anyone have any ideas why it might not be working as hoped? Could it be that the car reversing kit sends out a much louder ping?

Sorry, no idea why it won't work but others here have also tried and failed.
On the Devantech site they actually make a statement that it is NOT possible to replace the sensor with a waterproof one, so I gues they (the experts) have also tried and failed.

If you ever do get any success, please post how and what you did.

 

[Haku]

After doing some tests it's clear that the waterproof sensors are less sensitive than the non-waterproof ones which is why they won't directly work on the HC-SR04 board, and the sensors from the HC-SR04 don't work properly on the car reversing board as they're being over-driven and the receiving circuitry is then over-sensitive.

Turns out the car reversing board does indeed use the 12v source for driving the pings which is why it produces a louder sound than the HC-SR04 which only runs off 5v and to get the higher voltage to drive the TX sensor it uses a MAX232, which can only output up to 10ma.

I have a couple of ideas on what to try next, if I get something working I'll definitely post my results here.



edit: I hooked up the car reversing sensor box to a USB-TTL adaptor just out of pure curiosity and it turns out it's putting out 5 byte packets preceeded by a single fixed value byte at 9600 baud! Time to get sleuthing

 

[BeanieBots]

Check that both run at the same frequency. US transducers have quite a high 'Q' so any deviation from center frequency will significantly effect sensitivity.
If it's just a question of drive voltage, then that should be reasonably easy to fix.