OT:Distance to CV

Plinio Barraza plinio at mail1.orientation.com
Fri Mar 10 21:26:29 CET 2000


Hi all,

I am working on making a controller that senses 
proximity.  Like a theremin, but with two major 
diferences: 1. that it has far reach (maybe up 
to 8 meters), 2. and that it outputs a controll 
voltage instead of a frequency.

I thought of the following:  

A simple saw/pulse oscillator... saw feeding 
into a Sample&Hold.
The Sample&Hold recieves the cotroll pulse a 
certain delay after the pulse emited by the 
first oscillator.   Thus, the delay time 
determines how far the cap has charged.

I had two problems. 

First, I had this really simple schematic for 
the S&H.  I must have copied it by hand (no 
printer), from someone on this list (sory, can't 
remember who).  I couldn't get it to work... The 
circuit made me think it used 1/2 of a 13600 
(interlaced circles indicating bias IN [+5 
Sample &-5 Hold], followed by a darlington 
buffer [between opamp and buffer 100 Ohms and 
1000pF (?) in series to ground]).  The S&H input 
was on the + side of the opamp, and the feedback 
loop on the minus (all the way from buffer OUT), 
the feedback loop is connected to ground through 
a 1K resistor.

Every time I connected the oscillator to the 
Sample&Hold, the signal droped to zero.  I 
thought it was an impedance problem so I tried 
to put a buffer in front of the S&H, and it 
still would draw its input to ground.  So I 
think I got the circuit wrong.  Does this 
circuit sound familiar to anyone?

The second problem I have is in how distance can 
vary delay.  Would it be possible to do it 
accoustically?  

ie:  To amplitude modulate a high-frequency-
signal (say 40kHz) with the pulse of the first 
oscillator and output it through a tweeter.  
Then recieve the echo of the signal and feed it 
back into the S&H controll IN  (with necesary 
filtering, and amplifying or whatever is 
needed).  Is this too difficult to make this 
kind of short-range sonar?

Or would I be better off trying to make the 
dalay with a capacitance plate...  Ala theremin?  
I'm not sure how Theremin did it (and I haven't 
studied a theremin circuit) but I would guess 
one could make a sine and vary its phase with a 
variable capacitor, then convert (both original 
signal and phaseshifted signal) to pulses.  
Would this give a variable delay analogous to 
distance from plate.

I am sure I have made alot of ignorant 
assumptions, but Its the only way I can say what 
I am thinking.

Hope its not too off topic, any help 
appreciated.

Thanks 

Plinio
[Soon 2years lurking]


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