Sonar

SONAR POWER SUPPLY

The Polaroid 6500 series sonar unit requires a 5 volt, DC power supply which can supply enough inital biasing current (2000 mA). A simple power supply can be made quickly out of common parts from Radio Shack.

(3) 7805 Voltage Regulators
(1) Proto-typing board (your choice)
(1) 470uF electrolytic capcitor
(2) Terminal connectors

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All three voltage regulator should be wired in parallel, so the all input are wired together, all grounds are wired together and all 5vdc outputs are wired together. The cap is simply soldered to the ground and 5vdc output.

PARALLEL PORT SONAR APPLICATION NOTES

Interfacing the Polaroid 6500 series sonar unit to a PC parallel port is easy. It requires two input bits and one output bit. The first input bit is for an external oscillator, which will be used figure out how long it takes for a ECHO to reach its target and return. The second input is for the actual sonar ECHO signal. The output is used to trigger the sonar units INIT (which initiates a PING).

EXTERNAL OSCILLATOR

I use a simple 555 timer based oscillator and count the number of pulses between the time I activate the PING and the ECHO goes high (which means a target was detected).

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R1, R2 and C1 determine the frequency of the oscillator. Sound will travel at .9ms per foot. So to detect an object 1 ft (30cm) away it would take .9ms X 2 which equals 1.8ms. In most cases, I use inches to detect objects so for each inch (2.5cm) .15ms will pass. If you would like one cycle of your timer to equal one inch set the frequency to around 6000hz.

For a frequency of 3000hz, R1=10k ohms, R2=47k ohms and C1=.0047uF. This will get you close, I then set an object a known distance and add a "calibration" constant to the software. (Note this give about 2 inches (5cm) of resolution)

SONAR CONNECTIONS

The sonar VCC must be connected to +5volts. Note that there are several hundred volts generated across the transducer, so do not touch the transducer while the unit is in operation (it probably won’t kill you but it sure does hurt). Next, connect the INIT signal to the output bit you are using for the sonar unit. Finally, connect the return ECHO signal to the input bit, with a 4.7k ohm pull up resistor (connect this from the ECHO output to the 5 volt power supply. You will spend a lot of time trouble shooting if you forget to do this.

OTHER FACTS

The sonar unit has about a 1.33ft (40cm) deadband. This means that an object must be at least this distance from the sonar unit to be detected. You can get around this with some additional wiring and software. In most cases, I used the sonar to avoid objects and keeping them around 2ft (60 cm) is a good rule of thumb. If you needed more accuracy up close, I usually use IR optics to detect something a few inches from my robot. Some objects are harder to detect than others. For example, walls are easy to detect and chair legs are hard to detect.

I usually use three 7805 voltage regulators in parallel to power a sonar unit. If you have problems getting accurate and consistent distance readings, try adding a large capacitor between the VCC (5 volts) node and ground. This usually helps!

Make sure you take into account the 2.38ms dead time. Do not expect a signal within this time period after pulling the INIT high.

I have found that installing a .1uF capacitor in the C7 position improves the operation of the unit with the HC11 MC. This takes around 1 minute after the solder iron is hot and well worth the time.

information to interface the Polaroid sonar unit with a PC parallel port

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