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I'm using a digital multimeter (UT61E) and it doesn't have the "Wrong Probe Insert" alarm feature.

I mean when I select the Voltage Measurement with the knob but leave the probes in the Current Terminal, it will sound an alarm and vice versa.

I made this mistake all the time, so I want to build a circuit to do this and mod it on my multimeter.

I've been googling for a while but can't find out how this circuit is made. Maybe a simple idea or some explanation of the principle would help me a lot.

I really don't understand how the meter knows if the probe is inserted or not?

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    \$\begingroup\$ You've learned not to stick your fingers into a wall socket? Then use the same thought-process with your multimeter's current scales. I've learned to put probes back into the much safer "voltage" terminals, after measuring current. Every time. Some meters have no protection, and just assume a careful user. \$\endgroup\$
    – glen_geek
    Commented Feb 14, 2017 at 17:40
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    \$\begingroup\$ @glen_geek good for you. I wish my meters had that, and I've been an engineer for twice the time I've not been one. I usually remember to turn the lights off on my car as well, but do you know what, the one with the lights warning with the ignition doesn't get a flat battery as often as the one without! Some of us just need those doh!-proofing aids. \$\endgroup\$
    – Neil_UK
    Commented Feb 14, 2017 at 17:48
  • \$\begingroup\$ @Neil_UK fair comment. Am advocating a careful & thoughtful approach to measurement - still have 5 fingers on each hand. Have spent years trying to foolproof circuitry, which only seems to uncover a craftier fool. But I do admit that this meter warning system is pretty good. \$\endgroup\$
    – glen_geek
    Commented Feb 14, 2017 at 19:13
  • \$\begingroup\$ Thank you for your comments and I agree that being careful is always the best. But I think you both know that at the end of the day, when you're exhausted finding out what is wrong with your PCB or etc., mistake might occur. Therefore I just want to add an useful feature in bad situation. \$\endgroup\$
    – ReVl27
    Commented Feb 15, 2017 at 2:56

2 Answers 2

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The meter's microprocessor knows what position the knob is in. It can also know what terminal the probe is in by a switch at the bottom of the hole.

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They detect whether a testlead is plugged in and when that state does not match the selected range a signal is buzzed and displayed.

The implementation of the detection is dependent on the manufacturer. These two methods are very common:

  • electrical - splitting the input jack contact and detecting a current across both halves caused by the plug shorting them together

  • optical - shining light through the input and detecting the interruption of the light beam caused by the plug

Electrical (split input jack)

With the split input jack they can send a signal to one of the jack halves. When the testlead is plugged in, the signal is shorted over to the other halve and can be detected by the microprocessor.

See EEVblog episode #99 at 20:55 (direct download and other infos here)

Dave shows the input jacks of a Extech multimeter and explains why they do what you ask for.

This is the board viewed from the top:

Multimeter board (top)

(Source: EEVblog episode #99)

This is the board viewed from the bottom:

Multimeter board (bottom)

(Source: EEVblog episode #99)

Here's another example of a Keysight multimeter:

Multimeter board (bottom)

The mechanical construction is quite different but the jack is split too. Dave shows this a second later.

Source: EEVblog episode #832 at 17:10 (direct download and other infos here)

Optical

This type of input alert uses a (infrared) LED sending a lightbeam through a transparent input jack. When the plug enters the jack, the lightbeam gets interrupted.

In case of the Fluke multimeter this type of input alert is used to ensure the waterproofing.

See EEVblog episode #64 at 9:30 (direct download and other infos here)

Dave shows the board and input jacks of a Fluke 28 II multimeter.

This shows the LED and photodiodes on the board:

Multimeter board

(Source: EEVblog episode #64 info page)

This shows the transparent jacks of the case:

Case with transparent jacks

(Source: EEVblog episode #64 video)

so I want to build a circuit to do this and mod it on my multimeter.

I don't think that you should do that.

  1. Input protection on a multimeter is not trivial (Dave's videos on multimeter tear downs explain much about that). With bodging some input alert into the multimeter you risk to disturb necessary spacing and clearance.

  2. If the processor does not have a firmware that supports input alert adding such a modification is not done with just some wire. Not to speak about splitting the input jack.

Note: there might be other ways of input jack signalling like buttons or light barriers, but I don't think they're as reliable as real junctions.

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  • \$\begingroup\$ Thank you so much for your explanation. I'll consider your advice. \$\endgroup\$
    – ReVl27
    Commented Feb 15, 2017 at 2:58

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