I want to modify the internal beep sound pressure level of this bench DMM and make it louder.

I have a problem with my ears and I won't be able to hear the the beep sound well when making continuity measurements. The beeper is small (well, not so small, is a Murata piezo speaker.) Looking at the schematic, the Murata is continuously receiving 5VDC, and when you use the continuity mode shorting the probes, the circuit sends around 2.4VDC, with a mixed square wave to make the beeper/speaker beep noise.

I was thinking of using a kind of simple design using an opamp (the 34401A has a 12VDC rail inside,) or maybe with a couple of transistors will be better.

Maybe I'm getting deaf, but the only DMM I can hear is the Fluke 87, which uses a differend kind of beeping sound, bi-frequency, not a "piiiiiii" one (which, if you vary your head's position with respect to the DMM you won't be able to hear it well)

PD: Here is the part of the schematic which shows the piezo speaker:

enter image description here

  • 2
    \$\begingroup\$ It would be better to share the relevant section of the schematic and the transducer part number (or preferably datasheet) from the BOM if you have it. \$\endgroup\$ Apr 27 at 1:38
  • \$\begingroup\$ Alright. I will do it ASAP. Thank you. \$\endgroup\$
    – Fichamba
    Apr 27 at 13:08
  • \$\begingroup\$ Respecting the piezo speaker (E700 in the schematic), the manufacturer's reference is 9164-0173. In the HP parts list indicate "Alarm-audible 25V". \$\endgroup\$
    – Fichamba
    Apr 27 at 13:32

1 Answer 1


You could try something like this, only 5 jellybean parts:


simulate this circuit – Schematic created using CircuitLab

If it's too loud, add a resistor in series with C1. This gives about 21Vp-p rather than about 5Vp-p across the piezo.

It will function (at least for a time) without C1/R4 but C1 and R4 maintain ~0Vdc across the piezo when not active, which is recommended by manufacturers to prevent silver migration in the element, and subsequent failures.

  • \$\begingroup\$ Wow!, thanks a lot! It looks very nice. I will keep you informed. I will try it out in my breadboard andI hope this will definitely make a loder beep!. I'm anxious to try it out! \$\endgroup\$
    – Fichamba
    Apr 27 at 16:41
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    \$\begingroup\$ I finally done it!. And it worked. Done some mods, but much higher volume. Thanks a lot for your schematic. You make my day. I have a couple of DDMs with an extremely poor beepers (I think they are Rigols). I don’t have the schematic, but I hope it won’t be so complicated. Photo \$\endgroup\$
    – Fichamba
    May 3 at 21:33
  • \$\begingroup\$ Thanks @SamGibson. I don't know how to post an answer including a pic without adding as a comment. \$\endgroup\$
    – Fichamba
    May 4 at 23:26
  • \$\begingroup\$ @Fichamba - Hi, I think you said that exactly the wrong way round, didn't you? I think you meant that you didn't know how to post a comment which included a pic, without writing an answer (yes, it isn't obvious how to do it). || Since it wasn't really an answer, I converted it into that comment above. || If you want to include an image (e.g. schematic) in a comment, use this technique. Note that the draft answer (or question) must not be submitted. It is used only to upload the image & is then discarded. Ask on Electrical Engineering Meta if that's unclear. Thanks \$\endgroup\$
    – SamGibson
    May 5 at 2:33
  • 1
    \$\begingroup\$ Again, thank you for the explanation!. \$\endgroup\$
    – Fichamba
    May 6 at 15:54

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