Ultrasonic Sensor Boards
If you connect a cheap 4 pin Ultrasonic Module as per this diagram, then you can safely connect it to your Pi BUT ONLY WITH THE RESISTORS IN PLACE IF USING A 5V OLDER MODEL WITH A METAL CRYSTAL CASE IN-BETWEEN THE TWO TRANSDUCERS
There are two 2K2 and one 1K resistor in this diagram.
Using this arrangement, you only need one spare GPIO pin to trigger it and receive the returned pulse.
So now you simply use it (assuming connected to pin 23)
Use followed by a wait 1 and then just use the sensor item sonar23 to get the distance measured in cm. Any pin can be used for this purpose and if you had several of them you could broadcast each of them in turn.
Alternatively, you can use which will tell your Raspberry Pi to continuously do a distance check every 1 second and update sensor item ultra23 with the results without any further need to do another broadcast.
If you wish to have it update more quickly, you can define a variable called UltraDelay and make the updating more frequent. Not recommended to go any faster than 0.1 sec between updates
Update 3rd March 2019
With the advent of the new HC-SR04 that are 3.3V compatible – there is no need for the resistors and its easier to just connect the trigger and echo pins to seperate pins on the pi
Therefore to use them like this, use broadcast ultra xx,yy where xx if the trigger pin and yy is the echo pin or
S3GPIO command ultra xx,yy