HP 34401A beep level

[fichamba]

Hello everybody:

(you can call me the beep boy, because this is my second post about the same question, but regarding another bench DMM...hahahaa! ).

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

I was thinking in using any kind of simple design using an OpAmp (the 34401A have a 12Vdc rail inside). Or maybe with a couple of transistors will be better.

Great ideas accepted. Maybe I'm getting deaf, but the only DMM I can hear is the Fluke 87 (wich 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, almost me).

Thanks a lot.

[artag]

Interested to find you how this works out. I also have problems hearing the 34401A beep, noting particularly that it's very dependent on my head position. I guess one possibility is to try to emulate that fluke behaviour by adding a different piezo in paralllel, resonant at a harmonic of the first one.
Or perhaps replacing the simple piezo with a self-oscillating one which will then operate at a mixture of the two frequencies.

[iMo]

You may try to put a coil/inductor in parallel to the piezo (like 10-100mH, where it resonates best). Be careful as the sound level increases significantly, also you have to consider the voltage level at the driver's output goes up with the coil such you may smoke it.. This is how the alarm piezo buzzers are made. Use at your own risk..

[fichamba]

Just in case, I'm adding the part of the schematic wich belongs to the piezo speaker (E700). The Murata part number is 9164-0173. In the parts lists, you can read in the descrption "Alarm-audible 25V":

[fichamba]

Interested to find you how this works out. I also have problems hearing the 34401A beep, noting particularly that it's very dependent on my head position. I guess one possibility is to try to emulate that fluke behaviour by adding a different piezo in paralllel, resonant at a harmonic of the first one.
Or perhaps replacing the simple piezo with a self-oscillating one which will then operate at a mixture of the two frequencies.

Well, the problem with the self sounding ones is that they only use DC voltage in order to work. In the schematic, you can see that you always have +5Vdc (without any induced pulse, until you are in continuty mode and short the leads; then the voltage drops to around +2.7Vdc, but with the added square wave at 50% duty cycle). So, having +5Vdc constantly there, the self sounding piezo indicator will sound at all times!. That's the reason why we need to amplify this level (when modulated with the square wave, of course).

Remember the beep is also used when starting ut the DMM, etc.

EDIT: BTW, I also own a fantastic 121GW, and it has the same problem; the beep sounds very little. I need to make a test, because it has a 56 ohm resistor in series. Maybe jumping this resistor the beep level will increase a bit...or maybe not.

The problem is that this transducers are very directional, not to mention the fact that they are mounted inside of the device (and sometimes with another layer, like in the 121GW), so they are very covered. As I told before, only some of my Flukes are audible, even inside theis pouches. Well, you know...65 year old and still alive...and kickin! 

[iMo]

The usual way to increase the sound level is the parallel choke with the piezo element as I wrote above. It creates higher voltage spikes off the square wave and the piezo gets louder. Well, the issue is to try it in situ in the valuable meter.. From the schematics there are limiting/clamping diodes which does not allow this kind of modification.
Best is to use a separate npn transistor driving the piezo with the choke in the collector and the base capacitively coupled to the MCU's output (let the diodes and the resistor to +5V there).
A simpler variant, is to use an npn transistor coupled to the 1.78k resistor (MCU's output) with say 10k in collector, with the piezo wired to the collector, and wired to a nearby voltage higher than those 5V (mind there are separate inguard/outguard voltages/grounds!!).