Electronics > Beginners
SMD or THH or dead bug ? for Mr.Carlson's Super Probe ?
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lordvader88:
I'm not sure how the LED is supposed to work, it glow's a lot, and does turn on. I'll have to find some superbright ones too then replace it.
 
Just going back at it now. FOR THE PROBE I have a little tin can, or I could use a plastic pipe with some foil shielding from something I scraped.

I'm going to try SMD+TH, now I wish I had a big magnifier or goggles w/ mag.
floobydust:
SMT or TH is fine as long as there is a shield around the probe amplifier. I would say it is a finicky design, maybe hard for others to replicate.

The transistor biasing scheme depends highly on their hFE. You might want to check the collector voltages are reasonable - who knows what 2N3904 hFE he designed to vs what you have.
The LED driver transistor has nothing to deal with leakage current so a sensitive LED may never turn off. "3.2V 20mA" on the schematic. It might be the point though.
I don't see anything to limit bandwidth to audio frequencies, so the whole thing could oscillate at RF if layout not the best.

I would never hold a grounded metal probe while poking around anything.  I've used metal cigar probes before and it's too easy to bump into something live, and guaranteed your body is grounded which makes me squirm.

For a "super probe" I just use an LM386 signal tracer in high gain/high Z. That picks up anything I need to look for.
This super probe is much more sensitive and the "noise" pop detection I'm not sure what that's about.
Dacke:

--- Quote from: floobydust on June 25, 2019, 11:27:17 pm ---This super probe is much more sensitive and the "noise" pop detection I'm not sure what that's about.

--- End quote ---

I believe it's just a similar version of the "noise" function on a signal tracer but with a lot more sensitivity,  this being a non-contact probe.   But that noise function is not something I've found myself using all that much in the few years I've owned mine,  and getting the pop detection feature to work properly with the LED is a bit finicky so far (the LED I'm using does have a tendency to get stuck on as you stated).  I'm not even sure if I would find that feature particularly useful so I'm not too bothered about it.
Quarlo Klobrigney:
He's just re-inventing the wheel with bling lights, and feed my Patreon to see the schematic. LM386 with x200 gain & switchable diode detector, done.
Quote from: floobydust on Today at 18:27:17
For a "super probe" I just use an LM386 signal tracer in high gain/high Z. That picks up anything I need to look for.
This super probe is much more sensitive and the "noise" pop detection I'm not sure what that's about.

--- End quote ---
floobydust:
Voltage gain at 10kHz of the three stages is about 1,200 which is lots of gain, very sensitive. Layout would matter. Anything I've built like this oscillates.

I'm guessing it's an electrostatic probe, with the LED showing detected RF. These are very useful sometimes, pays for itself in saved time.

For many decades, I've used the crapola LM386 Av=200 in a (contact probe) signal tracer with loudspeaker, or headphones if I need to track interference or listen for ghosts. The LM386 is a true piece of shit, high distortion, loves detecting AM radio, prefers to oscillate, so finding a malfunctioning stage is difficult with it. But it does the job and I'm too lazy to build something better.

I did an LT Spice sim on the Super Probe and it kinda confirmed what I was thinking.
Common emitter stages with no local feedback do have high distortion and are finicky about transistor hFE for clipping.
2N3904 beta=300, collector is at 0.25V
BC547  beta=458, collector is at 0.15V
BC847  beta=182, collector is at 1.4V

But the input stage has a low input impedance, it loses a lot of signal when you have high Z capacitive-coupling from the probe. I'm saying the probe loses much "in the air". A JFET would be ideal there and then you need less circuit gain to make up for air losses.
I would use that and add some DC feedback for biasing the BJT's.
An SMT version, would be nice to stuff the probe in a pen with foil shield inside.
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