Ultrasonic Sensor

 This article shows an ultrasonic sensor measurement

experiment that I have done recently.

Step 1: Design the Circuit

The circuit below show a biased ultrasonic sensor connected to two USB oscilloscope channels (each with input resistance of 10 Megohms):

Unlike microphone, infra-red or light dependent resistor (LDR), the ultrasonic sensor does not need biasing. Biasing is only used to offset the voltage because my USB oscilloscope cannot measure negative voltages.

I could have modelled the ultrasonic sensor as a voltage source. I can convert using Thevenin's theorem. However, I did not assume the sensor output impedance. Yet the second video above proves that the output impedance is high and thus can be ignored. Otherwise the average output voltage or the DC voltage would not have increased with supply voltage (shown in the second video).
The Co capacitor is used to remove the DC component of the output signal.

Calculate the high pass filter frequency:

fh = 1 / (2*pi*((Rbias + Ro)||Rl1 + Rl2)*Co)

= 1 / (2*pi*((100,000 ohms + 1,000 ohms)||(10,000,000 ohms) + 10,000,000 ohms)*(470*10^-9))

= 1 / (2*pi*((101,000 ohms)||(10,000,000 ohms) + 10,000,000 ohms)*(470*10^-9 F))

= 1 / (2*pi*(10,099,990.1 ohms)*(470*10^-9 F))
= 0.03352751192 Hz

Step 2: Simulations

I used the old PSpice simulation software to reduce the drawing time:

Step 3: Connect the Transmitter

Step 4: Connect the Receiver Circuit

However, you can clearly see the resistor colour coded values in the photo above due to large resistor sizes.

Step 5: Testing

I adjusted my signal generator to 40 kHz sine wave output.

The USB oscilloscope is showing 1 Vpeak amplitude signal (0.5 V / square). The period of the sine wave is supposed to be 1 / 40 kHz = 25 useconds or 0.025 mseconds. The width of the screen is 1 msecond. Thus there should be about 1 / Ts = 1ms / 0.025 ms = 40 periods in the plot below:

You can see my experiment in those videos:

In practice increasing the biasing resistor value will NOT increase the circuit sensitivity. However, biasing resistor values below 100 kohms will reduce the circuit sensitivity. Ultrasonic sensor modelling is beyond the scope of this article. The AC output resistance/impedance is be different from DC output resistance.

The transmitter and receiver components are the same.

I used high power resistors that are not needed for this circuit.

You can also see a frequency spectrum that is not showing a clear sinusoidal spike due to low sampling rate of the USB oscilloscope that I used.