Interfacing a MaxBotics ultrasonic range sensor to a WiSense node

MAXBOTIX makes highly reliable ultrasonic range sensors. I have used them in the past for vehicle detection in covered parking lots.

This sensor can output ranging data serially. The serial parameters are –

• 9600 bps
• 1 stop bit
• No parity bit
• The serial output on the sensor’s TX pin needs to be inverted. I did this by using an opamp.

In the block diagram (above), the +ve input is fixed at Vcc/2. The -ve input is connected to the MaxSonar’s TX pin. When the TX pin is low (0 Volts), the -ve input is lesser than the +ve input (Vcc/2), as a result the Op-Amp drives it’s output to Vcc volts. When the TX pin is high (Vcc volts), the -ve input is higher than the +ve input (Vcc/2), as a result the Op-Amp drives it’s output to 0 volts.

If the RX pin on the sensor is pulled high for at least 20 micro-seconds, it will trigger the sensor to perform a ranging operation. This will take around 49 milli-seconds.

I added an FFD app (ffd_app_wl.c) which periodically gets range data from this sensor and sends it to the external gateway app via the coordinator node. I used this app to measure the water level in the 1000 litre tank on the terrace.

The sensor driver is implemented in two files.

• Pltfrm/src/max_sonar.c
• Pltfrm/inc/max_sonar.h

Here is the code from ffd_app_wl.c.

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You can take a look at the code on bitbucket.

Here is a snapshot of the gateway app running on the laptop connected to the coordinator node.

Some pics of  the test setup –

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References:

• http://www.maxbotix.com/documents/LV-MaxSonar-EZ_Datasheet.pdf
• https://bitbucket.org/rkriskros/lpwmn_code
• http://www.wisense.in