Diode replacement

One method that is commonly used to get low-power audible signals is to produce capacitive discharge into the sound emitter, through a diac or similiar breakover device. A larger numer of 'ticks' can be generated on each half of the line cycle as the sensed voltage is increased.

Your big problem may be getting a small enough transducer to fit into a screw-driver-sized housing. You'd want to enclose the circuit - like the original screw-driver was - to maintain safety, as the circuit components are live.

RL

I assume you mean a light emitting diode i.e. LED

It is most likely possible but I can't offer any advice without know more about the existing circuit

You need to know what voltage and current is available to drive the buzzer so that you can choose the appropriate type of buzzer.

-- Dan Hollands

I would presume that the diode you are talking about is actually a neon disharge lamp (glows orange when you touch the mains with the screwdriver).

If so, DO NOT modify it the way you are talking about unless you want to give yourself a very serious electric shock.

The typical mains tester screwdriver is a neon discharge tube in series with a high (470 kilohm?) resistor to limit the current flow to a safe level.

Once again DO NOT TRY THIS AT HOME!

Alan

Thanks for all the replies.

The components do not need to go back into the screwdriver casing although all the components will be encased.

I may be being really stupid here but could I still use the high resister and use a buzzer instead of the NEON? Basically my end product will be a mains tester that buzzes if whatever its touching is live?

Thanks Colin

Right ok... made me laugh a bit that... seems its not a good idea then. Maybe I have to follow a different design to get an audible signal if

Thaks for all the help...

A lucky Colin...

Just butting in to comment that what Colin proposes might be very dangerous - if, as I gather from his comment about having a 240v supply, this volatage tester is one that uses a neon indicator to test for voltage, then replacing it with a 'buzzer' is a complex issue filled with the possibility of electrocution.

The neon indicators work with only one point (the tip of the screwdriver) connected to the circuit to be tested. The return circuit is by way of body capacitance, and is sufficient to light a neon globe. Using the same technique to sound a buzzer is dangerous in the extreme, and would require knowledge and experience which is probably beyond the OP's current level.

Of course if my surmise is wrong and it is one of those 12v testers then it is merely a matter of choosing the right buzzer

But I needed to post just in case my concerns were real

David

Dan Hollands wrote:

NO NO NO - this is NOT a good thing to do - for one it will not work, and if it did work, that would also mean you would not hear the buzzer because you would be DEAD as the current for the buzzer would be going through your body - ZAP - one fried Colin

If you really want to make a voltage tester, then make one that tests batteries, not the mains.

David

Col> Thanks for all the replies.

A buzzer needs enough current that you'll be electrocuted !

The neon requires so little that it's basically safe.

That's before looking at any of the other practical issues.

Graham

They involve 'active circuitry'.

Graham

Not for the OP to try!

I wonder if we put the right piezo electric device across the neon bulb if you could make one that would work (sort of).

The 470K and the capacitance (including the motion) would make a ramp on the piezo. When the NE-2 strikes, there will be a sudden drop in the voltage.

With luck, the mechanical resonance and the RC osillator effect would line up. This would allow the NE-2 to quench and the cycle to repeat at a rate that makes the most sound.

I wouldn't expect the result to be very loud though.

with ~70V on it it won't be inaudible....

with the right series inductance it may be possilble to get the thing to oscilate at an audio frequency, like a spark-gap transmitter. or it may be possible run it at a higher frequency and drive a step-down transformer to power a sounder...

Thanks for the replies, this group is very good....

I have changed my design plan and am hoping to add a single 'aa' battery to sound a buzzer rather than use the mains. Just need to work out how to do it?

Thanks Colin

In article , Jasen Betts wrote: [....]

You don't need the inductance to make it oscillate. Many an oscillator has been made with just an NE-2 and a single RC.

There won't be 70V steps on the piezo. The steps would be ~30V. The 60Hz component won't lead to much sound because a small device would not be very effective at low frequencies. What you would hear would be a buzzing sound.