TD3X ( pi )

[DOOLEY]

just looked at that farnell link,

god,
some of them are expensive !!!


price varies from about £1.60 each to think about £12 !!!

home made is a good option then.

like I said, I just picked out a 25K LDR with a domed top,
filled the domed top flat,
same with an IR led,
dabbed with superglue and stuck e'm together.

finally covered with 2 tubes of shrink wrap and nipped the ends while
still soft so no external light could enter.

1st couple I made worked well.
so well they only needed a 33k resistor on the IR led.

opto-couplers I think are quite power hungry,
didn't want to waist 10ma driving the IR led just to alter the pitch.

so was quite happy when the homemade device was perfect with the 33k inline.

[Old cart]

A couple more comments, you will probably have to adjust VR2 to adapt to any individual VACTROL (LDR). You are looking for one that is about 25K ohm with 1.5 volts applied and is fast, maybe on the order of 10 ms or so. These turn off much slower than they turn on so the target response will show a quicker change as you approach the target. This class of products may become hard to find as they use materials that are being phased out.

The slower version of these might work very well to adjust ground balance on an auto ground balancing PI, but that another whole topic.

[Old cart]

The spec sheet for the VTL5C3 is here.

http://www.alliedelec.com/images/products/datasheets/bm/PERKINELMER/70219697.pdf

The key here is that these use a photocell ( not a transistor) that is optically coupled to an LED. This gives a voltage controlled variable resistance.
These have a response time of about <40ms which equates to a time smear of about 40mm at a coil sweep speed of 1 m per second.
Looks like these could be used for this application if you don't want to make your own.

[eclipse]

You're right, my mistake. Haven't dealt with opto components yet. Here are options for LDR output optocouplers from Farnell:

http://uk.farnell.com/photocell-ldr-output-optocouplers

[Davor]

Few optocouplers for consideration: ...

None of these are vactrols. If you wish to obtain a vactrol, the easiest way is making your own, as DOOLEY did. Popular ommercial version is a vtl5c3/2 but you'd need those only if you wish to have equal behaviour from several devices, which is not the case here.

[daverave]

do the opto couplers work well with non vco audio with more stable threshold????

[eclipse]

Few optocouplers for consideration:

SFH615AA

http://www.vishay.com/docs/83672/sfh615aa.pdf

SFH610A

http://www.vishay.com/docs/83666/sfh610a.pdf

VO615A

http://www.vishay.com/docs/81753/vo615a.pdf

Probably I'll get couple of to test which works best, thanks Dooley for the tip that not all optocouplers are created equal

[DOOLEY]

hello old cart,

yup ,
this is also true,
tiny spikes don't affect audio pitch in a notable way.

[DOOLEY]

hello all.

davor is absolutely correct,
it's a homemade VACTROL.

it's only purpose is to alter the sound pitch from the audio 555.

the sound channel is one volume, either on or off,
target intensity is indicated by pitch using the homemade vactrol.
it raises the pitch as the target get closer.

as the human ear can detect pitch changes better than volume changes I
did the sound this way to try to portray more target information via sound.

tried many many ways to alter the pitch in a "NICE" sounding way on a 555,
homemade part was the simplest solution.

tried transistor opto-couplers but to be fair they conduct more in one direction than the other when "on"
so waveform doesn't stay 50/50 duty.

yes,
tried 2 x transistors but this also altered duty.

tried 2 opto-couplers in opposite polarity's this also didn't maintain 50/50 duty.

and varying voltage on pin 5 CERTAINLY doesn't maintain 50/50 for nice audio !!

an LDR rated at nominal 25k will work well,
just the cheap ones seam slow,
too slow to keep up with a target passing so the pitch change lags behind.

the "fast" ones out of my bits boxes were the resin encapsulated ones,
domed top ones,
poss 20 years old,

the new "SLOW" ones off ebay were only lightly sealed flat top ones.

poss my bad luck getting "slow" ones.

[Old cart]

Main use of an optocoupler is to isolate one part of circuit from another. In this case it keeps small noise voltages from getting into the vco circuit , making it quieter and more stable.
They are normally much slower than direct coupled circuits which also contributes to the smoothness of the response.
My guess is this is one of the main reasons this doctor sounds so nice.

[daverave]

but what is the purpose of the opto coupler ??does it make the detector more immune to noise and more stability ???

[Orbit]

Try to do that 2x bc107 ,insert in housing without light !

[Davor]

Proper name is a vactrol, or resistive opto-isolator, a component that is recently regaining its popularity in audio consoles etc.

[daverave]

what is the purpose of the opto coupler ????

[sinclairuser]

hi dooley, nice to see your project getting some interest, i looked at your circuit for my own winter project(PI+VLF in the same enclosure), but in the end i went with minipulse for PI but only due to A-already having a board (silverdog), B- size and shape of board and C- positive ground of the MPP.
when i have done this i may give yours a go.