Fuse Finder.

if you put a meter on volts where the sounder used to go, do you see a DC  or an AC signal ?


Those little sounders either need driving at AC (test that with audio from a headphone jack) or the DC models contain a one transistor oscillator,  but then there are a number of variations on the oscillator  same theme, the loudest  making the piezoelectric disc into a 3 terminal device, so that it i s driven by the collector of the transistor, but there is a pick up  electrode that acts as a crude microphone and drives the transistor base - these are automatically on-tune for the self resonance of the disc.

moving violently back and forth sounds a bit worrying when it used to work was the audio tone pulsed or continuous.? From what you say the voltage is pulsing now.


In any case, it sounds like the electronics on the board is chopping up a DC supply of about 5V to create a square wave to drive the buzzer.

This is in effect rather like a DC of half the supply voltage on average,  but with an AC superimposed that takes the total down to zero on the negative half cycles, and up to the full supply on the positive ones.

Be aware that some older moving coil meter AC ranges are not AC coupled but are just a DC range with a diode in series, so they will still read on  a DC input, but rather over estimate, as the scale is compensated for the AC case when curretn ony flows in the meter movement for half the time.  The clue to DC present on such a meter is that the 'AC' reading is not the same if you reverse the polarity of the meter probes.


So you may not need the type of sounder that takes a DC input, as if the board is already generating AC, then a simple transducer will do, with no electronics in. 

 example of bare piezo electric crystal


 made up units with electroncis inside


Connecting a pair of walkman style headphones via some limiting resistance ( - which could be your moving coil meter - it is after all a box of switched resistors in series with a meter mechanism) will allow you to be totally sure if the audio frequency AC is created on the PCB. Equally I think it will do no harm to fit a cheap DC sounder and see what it does. Or can you open the broken beeper and see if there is electronics inside or just the bare disc of crystal that changes shape with applied voltages.

Thanks Mike,

                        I will fit a new sounder and see what happens. The old one is encapsulated with just two leads and I can not get into it. It also has no markings on it. The circuit board has an NE555P chip on it and a few resistors, diodes, capacitors and a transistor at the detector end. The booklet states that the switching frequency is 5Hz. approx. That seems to be about the rate at which the L.E.D. flickers on and off.


Bye,  Z.

The ' 555 is a venerable chip design (*) and creates a controlled time delay, set by an external R and C,  and by linking the output to the reset input it is often used as a free-running pulse generator - if there is just the one, it may be doing the 5Hz for timer, and that would imply the audio frequency is generated within the buzzer module.  If the buzzer is in parallel with the LED, that would clinch it as the sort of buzzer needing DC.



(*) so venerable in fact it has its own web page on wikipedia and is apparently the most popular chip on the planet.

Thanks Mike. I have replaced the small sounder with a device called a 5 Volt mini PCB magnetic active buzzer (continuous beep) and it works well. It beeps at about 4 beeps per second. So I can now use the fuse finder. The new part was very inexpensive.


Bye,


Z.