Blind Squirrel PI.

[DanZ]

I've been gathering up the parts needed to build one of these and keep seeing something in the schematics that's a little confusing. Good chance one of the many experienced guru's on here can help clear it up for me.


U6 (555 for audio out stage) seems to be excess baggage, that is, why couldn't you just connect the cap. on pin 3 directly to the transistor's emitter that's driving U6 and do away with the 555 altogether?
Appreciate any insight.
Regards,
Dan

[Qiaozhi]

It's because U6 is used to generate the audio tone. Also, it appears to be a VCO, whereby the audio frequency will vary depending on the strength of the target signal. Without the 555, it would either just give a "click" in the speaker, or you may not hear anything at all.

[DanZ]

Thanks. After I posted I thought about it more and went back to hit a "delete post" button but didn't find one. Makes sense.
Appreciate your help! Cheers,
Dan

[sinclairuser]

the version you posted differs slightly from this http://oldradiobuilder.com/mdetsp.html
looks like yours uses more common parts(depending where you live!).

[6666]

WARNING That "new" version looks like a revamp of a schematic that Dooley and myself where experimenting with a year or two ago
use at own risk.

[DOOLEY]

how do 6666, and all.
yes,
i remember it well,
the "blind squirrel" jpg is the modified for simplicity one that was posted on another forum.
i used a cd4098 instead of the 4538 so timing caps will need to be differnt values.
we never got it finished,
theres no "ballance" on the tl 071,
r12 wants to be 2.2k,
and i ended up opting for an opto intead of the npn to the 555 vco.
im quite happy with mine,
use it regularly no problems,
poss best sticking to original design from old radio builder and just substitute clock chip with the 555 arangement though.
its non-motion but quite stable.
recomend this easy logically aranged project to anyone for a 1st time build.
mines a little bit over modified now though.

[DanZ]

Appreciate the heads up and previous experience tips with this altered version.

[DanZ]

the version you posted differs slightly from this http://oldradiobuilder.com/mdetsp.html
looks like yours uses more common parts(depending where you live!).

Yes it is different than the "oldradiobuilder" design. Yesterday I got it built to the point of firing it up and after correcting a little mistake I made it appears to be working. I just dangled the coil off the edge of a box to keep it in the air and it detected my wedding band at about 10". I don't have an o'scope any more but could use a piezo buzzer to go through the circuit to detect oscillations where they should be and hear the frequency adjustment/width working. So far so good. Now on to building something to hold it all together. I think that I'm going with a waist mount battery (3X6volt) using SLA's. I see, depending on the frequency/pulsewidth adjustments, the current draw can be anywhere between 200ma and 500ma. An internal pack of rechargeables probably wouldn't last too long.
Cheers!

[DOOLEY]

hello danZ,
well done on the build,
years ago mine fired up straight away also, no problems.
quite a straight forward logical build isn't it.
not the most advanced, but ok.
more stable with a wide ish sample window, 20us ish,
and delay at about 20 - 30 us,
handy if youve fitted a pot for this on the front panel, esp at the beach.
notice youve used "magnet" wire, enamaled copper for your coil,
poss best using 0.6mm pvc covered "equiptment" / hook-up wire,
the insulation space between turns will give you a "faster" coil.
the power usage,
i opted for about 300 pulses per second.
i was finding 200 - 250 ma for a nice compromise on depth and battery life,
didnt gain a right lot of distance after that.
also good for about 10 hours theoreticaly on a pack of ni-mh 2000ma cells.

[DanZ]

Thanks, I'll try the 0.6mm pvc insulated wire to see how it compares. Also want to try a smaller 7 or 8 inch on it.
I need to get a feel for the thing as to how it responds with each of the Freq, Pulsewidth, Delay, RX-bandwidth controls and the different metals. Lot to learn.

[6666]

G'day Dooley


Danz Good to hear you you got the "blind squirrel" working have fun .

[reiv]

Hi
I`m finishing Blind too. I have changed only transmitter to get linear frequency regulation by poti. It is from Delta pulse and gives range about 196-600Hz and tx width from 30-150us.
Chears

[DanZ]

Reiv,
Looks good! I'm limited to some parts availability around here so I went with the 'generic' route using the 741's and all but it seems to be working. A bit overkill on the power section with mine. I used a 2n3055 (TO-3 case) regulated to 15 volts. All the articles I read indicated over an amp current draw so that ruled out using a 7815 but after seeing it come up and draw just over 1/2 an amp max. (depending on frequency/pulsewidth settings) made realize a 7815 would have been fine. Oh well.
I've been working on the coil cover and strut today. Getting close to field testing soon.
Good luck with yours!
Cheers.

[DanZ]

Need to run out and buy a couple SLA's then take it outside to try but it seems to be working with a bench pwr. supply.

[sinclairuser]

nice job mate, what does the meter respond like?, i mentioned to a mate of mine who works in parts aquisition that i wanted a small u meter for a project and he gave me a box with 4 in i think they are samples as they are all slightly different but the same company, the box says no resale on it, anyway i did not want to add one to what i'm building if it just flicks to the end pointlessly.
so is it readable on the pi?.