Ever wanted a REALLY sharp continuity probe? - Page 1
Forum section: electronics / projects

author: TerraHertz
author_rank: Super Contributor
timestamp: September 21, 2013, 01:03:47 am
content: Tracing out a PCB recently I got sick of not being able to probe directly to tracks, through the solder mask using a standard multimeter probe. The point is too blunt to puncture the mask. Something with a very sharp point was needed.As it happened I had an old pair of Fluke multimeter probes, in which the red probe's plastic was disintegrating due to some age-related de-polymerization. So I cut the old probe off and replaced it with an improvised probe constructed quickly from a biro and an old gramophone needle. I had a box of the needles, but I see on ebay that new gramophone needles are cheap, if you don't have any lying around - you know, for your gramophone. About $4 to $10 per hundred. So they're an available way to get a very sharp and robust point.After Robrenz's impressive vacuum tweezer I'm almost ashamed to post such a pathetic construction. But perhaps someone may find it a useful idea. See pic.The 2cm long rod extension is needed because otherwise the biro plastic tip isn't rigid enough to hold the needle securely against forces typically used when probing a board. It's the hotmelt glue between this rod and the main plastic body of the pen that provides the rigidity.If you wanted a point that was insulated except at the very tip, just add a little heatshrink tube over the needle.Another reason I used the old Fluke probe was that it has a good thick copper core. So the resistance of the probe leads isn't swamping the small resistances typically found when track-tracing.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: September 21, 2013, 02:06:42 am
content: As an alternative you could try thisThrow the multimeter into your car as you rush to a job.Close the car door so as to lead a probe hanging outside the door and dragging on the ground.Drive 15km, ignoring a funny noise because it is not worth investigating.Voila.ps. You may compromise your CAT rating doing this.

author: IanB
author_rank: Super Contributor
timestamp: September 21, 2013, 02:13:42 am
content: Why not use an ordinary sewing needle?Advantages:They come in a wide range of sizes from very small to very largeThey are guaranteed to be made of metal and conductive (gramophone needles could have sapphire tips and be insulating)They are very,verysharpDisadvantages:None that I can think of.

author: TerraHertz
author_rank: Super Contributor
timestamp: September 21, 2013, 02:39:28 am
content: Quote from: IanB on September 21, 2013, 02:13:42 amThat was my first thought too. I had a choice of standard sewing machine ones, or some great big ones used in sack-sewing machine.[Edit: Doh. I thought you meant sewing machine needles, but you meant hand sewing needles. Uh... because I didn't have any that were thick enough?]But there are disadvantages:- They are all too long. I'd have to cut one short, and still have exactly the same mounting problem.- The ends aren't round - they are sort of blade shaped. And there's the thread hole and channel.- I only had a few needles for my sewing machine, and would rather keep them for their intended use. But although I do have an old gramophone (a restoration project) and a stack of 78s, I'm unlikely to ever need all the full box of needles I have.Gramophone needles are just right.Btw I've never seen a gramophone needle that wasn't hardened steel. Never one with a jewel tip. You're thinking of 'modern' phonograph pickups perhaps?In other improvised spikey probes in the past, I generally used hard steel dressmaker's pins. But they are a bit too thin.For example there was an "is it tristate?" probe. Used with a scope to look at uP bus lines to make it clear when the bus was being driven by something, as opposed to just floating. It's a probe with two connections to the point. One is direct, and that goes to the scope. The other is via a 10K resistor and connects to the output of a triangle wave generator, outputting a 5Vpp signal (between GND and +5V.) The result is that floating intervals in the trace become obvious 'bands' of solid colour.Normally with bus signals you can't tell, since over time intervals of a microsecond or so the bus line just holds its last voltage, more or less. The 10K resistor is high enough impedance that the injected triangle wave has no effect on operation of the system. (If it does, there's something wrong with the system's noise margins!)

author: robrenz
author_rank: Super Contributor
timestamp: September 21, 2013, 02:50:27 am
content: Very nicely done there TerraHertz

author: sleemanj
author_rank: Super Contributor
timestamp: September 21, 2013, 02:53:44 am
content: As a type 1 diabetic, I have a constant supply of very sharp fine "probes"not refillable, not recyclable, and I accumulate about 3 extra empties a month. Good for piercing through SMD reel tape covers, wire insulation etc. Break em apart and they can sort of work as a syringe plunger for flux etc.

author: TerraHertz
author_rank: Super Contributor
timestamp: September 21, 2013, 03:16:39 am
content: Quote from: HackedFridgeMagnet on September 21, 2013, 02:06:42 amEvery time when I see people driving along with ropes hanging off the back of their ute and dragging on the ground, I do my best to drive a wheel over the rope. So far never actually managed to do it.

author: fpliuzzi
author_rank: Regular Contributor
timestamp: September 21, 2013, 05:30:31 am
content: Just to add another variation to the interesting sharp probe ideas presented above...A few years ago I took a couple of Tip Jacks that I had in my parts drawer and soldered some very sharp sewing needles to the pcb mounting tabs of the Tip Jacks.I snapped the needles to shorten them (wearing safety glasses) and bent the two mounting tabs on the Tip Jacks flat to the plastic body before soldering the shortened, pre-tinned, needles to them.Afterwards I slipped some thin teflon sleeving over most of the needle's length and put some black heat-shrink tubing over the Tip Jack's body.Now, whenever I need to probe an extremely tiny spot on a pcb, or pierce conformal coating or the insulation of a wire to make a measurement, I slip these pointy adapters onto the probe tips of my DMM. These adapters are mostly used for doing continuity and when making low voltage/low current measurements in situations where their extreme sharpness would be beneficial.Regards,Frank

author: jancumps
author_rank: Unknown
timestamp: September 21, 2013, 07:59:06 am
content: Anyone tried cactus needles?

author: ResR
author_rank: Regular Contributor
timestamp: September 21, 2013, 08:09:18 am
content: Quote from: jancumps on September 21, 2013, 07:59:06 amCactus doesn't conduct electricity and is too brittle to use for probing. I tried and end up with needle in my thumb. It hurt like hell.

author: envisionelec
author_rank: Frequent Contributor
timestamp: September 21, 2013, 05:09:25 pm
content: Back in the day I was a car stereo installer and did dozens of security alarms and remote car starters. I had an X-Acto knife made into a probe. The body of the knife was insulated with shrink tubing. Best of all, the "tips" were replaceable.

author: joun
author_rank: Contributor
timestamp: September 21, 2013, 06:27:52 pm
content: Hello to everyone..I have something like in picture

author: ju1ce
author_rank: Regular Contributor
timestamp: September 21, 2013, 09:07:42 pm
content: I haven't encoutered a situation where my Pomona 6342 leads haven't been sharp enough:http://www.pomonaelectronics.com/pdf/d6341-6342-6375_100.pdfMuch neater than a home-made solution, and not too expensive (about $20). I think there's an EEVblog episode where Dave receives a pair as a gift.

author: notsob
author_rank: Frequent Contributor
timestamp: September 22, 2013, 02:40:19 am
content: I've been using the HCK (now multi-contact) 4mm plug-on probes for yearscouldn't find a quick reference to it, but here's something very similarhttp://uk.farnell.com/hirschmann-test-measurement/972327101/probe-pin-4mm-red-mvl-s/dp/2098492

author: echen1024
author_rank: Super Contributor
timestamp: September 22, 2013, 04:25:23 am
content: I have used unused syringes before. Works a treat, and, if you fill them with salt water, you can destroy the circuit after probing.

author: eKretz
author_rank: Frequent Contributor
timestamp: September 22, 2013, 07:11:48 am
content: I just took the extended insulated "needle" probes that came with my Fluke accessory set and used my knife sharpening stones to needle point them and then I lapped them with 1 micron diamond compound. They are wicked sharp now and have a mirror finish.