Multimeter Mod! Soft Power & Auto Off

[EpicIntelGamer]

Hello!

Since I'm still a newbie in electronics I've yet to obtain a nice DMM and use a $10 CenTech DMM. I usually find myself using it in short bursts, just to test something here and probe there and I always seem to forget to turn it off.

I decided I should make a button that when pressed, would turn on the DMM for about a minute, then turn it off.

I knew there was a multitude of ways to make soft power buttons with auto off like this but I wanted something that was very simple and could be done with the components I have laying around.
I thought up a little circuit where I use a transistor to control the power of the DMM and simply have a button to apply power to charge a large capacitor which acts as the trigger for the transistor.

This is how the DMM looked before the mod:




I found that with the transistor I was using (MPS5172 NPN) that a 3300uF capacitor seemed to keep the multimeter at full power for almost exactly 60 seconds, then another 20 or so with the readings getting more and more inaccurate and the low battery indicator coming on.

But that was perfect for me, 1 minute of reading and then a nice power off, so I put the components on a little bit of protoboard and hot glued it into the case in the big empty space behind the LCD.

And here it is with its little mod:




Thanks for viewing and feel free to leave any comments or suggestions below!

[Lightages]

Nice idea. It is sad when you end up paying more for batteries in a year than the meter cost you. Especially now that  you have made that mod, you really should keep that thing out of the wall sockets!

There is a lot of information and help here to help you decide what to get for a safer reliable meter. I hope you stick around.

[EpicIntelGamer]

Nice idea. It is sad when you end up paying more for batteries in a year than the meter cost you. Especially now that  you have made that mod, you really should keep that thing out of the wall sockets!

There is a lot of information and help here to help you decide what to get for a safer reliable meter. I hope you stick around.

Any reason (other than the fact that it and its leads cost $10) that I shouldn't use it in the wall now? The input jacks are far away from where I did the mod.

Yeah I've been waiting for the right time to snag a nice fluke on ebay, it'll happen sooner or later. And you can be sure I'll be here a while, I like to be able to get some external recognition and suggestions on the projects I do.

[Lightages]

Any reason (other than the fact that it and its leads cost $10) that I shouldn't use it in the wall now? The input jacks are far away from where I did the mod.

Some people here might think I am being a little zealous when it comes to the safety of some multimeters, but there are some that see things the same way I do. Basically that meter should only be used on circuits maybe up to 50V and only small battery power as well. The chances are that it will not blow up in your hand but it is certainly not designed or built with any regard to the proper construction for any kind of high energy circuit use. It could literally blow up in your hand or send a small piece of plastic shrapnel into your eye. How likely is this to happen? No idea. I do remember seeing images and accounts of these very cheap meters doing so, or just outright failing to read correctly. If a meter does not show a live circuit when it should, and then you touch it....

Anyway, it would be in your best interest to learn a bit more and not just listen to me. The following thread has information and links to other threads discussing meter safety and design.

https://www.eevblog.com/forum/testgear/a-list-of-multimeters-that-do-not-appear-to-meet-their-claimed-safety-specs/

Yeah I've been waiting for the right time to snag a nice fluke on ebay, it'll happen sooner or later. And you can be sure I'll be here a while, I like to be able to get some external recognition and suggestions on the projects I do.

Fluke is not the only meter that is worth buying. You can get a very safe, very well built, and very versatile meter for under $150 new from other manufacturers. Fluke is not usually the biggest bang for the buck. A used one can be had for a good price of course.

If you are willing to spend $135, IMHO the best buy in a multimeter right now under $150 would be the Brymen BM257. It can be purchased from a member here, iloveelectronics, real name Franky Tong. He has an ebay store and ships for free or $2 more with tracking. Contact him here by PM to maybe get a better price.

But like I said, don't take my word only on anything, read up and see what others have to say too.

[EpicIntelGamer]

Any reason (other than the fact that it and its leads cost $10) that I shouldn't use it in the wall now? The input jacks are far away from where I did the mod.

Some people here might think I am being a little zealous when it comes to the safety of some multimeters, but there are some that see things the same I do. Basically that meter should only be used on circuits maybe up to 50V and only small battery power as well. The chances are that it will not blow up in your hand but it is certainly not designed or built with any regard to the proper construction for any kind of high energy circuit use. It could literally blow up in your hand or send a small piece of plastic shrapnel into your eye. How likely is this to happen? No idea. I do remember seeing images and accounts of these very cheap meters doing so, or just outright failing to read correctly. If a meter does not show a live circuit when it shoud, and then you touch it....

Anyway, it would be in your best interest to learn a bit more and not just listen to me. The following thread has information and links to other threads discussing meter safety and design.

https://www.eevblog.com/forum/testgear/a-list-of-multimeters-that-do-not-appear-to-meet-their-claimed-safety-specs/

Yeah I've been waiting for the right time to snag a nice fluke on ebay, it'll happen sooner or later. And you can be sure I'll be here a while, I like to be able to get some external recognition and suggestions on the projects I do.

Fluke is not the only meter that is worth buying. You can get a very safe, very well built, and very versatile meter for under $150 new from other manufacturers. Fluke is not usually the biggest bang for the buck. A used one can be had for a good price of course.

If you are willing to spend $135, IMHO the best buy in a multimeter right now under $150 would be the Brymen BM257. It can be purchased from a member here, iloveelectronics, real name Franky Tong. He has an ebay store and ships for free or $2 more with tracking. Contact him here by PM to maybe get a better price.

But like I said, don't take my word only on anything, read up and see what others have to say too.

I do completely agree I'd not like to put this meter anywhere near mains potential (as I said I have a good amount of electrical knowledge). The only reason why I was asking is because it seemed like you were saying my mod had made it unsafe, not that the DMM itself was just horrible to begin with.

Yeah all of those are a tad bit over my price range. To be honest the only thing I'll bite on is something under $50, definitely used, and I prefer eBay.

Thanks for your reply.

[Lightages]

Yes you are right, your mod probably did nothing to lessen the safety of that meter.

I hope you can find a safe meter for higher energy circuits for under $50. It is possible to find somethings used and even a Fluke once in a while too for that price.

[Lightages]

You might want consider this:
http://www.ebay.com/itm/Amprobe-AM-520-HVAC-Multimeter-750V-AC-1000V-DC-/321158107226?pt=LH_DefaultDomain_0&hash=item4ac683d05a#shId
or from the same company but direct from their website:
http://www.testequipmentdepot.com/amprobe/multimeters/am-520.htm

[alm]

The switch may be far from the input terminals, but the battery terminals are directly connected to one of the inputs. I would argue that the button provides less insulation than say the rotary dial, and that therefore using this meter on high energy or higher voltage circuits is an even worse idea than before.

[EpicIntelGamer]

Where exactly is the battery connected directly to the inputs?

Also maybe you guys are different but usually when I'm probing around I'm not holding the dmm in my hand.

This meter might rarely see the 120v mains but is mainly for little projects.

[EpicIntelGamer]

You might want consider this:
http://www.ebay.com/itm/Amprobe-AM-520-HVAC-Multimeter-750V-AC-1000V-DC-/321158107226?pt=LH_DefaultDomain_0&hash=item4ac683d05a#shId
or from the same company but direct from their website:
http://www.testequipmentdepot.com/amprobe/multimeters/am-520.htm

That does look like a very nice multimeter, not a bad price either.

Thanks for the suggestion.

[c4757p]

I agree, it's not safe like this. Those switches offer very close to no protection. If you were to probe mains, you might find it on the button. You say you don't use it like that, but you've now obligated yourself to set it down every single time you probe anything high-ish energy. Times you need to do that are uncommon and tend to come up suddenly if you're not an electrician - are you sure you'll remember?

There are long-shafted tactile switches - I've got a bunch with a shaft maybe 3cm or 4cm long. Get one and mount it so that the top of the shaft just pokes out of the case, and the switch itself is far back. Still might not be as safe as the meter was, but it's much better than this.

Where exactly is the battery connected directly to the inputs?

Set it to measure resistance and use the positive input to probe the negative terminal on the battery (leave 'common' disconnected). I suspect you'll find a rather small number.

[Lightages]

My point is that those cheapo meters are so unsafe that what he did could not make it significantly any worse.

[c4757p]

They're not usually as unsafe as people make them out to be. They at least don't take the mains voltage you're probing within millimeters of your hand.

[EpicIntelGamer]

I agree, it's not safe like this. Those switches offer very close to no protection. If you were to probe mains, you might find it on the button. You say you don't use it like that, but you've now obligated yourself to set it down every single time you probe anything high-ish energy. Times you need to do that are uncommon and tend to come up suddenly if you're not an electrician - are you sure you'll remember?

There are long-shafted tactile switches - I've got a bunch with a shaft maybe 3cm or 4cm long. Get one and mount it so that the top of the shaft just pokes out of the case, and the switch itself is far back. Still might not be as safe as the meter was, but it's much better than this.

Where exactly is the battery connected directly to the inputs?

Set it to measure resistance and use the positive input to probe the negative terminal on the battery (leave 'common' disconnected). I suspect you'll find a rather small number.

Well I guess I'm the only human being that lacs a third hand.

I am interested in finding 5k ohms between positive input and negative battery terminal however everyone forgets the switch is connected to the positive terminal.

Everyone also forgets that the only time my hand is near the button is when it needs to be turned on.

And really I've said it how many times, this meter never has and probably never will see the mains.

Wow you guys would probably have a fit if you saw the leads for this dmm if your so upset about a button.

[EpicIntelGamer]

They're not usually as unsafe as people make them out to be. They at least don't take the mains voltage you're probing within millimeters of your hand.

Actually maybe it's just me again but when I grasp a dmm my fingers are pretty close to the input side of it and actually very far away from this button.

[Lightages]

Like I said, some of use can be a bit zealous about this 

Most people never have a problem with the most dangerous methods or actions, but some do. That is why regulations and rules get made, like the CAT ratings. All you need to do is see someone who is permanently scarred and badly so to realize that it can happen. My warnings are just that, warnings. Take them as you want.

[EpicIntelGamer]

I'll never use this on the mains.

There, can everybody stop now?

[Lightages]

Fair enough EpicIntelGamer.

Yes I know what clean and "normal" wall socket voltage is in the US. I am a Canadian....

It is not what is normal that causes problems but what is abnormal. I will not go on about it, just read the links in the thread I posted. That is all I ask. Make up your own mind from the information, but be educated and be safer. That is all I hope.

And in the end, yes I like your creative thoughts on how to make your multimeter better. That is what is satisfying and fun about this hobby.

[eKretz]

Heh, best to just say, "I will never use it in an unsafe manner," and back away slowly. LOL    

The longer switch shaft mounted internally is a good idea though.

[brainwash]

If you'd like to replace that meter with a cheap yet (a bit) safer one you can buy a UT10A. It lacks current measurement but that should be ok and its inputs are supposedly rated for 250V on all ranges. I haven't opened mine yet but it is anyway going to stay well away from anything >50V.
The problem with those harbor freight meters is not only design but also build quality: dry solder joints, loose solder balls, improperly crimped cables with loose strands, ...

[EpicIntelGamer]

Fair enough EpicIntelGamer.

Yes I know what clean and "normal" wall socket voltage is in the US. I am a Canadian....

It is not what is normal that causes problems but what is abnormal. I will not go on about it, just read the links in the thread I posted. That is all I ask. Make up your own mind from the information, but be educated and be safer. That is all I hope.

And in the end, yes I like your creative thoughts on how to make your multimeter better. That is what is satisfying and fun about this hobby.

Let me ask you this, how far is your fingers from the electrified prongs when you plug in an outlet? Probably reasonably close. Does this mean that just because they are close and you never know when the "abnormal" will happen that you should never touch a plug again?

I don't know why people say longer shafted buttons are better when I'll never use this on mains, and definitely will never touch the button while its connected to mains.

The leads for this thing, they are in terrible condition. I wouldn't think for a second to use this in its current condition with its current leads to test mains.

Honestly though my point is, the abnormal is the abnormal. How reasonable is it to think that one day I might probe around with this thing and suddenly find enough voltage to actually jump a gap whether it be from the leads to me, from the button to me, from the input jacks to me, in a wall socket? I don't think its very reasonable. It takes a considerable amount of voltage to jump gaps and I'm sure if many times the 120v normal was passing through the wires in the walls, there would be other indications.

I really wish you could see how paranoid I am with electricity already.

 I give it more than enough respect, I assure you, but I can't sit here and worry about every abnormality ever known to man or I'd never go within 100feet of a socket again, I'd never go in a car again, etc.

I'm not saying that that warrants stupid practices but I'm saying if your being as safe and prepared as you can be than that's it, fate will follow its course and  you cant do anything about it.

And once again just for good measure: I'll never use this on the mains.

[EpicIntelGamer]

If you'd like to replace that meter with a cheap yet (a bit) safer one you can buy a UT10A. It lacks current measurement but that should be ok and its inputs are supposedly rated for 250V on all ranges. I haven't opened mine yet but it is anyway going to stay well away from anything >50V.
The problem with those harbor freight meters is not only design but also build quality: dry solder joints, loose solder balls, improperly crimped cables with loose strands, ...

Actually to be honest not having current measurement would be a big problem for me. I use the dc current measurement on my DMM just as often as I measure voltage or resistance.

Yeah the rotary switch on it doesn't look too good and I do spot some rather poor design in how they have to stretch the solder from the input pcb to the main pcb.

[eKretz]

This is why I recommended the "back away slowly" approach. It seems less of a headache for everybody. I have had the safety troop jump on some of my posts at different sites in the past and have learned to heed the advice and let the rest run like water off a duck's back. Getting defensive seems just to raise hackles all 'round. I think I wouldn't have mentioned the leads, heh. As for the modification and the short vs. long button shaft: an argument could be made that someone else might one day pick up your meter and not apply the same limitations that you do. If they should be hurt, you could be held liable should they file suit (the U.S. is a ridiculously litigious country, and I live here, so not just saying it to be insulting).

[EpicIntelGamer]

This is why I recommended the "back away slowly" approach. It seems less of a headache for everybody. I have had the safety troop jump on some of my posts at different sites in the past and have learned to heed the advice and let the rest run like water off a duck's back. Getting defensive seems just to raise hackles all 'round. I think I wouldn't have mentioned the leads, heh. As for the modification and the short vs. long button shaft: an argument could be made that someone else might one day pick up your meter and not apply the same limitations that you do. If they should be hurt, you could be held liable should they file suit (the U.S. is a ridiculously litigious country, and I live here, so not just saying it to be insulting).

Yeah this is getting really annoying. Just because I'm new on a forum doesn't mean I'm new to safety.

I'm not actually sure if I would be held liable, I never let anyone use my stuff so they would have to be stealing it to use it. Either way, I don't think anyone's going to get a hold of my DMM and hurt themselves.

[Lightages]

EpicIntelGamer:

I sent you a PM. I am not trying to insult anyone. Like I said I was just trying to point to information.