Ultrasonic proximity detection for PIC-based robot.

[the_merovingian]

ultrasonic proximity detector

I'm trying to design a proximity detector for a PIC-based robot (I just need to know if something gets close, not how close it actually is). I've bought a pair of matched ultrasonic transducers from Maplin, but I'm having trouble with the circuit (I don't have access to a 'scope). I want to build something from scratch rather than but 'module'.

I've come across this design to use as a basis for the reciever:

https://3.bp.blogspot.com/_ycHwJEos...BfBACQ/s1600-h/ultrasonic-switch-receiver.jpg

I understand that Q3 and Q4 provide amplification and that D4, R14 and C5 provide 'rectification / peak following' (not sure of the correct term) for the ~40kHZ waveform.

Could someone explain the purpose of D3 please?

TIA.

 

[FvM]

ultrasonic proximity detector circuit

Curiously, D3 has no function in the said circuit, because it's permanently reverse biased. If you put a capacitor in series with R13, it would have a purpose.

As a general remark, it's a very simple proximity detector, because it only measures intensity rather than transit time of ultrasonic sound.

 

[the_merovingian]

ultrasonic pic

Thanks, I thought as much.

Indeed, I'm aware of the limitations of the circuit, I don't need accurate (? - a relative statement due to the speed of sound through air) measurements, just an indication of whether something is in the way or not.

Would it be simpler to do this with a modulated IR source? Wouldn't I need additional circuitry in this case, to provide the (narrow) band-pass of the signal frequency (whereas with the ultrasonic transducer this is essentially inherent due to the resonance affect)? Which frequencies for the IR would people recommend (and more importantly why)?

Additionally, I'm trying to understand how the values for R6,7,8,9,10,11,12 were derived in https://3.bp.blogspot.com/_ycHwJEos...BfBACQ/s1600-h/ultrasonic-switch-receiver.jpg.

Could anyone suggest the reasoning / maths behind these values in terms of the gain it provides, for me to understand?

TIA.