SMD or THH or dead bug ? for Mr.Carlson's Super Probe ? - Page 1
Forum section: electronics / beginners

author: lordvader88
author_rank: Frequent Contributor
timestamp: June 20, 2019, 04:18:51 am
content: I built this on a breadboard and used some coax cable, and it works great. He says to use a 'super bright' green LED. What does he mean by that ? I have average grn LED w/ Vf=1.8V or whatever. I used a blue 3V LED, but never changed anything else.I have a little metal case, that can fit about a 2-3cm wide prototype board. I salvaged most the SMD parts I would need, even the capsFor dead-bugging, I'm brand new, using a knife to cut isolated islands, and I have no glue. I wondering what would be cleaner, to make a dead-bug version on a gnd plane.OORRR to use perf.board and make a mix if smd/THH ??I'm sure I won't notice the difference, but which is better , both would be pretty small?What about TO92 BJTs vs sot23? I could either in any prototype..And would a USB cable make a good cable to go from the probe-handle to the box w/ the audio and knobs ? Or does that have to be coax

author: Bud
author_rank: Super Contributor
timestamp: June 20, 2019, 04:44:45 am
content: I think the use of a superbright LED is to save battery power. The LED appears to be powered via a 1K resistor and transistor from 5V supply, which would produce 2.5..3mA current through the LED. This would be too little current for normal LED (10-20mA) but a superbright LED would work at normal brightness with this small current.

author: lordvader88
author_rank: Frequent Contributor
timestamp: June 20, 2019, 08:47:11 am
content: what would the voltage drop be ? I haven't figured out how the settings, and inputs, affect the LED, it's works enough tho. Plus so far I mainly used it UN-sheilded.I just salvaged a SMD LM4808LowVoltageHighPowerAudioPowerAmplifier, which is also tated for under 6V, so it's probably like Mr.C's chip. Right now I'm just using a LM386.

author: bson
author_rank: Unknown
timestamp: June 22, 2019, 04:08:46 pm
content: A green superbright will light up enough for indicator use at ~50µA. Also called low-current or ultrabright LEDs.

author: Dacke
author_rank: Regular Contributor
timestamp: June 22, 2019, 04:38:20 pm
content: I just used a cut piece of protoboard and mostly through hole, a couple things were SMD, and a LED I found in my LED bin. Took a few tries to find an LED that would work with that low current. Also using a 9V battery with a 7805 instead of the 4 AAs because they wont fit in the probe body I have. Most of these components I already had. I'm not using the amplifier box he used, as this probe is used directly with a signal tracer. The 7805 is not really necessary, but there just in case I modify this to use a different power source and I have tons of them anyway. The probe works, I just have to finish the probe body, it's just a piece of polycarbonate pipe with copper tape around it and vinyl end caps. If you're looking for the coax he used for the probe tip, I believe it's RG142.

author: lordvader88
author_rank: Frequent Contributor
timestamp: June 25, 2019, 10:56:35 pm
content: 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.

author: floobydust
author_rank: Super Contributor
timestamp: June 25, 2019, 11:27:17 pm
content: 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.

author: Dacke
author_rank: Regular Contributor
timestamp: June 26, 2019, 02:16:44 am
content: Quote from: floobydust on June 25, 2019, 11:27:17 pmI 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.

author: Quarlo Klobrigney
author_rank: Super Contributor
timestamp: June 26, 2019, 02:47:25 am
content: 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 onTodayat 18:27:17

author: floobydust
author_rank: Super Contributor
timestamp: June 26, 2019, 03:59:08 pm
content: 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 crapolaLM386Av=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 0.25VBC547 beta=458, collector is at 0.15VBC847 beta=182, collector is at 1.4VBut 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.