Probe or logic pen enclosure

[sky_123]

Hi,

Anyone know of a nice small enclosure ideal for making an analog probe, or logic pen, etc?
Something just barely big enough to fit a few surface mount devices on a narrow PCB,
and with a probe tip, and pen or marker shaped.

I know it is possible to fashion these out of fat pens and so on, but if there is
a ready-made enclosure it would be quite good.

I suppose a solution could be any piece of tube, but creating a nice probe tip might be
a challenge.

 

[erikl]

A ballpoint pen with a plastic refill with a metallic ballpoint?

 

[sky_123]

Yeah it's a tough one, doesn't seem to be any commercially available enclosure suitable for probes. The idea I was thinking of was a plastic test tube (you can get them with end caps/plugs) and then drill a hole and insert a test-bed test pin (like this type of thing) without the spring portion.
However, I'd still want to maybe have it shielded on the inside, and have some access to the ground near the probe head. So the search continues : (

 

[srizbf]

suitable enclosure i can think of are:

some ink/ballpoin pens are sold in a plastic box. that box will be correct size for logicprobe pcb and aa cells.

or , some toothbrush are sold in a similar plastic box . you can use that one also.

for tip , take a ballpoit refill (used) and insert wire and solder thro the tube.
even metallic refill will do the job.

 

[krp]

Once-upon-a-time this kind of enclosure could be bought; Jabel was one manufacturer I recall.

But not for decades.

I have used the toothbrush case idea; it's OK if shielding is not required.

I've also used brass "hobby" tube but, as you say, it's difficult to make nice ends.

Another idea might be the aluminium tubes that vitamins and other things come in; I've seen them in supermarkets. They have a plastic "plug" in one end.

Small die-cast cases can be used;

https://i40.tinypic.com/345y2j7.jpg

Not exactly a "probe" but a tip could be fitted using a BNC(f) connector; I have seen this kind of probe tip made with SMAs.

The case above is a Pomona; not cheap new but I bought several surplus for not too much some years ago.

I solved the problem some years ago by buying a lathe and a milling machine;

https://i44.tinypic.com/kdwzlt.jpg

https://i42.tinypic.com/2ynsb42.jpg

 

[sky_123]

They are lovely machined cases, and I like the cap-head screws ; )

I'm going to go the way of a die-cast case too, like the Pomona ones. By the way, how are you getting power into the circuit, in your pomona photo? Is that a normal coax, or
do you have some combined cable with a power supply wire?

I think I've solved the tip issue, with your help. I think a chassis-mount SMA socket (as shown in this image) would be perfect. All I need to do is replace the center pin (which is actually a
sleeve since this is a socket) with a long pin! Then I also have a nice ground connection around the outside, to which a normal solder ring (maybe M6 sized) can be fitted. Thank you for the idea.

 

[krp]

That particular device is a diode detector so there is just DC out to the meter.

You could make a very neat job by using the socket you showed and using a plug with a stiff wire as the probe tip; fill the space between the wire and the plug body with epoxy for strength.

That works particularly well if you want different kinds of tip or want to connect a co-ax.

Another idea might be to use an oscilloscope probe if you don't need the active circuit close to the tip; I did this with a home-made audio voltmeter using a $10 ebay probe (new) that was surprisingly well-made.

https://i41.tinypic.com/1zchj7s.jpg

Note the label about the resistance of the probe; 'scope probes use resistive wire to "damp" them and this must sometimes be taken into account.

 

[sky_123]

Good info.. All of my SMA connectors at hand are right-angled, but I'll get a straight one to experiment with that. Regarding the stiff wire, looks like Pomona sell a (reasonably priced for a change) set of probe tips (**broken link removed**) that contain some non-retractable ones. They are 1mm diameter, which is spot on. When I've assembled a probe tip, I'll report back : )

Also, just noticed, these guys here sell small ABS plastic end-caps which would be ideal for plastic or brass tube enclosures (but I'm going with the Pomona enclosure).

 

[krp]

Try Caplugs https://www.caplugs.com/ for a wide range of caps.

They are very good with free samples; I wanted a few of a particular kind and Caplugs Australia arranged for Caplugs US to send them to me at no charge.

 

[sky_123]

I tried assembling a probe, and am very happy with the result. See here for the steps I used. I used the SMA connector method, but I used the SMA socket, not the SMA plug. This is because for my particular application, I'm unlikely to need to extend it with a plug.
I've got a straight SMA plug to experiment with, but I fear it is a trickier method, because I need to solder the SMA plug center pin to the probe and keep them aligned axially while soldering them, which could be tricky. I might need to create some jig for that.
The SMA socket method I found to be very easy.