# Ultrasonic range meter - problem

#### mlodyfn

Joined Nov 4, 2012
6
Hello,
I know that are many circuits about ultrasonic range meter but i have made different one.

The circuits works but with some problems:
1. Range is very small about 3 inches
2. In transmitter part I used totem pole circuit to sent 0-12 V f= 40k Hz wave but on trancuder I can see 40k Hz with noise

I am not sure about the Band-pass filter

Can someone check that circuit ?

greetings

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#### DickCappels

Joined Aug 21, 2008
5,886
Just to help us out here, what frequency does it operate at and what kind of transducers are you using?

#### Audioguru

Joined Dec 20, 2007
11,251
Your schematic has its parts spread too far apart. Its lines and text has no contrast.
I cropped it, added contrast and folded part of it so it is not as big as my neighbourhood.

The resistor values in highpass filter are much too low. Look at the opamp's datasheet, its max output level drops with a 2000 ohms load.
The gain should be evenly divided between the two opamps because one opamp cannot have a gain of 100 at 40kHz.

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#### mlodyfn

Joined Nov 4, 2012
6
Just to help us out here, what frequency does it operate at and what kind of transducers are you using?
I am using 40ST-12 as transmitter and 40SR-12 as receiver with frequency 40,2 kHz

Audioguru - I have simulated it in LTspice, it worked. Now with new values in high pass filter

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#### Audioguru

Joined Dec 20, 2007
11,251
Audioguru - I have simulated it in LTspice, it worked. Now with new values in high pass filter
Your new resistor values will work properly with opamps.
But I calculate that the highpass filter cuts frequencies below 48.8kHz and the lowpass filter cuts frequencies above 33.6kHz which is not what you want. Maybe their cutoff frequencies should be swapped.

Sorry, I was wrong. The cutoff frequency for the highpass filter is 33.6kHz and the cutoff frequency for the lowpass filter is 48.8khz.

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#### mlodyfn

Joined Nov 4, 2012
6
Low-pass
C1= 1 nF
C2 = 2,2 nF
R1=R2 = 2,7k \Omega

1/( $$\sqrt{2}$$ *2*3,14*c1*f) ≈ 41 kHz

Hi-pass
C1= C2 = 1 nF
R1= 6k8 Ω
R2= 3k3 Ω
f ≈ 33 kHZ

I will try new values tomorrow

#### Audioguru

Joined Dec 20, 2007
11,251
You are almost correct and I was wrong. I corrected my last post.

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#### mlodyfn

Joined Nov 4, 2012
6
New measurement distance is a little bit wider but something is still wrong. I have to check it with use oscilloscope

BTW: Can You send me web with low-pass calculator

#### tubeguy

Joined Nov 3, 2012
1,157
Looking at the specs on your transducers, it appears the receiver has max sensitivity with around 50k "loaded" resistor. I think with the resistors shown for the high pass the impedance is to low, it loads down or drops the signal considerably. You might recalculate the hi pass using resistors 10-20 times higher.
You are driving the receiver with a 12vdc supply. It appears the transmiitter needs 12v RMS to begin operating efficiently. 12vdc with no drops for transistors etc is less than 4 v rms.
You may have to increase the 12vpower supply considerably. The opamps will handle +/- 15 vdc. Should check the transistors voltage ratings

#### tubeguy

Joined Nov 3, 2012
1,157
Oops.
I meant to say you are driving the transmitter with 12vdc.
and the RMS voltage is less than 4v with transistor drops.

Another but more complicated way to get more power to the transmitter using just 12vdc would be to use two amplifiers in a bridged configuration - common in the audio industry.

#### Audioguru

Joined Dec 20, 2007
11,251
Are you detecting a reflected signal?
Then the object must be hard, smooth and flat. The object also must be at a perfect angle left to right and up to down.

#### mlodyfn

Joined Nov 4, 2012
6
I think the problem is with gnd on highpass it wasnt virtual ground like on lowpass i have changed some things
1. New opamps NE 5532
2. I increased the R in virtual grund and added capecitor 100n
3. C20 on virtual grund

I will try change it or i will choose the LT1264 from filtercad

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#### Audioguru

Joined Dec 20, 2007
11,251
i have changed some things
Now the highpass filter WILL NOT WORK! Read my attachment.

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#### tubeguy

Joined Nov 3, 2012
1,157
Another idea.
You could move the IC3b circuit with an input coupling cap, to the input ahead of the high pass as a buffer.
Then you could keep the filters as-is. Noticed the gains are (maybe inadvertently) back to 10 and 100. They should be balanced as pointed out by audioguru.
If you try this I would also suggest increasing IC3b resistors to at least 1meg and 33k.