Problem with ultrasonic receiver

[mkoijn]

Hi,

I've built the following 40khz ultrasonic receiver as part of a project to measure the speed of sound in solids (wood etc.) The corresponding transmitter works well and the receiver is also partly working. There is a problem with the comparator part of the circuit (ic 5). I Measure 0.3 V with a multimeter on pin 3 of ic5 when the receiver isn't receiving any signal. And the potential divider on pin 2 is set at 0.41 V. But pin 1 is outputting 5.7 V, which should however be at 0 V when pin 3 is lower than pin 2?
Strangely, when I hold the multimeter on pin 3 and ground, the circuit works as expected and measuring with a second multimeter on pin1 I see 2mV. I would greatly appreciate any help to get this working.

Many thanks

Albert

 

[PRR]

> 0.3 V with a multimeter on pin 3 of ic5 when the receiver isn't receiving any signal. And the potential divider on pin 2 is set at 0.41 V.

I see a voltage-divider on pin THREE, not two.

So I dunno if you typed wrong, wired wrong, or what. I'm also too lazy to look-up pinout on LM358.

As-Drawn, the idle output should be high.

However "idle" may be hard to get because the gain is so high it's never really "silent".

I also suspect the rectifier and C11 need some pull-down resistor. To bleed C11 after a sound is gone, and to bleed the LM358's non-trivial 0.1 microAmp input current (which may be why you have 0.3V where we'd expect zero).

> measure the speed of sound in solids

IIRC, '358 is an opamp and not particularly fast. Between saturation and slew, its output may not change until "long" after the wave has passed. LM311 is the old-old-OLD part for such uses, though it would need a negative supply or circuit re-working.

 

[mkoijn]

Many thanks PRR. I mislabeled the schematic; pin 2 and pin 3 were switched. 
Regarding your suggestion of a pull down resistor after the rectifier; is my dmm acting as such when I measure on pin 3 and ground ? what value would be a good place to start, Mohms ?

Many thanks

Albert

 

[joaquins]

Most DMM (measuring voltage) are around 10MΩ.

I'd go with something more like 100KΩ that won't be a problem, a time constant of 100µs with 1000pF in a ultrasonic aplication seem good, 1600Hz cutoff.

JS