QM1323 Multi meter -What is the red light built in used for ?

My apologies to you both. The only neon tester I have had experience with is the screw driver type.


That is why I asked for clarification.

A point of interest: I built a power supply when I was younger that used a neon to indicate when power was supplied. The neon did not light up when I turned on the 240 vac.
I sought help and my foreman who told me to double the value of the in-line resistor. This done; the circuit worked correctly. This surprised me as halving the current through the neon
allowed it to glow. Hence my skepticism in using a neon as a voltage tester. Yes... the original design was for 110-120 vac.

Your tester must have additional circuitry that gives it such a range of detectable voltages.

I have this meter. It's definitely an non-contact AC voltage detector circuit that is on all the time. There is circuitry on board that drives the red LED. This runs off the 9V battery in the device. The unit is really sensitive pretty much any EM field, and any AC voltage or moving magnetism will trigger it to some extent. This is why bumping it so often makes it flash, and those that have seen it flash regularly (eg: every 30 secs) probably had a mobile, cordless phone or some other wireless device (cordless mouse/keyboard, wifi, etc) near the device at the time.

The big killer for me is that I carry this thing around everywhere and by the time I go to use the meter, the battery is flat. The battery is behind a plate held in with 2 (small) Phillips screws, so it's non-trivial (not to say annoying) to have it in your kit without a battery in it. I've had brand new 9V batteries go flat after going through 3 airports (and a bit of driving) in check-in baggage, which renders it useless on-site. I now use rechargeable 9V batteries, but they still take time to charge after going flat in the device.

I've "pretty much" figured out the trace that runs the whole detection circuit and found a switched version of the battery line from the knob, so I'm going to try and see if I can make it work only when the unit is not in the OFF position. I've taken some before photos and will post a few pics of what to do if it works.

FWIW: I can't see how such a change would affect the CAT III-600V rating, as this circuit is directly connected to the 9V battery all the time. Applying a high voltage to a 9V battery is more likely to cause problems than putting it through the rotary switch on the device. Also, the rotary switch is connected directly to the 9V battery + line (on a common pin to all settings, even off), means it's connected anyway. That said, I don't deal with anything that high in voltage, so I don't see it being an issue for me.

Notes:
It's definitely not documented in the manual, even though that's what it does.
My guess is it was a "last minute" feature that someone added in, and the person adding it either couldn't be bothered running the trace to the switched line, or found it difficult to do without re-routing a significant section of the PCB (and was deemed too hard to resolve).

PS: I didn't really care too much about this for the 2 years I've had it, but my primary meter (which is also larger and heavier) has developed a fault with the DC/AC volts range, so this itch must be scratched to fix this so I'll have something I can use. Otherwise it's a nice little meter and the size/weight is great for travel.

Why have a LED with such an exotic function and not have details about its "amazing capability" in the manual ?????

I can only assume that (as per the notes in my post) it was a last minute feature that wasn't fully fleshed out.

I really don't ever expect much out of manuals for stuff like this at the cheaper end of the spectrum. At the rate they get churned out, I doubt there's much time to even slip an errata page into the box.

Also, since the QM1551 is now available (which definitely has non-contact listed in it's specs, but otherwise is the same), I suspect the QM1323 has been superseded.

Success! I've got it disabled by cutting one track and running a wire.

The track to cut is on the LCD side of the PCB. You need to remove the battery door (2 screws), the 4 case screws, and then 6 screws that hold in the PCB. There are 2 screws that hold in the LCD, so you don't want to remove those. The LCD screws are the second set down from the top (LED) end of the meter.

Here's the track I cut (circled).

Pic 1:



...and here is the wire I ran.

Pic 2:



The anode of the diode D8 goes through the rotary switch to the 9V battery positive. It's connected for every rotary switch position except the middle (OFF) position. This seemed the easiest place to tap it off after the switch.

I ran it to the common pin on U4, which appears to be a FET. This is where the cut track runs to, and is where the non-contact AC detector circuit gets power.

I've tested it and am happy with the result. The non-contact AC detection works fine as long as the switch is not in the OFF position (doesn't matter what mode it's in), and all other modes work (see note re: Buzzer).

Note: If you cut the track but do not connect the wire you will have no buzzer, even for the buzz test mode. I wasn't willing to take components off the board to figure out where the track ran. Anyway, this stops the unit "squawking" when the battery is low (which just makes the battery flat faster - and really annoys airline security staff, let me tell you!), which I've heard it do even when the switch was in OFF position. As a consequence, I'm expecting to get a lot longer battery life out of it now, and annoy a lot less airline security.

Anyway, hope this helps someone.

Have fun!

The light lights and the buzzer buzzes when the meter is turned off. Was it made by a battery manufacturer so that you need to buy a new battery often?

Oh, frequently a flat battery leaks and will destroy the meter. Then you buy a new meter?

Cefair - thanks for you mod picture. I've just done the same on mine. The tracks you suggest test out correct and you've saved me a ton of batteries

oldbill said:

Cefair - thanks for you mod picture. I've just done the same on mine. The tracks you suggest test out correct and you've saved me a ton of batteries

Click to expand...

No problem! Glad that someone else got something out of my hack.

I've had really good battery life now with the meter pretty much living in my car. I've got a ton of interstate trips coming up in the next year and no doubt I'll have times where I'll need the meter to work. The less fussing about I'll have removing/inserting batteries, the better!

May be right - do your keys have a battery in them that might have pulsed?

audioguru said:

The light lights and the buzzer buzzes when the meter is turned off. Was it made by a battery manufacturer so that you need to buy a new battery often?

Oh, frequently a flat battery leaks and will destroy the meter. Then you buy a new meter?

Click to expand...

No the buzzer only buzzes when switched to Ohms and mode changed twice . A fine meter really , very happy with mine had it for 4 years now not a problem anywhere . Changed the battery twice probably . Dont often carry it around in the car though. Lives on a boat or a shed mostly

Can't help commenting on non-contact testers, as there is so much ignorance and misinformation flying around in this thread on that subject.

Where have y'all been the last ten years or so? The non-contact testers that folks use nowadays (speaking as an electrician, i.e., someone who works with 120 vac), do not use neon bulbs. Those are the old-school types. The newfangled ones (which actually aren't all that new, having been around for at least a decade or so) are little pen-shaped thingies with a bunch of electronics inside which light a LED and beep a beeper when they detect nearby live wires. All the ones I've seen run on a single AAA cell. They have no exposed metal parts; the "probe" tip is plastic, and they don't need to be placed in contact with a live conductor. Putting them up to an energized outlet is enough to set them off (or next to a "hot" wire). Safest way to determine if something's live or not; I call mine "the lifesaver". So that's what's inside those multimeters, not a neon bulb.

Sheesh ...

A "lifesaver" with a little AAA cell that soon goes dead will KILL you. I trust my neon bulb testers that use no battery.

audioguru said:

A "lifesaver" with a little AAA cell that soon goes dead will KILL you. I trust my neon bulb testers that use no battery.

Click to expand...

Well, duh, you make sure it's working first before you trust your life to it, by putting it up to a known live device or wire, and replacing the battery as needed.

Your neon-bulb testers aren't anywhere near as sensitive (nor even as reliable) as the non-contact testers everyone else uses nowadays.

Seems you've got something snarky to say about everything here, "guru" ...

Some electricians use a finger or two to "feel" if a circuit has high voltage.

Ah, yes, the old 3-fingered electrician ...

Thanks but no thanks.

Thanks Cefiar
I have just applied your hack and it works (You knew that).
My light was on and flickering constantly and rapidly.
It is now off until I bring it close to an AC wire.

Interesting aside with these I just found out after using it for 5 or 6 years . It is BACKLIT ! press and keep down the hold button and up she comes . Just ordered a new one because I could not see it , needed a backlight. Still bought a couple to try at $10 each

Great mod Cefiar, you saved me having to chase tracks

What a ridiculous way to wire a detector (ie, always on) - although considering the price one can't be too critical. Quite happy with this meter otherwise, and now it will have long battery life as well.

I have had but one battery change in 5 years so I don't see what you have to complain about

i have the same meter as you and it is a non contact voltage detector