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I was trying to test my ultrasonic sensor by measuring a distance of 10cm until 100cm and using an oscilloscope to look at the echo pin output signal. My oscilloscope is set at 0.5 time/div.

At first, my ultrasonic sensor couldn't work properly, but after some tweaking around, I can finally get my ultrasonic sensor working. But the strange thing was, the measured distance I got was always less than a half of the real distance value. For example, for a distance of 70 cm, I got 27.2 cm from the calculation. This is the output signal for a distance of 70 cm (3.2 DIV * 0.5 ms/DIV = 1.6 ms).

Echo output signal at a distance of 70 cm with TIME/DIV of 0.5

Basically, all I do is, I calculate the HIGH time by multiplying how many DIVs appeared and then I multiply it by TIME/DIV (I set TIME/DIV = 0.5 for all measurement). Then I multiply the time with velocity of sound (340 m/s)and then divide it by 2. Because the HIGH time appeared in oscilloscope is the time to travel back and forth.

Distance = (T*Vsound)/2

Why did I got such a large discrepancy between the experiment result and the actual value? I've followed the standard procedure. I connected the Trig pin with function generator of 40 kHz frequency, the Vcc pin with 5 Volt, GND pin with ground, and the echo pin with the oscilloscope. Also my ultrasonic sensor is a brand new one, never used before.

So what possibly I did wrong?

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enter image description here

Figure 1. Oops!

It appears that your scope horizontal trace is not in cal mode so all your readings are unscaled.

There should be a cal test point - usually a 1 V, 1 kHz signal. Hook up the probe and check that everything is in order.

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  • \$\begingroup\$ good eye! The "cal" position often includes a tactile "click", but that's not always the case. I suspect that OP's time-measuring method may have a flaw as well. \$\endgroup\$ – glen_geek Oct 27 '17 at 16:20
  • \$\begingroup\$ Ah... I just noticed that. I just remembered that I have calibrated the oscilloscope before. But then, because I can't get the sensor working, I was quite frustrated and tried to turn every knob I can. Hahaha. So if the horizontal position (SWP.VAR) wasn't calibrated correctly, the reading got real bad like that? I don't know about that before. Thank you for pointing that out. \$\endgroup\$ – Muhammad Arifin Effendi Oct 28 '17 at 0:57
  • \$\begingroup\$ I just got this from a document about oscilloscope "When SWP UNCAL is out the SWP VAR control reduces the sweep rate." I catch what you really mean now. Thank you so much :)) \$\endgroup\$ – Muhammad Arifin Effendi Oct 28 '17 at 1:09
  • \$\begingroup\$ Good. Are your readings correct now? Don't forget to accept the answer if it is correct. \$\endgroup\$ – Transistor Oct 28 '17 at 8:29

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