Ultrasonic Range Finding.

Transducer mounting

For obvious reasons, both the transmit and receive transducers should be facing in the same direction. But there is another phenomena that can cause potential disaster for your experiment, that of ‘ringing’.

Each time the transmitter sends out a 40KHz ping, the receiver physically vibrates (rings) in sympathy. This ringing can cause the receiver’s sensitive amplifiers to see a false reflection immediately after the ping, especially if the sensitivity control (VR1) is set high. In order to help alleviate this problem, both the transmitter and receiver transducers must be padded in order to isolate them somewhat from the medium on which they are both attached. i.e. the PCB. This was accomplished in the prototype by placing a piece of felt on the bottom of the transducers where the connecting wires protrude. The illustration below shows the prototype’s arrangement.

Because we’re not transmitting a very powerful signal from the send transducer, the ringing effect if not too much of a problem, but it is important to be aware of the phenomena.

The receive transducer should be connected to the pre-amplifier’s input using a single core screened cable (small diameter microphone cable). After all, the pre-amplifier’s input is of a high impedance, and the transducer is a form of microphone, so the same precautions should be carried out as if we were dealing with an audio signal. This is especially important if, like the prototype, the transducer setup is a few inches away from the receiver’s circuit. If the transducers are mounted on the same PCB as the receiver, then this precaution is obviously eliminated, but you must still treat it as if it were audio, and keep the circuit away from strong AC signals, such as transformers, television sets etc. If strong AC signals enter the pre-amplifier, then false readings may occur as they swamp the tiny signals produced by the transducer.

The transmit transducer’s hook-up  is not as critical, so standard multi-core wire can be used.