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ArnoudC, I’m not sure which Maxbotix sensor you’re using, but on the models we usually use, it’s pin 5 on the sensor that gets connected to the Mayfly digital pin (pin 5 in my example code above, but you can make it whatever you want). I think on most of the Maxbotix sensors, Pin 6 and 7 are the power and ground pins, and pin 5 is the serial TTL or RS232 output pin that you can easily read with the Mayfly using the capture code above. Sensor pin2 is the PWM output and pin3 is the analog signal, but I prefer the serial output. The code above only works for capturing the sensor serial output. Check the datasheet for your sensor model to verify that these are the correct pin numbers.
You can connect the sensor power and ground pins to any power or ground pin on the Mayfly or any other Arduino board. The Maxbotix sensors draw just a few milliamps of current so I usually just connect the sensor power pin to an available Mayfly digital pin and switch it high or low to control the on-off cycle of the sensor, which is handy for situations where the logger is programmed for sleeping between measurements. However, I have found that some of the sensors don’t like having their power pin connected to the switched 3.3v bus on the Mayfly. So if you’re running the Mayfly without sleeping, just connect the sensor power to the 3.3v constant bus. If you’re sleeping the Mayfly between sensor readings and you want to shut off power to the sensor, then use a digital pin to provide the power.