hi ,
i can read simple schematics, but would it be possible for someone to draw me a simple circuit to amplify the signal from a piezo buzzer beeping to trigger an scr and turn an led or relay on?
i am thinking a darlington transistor to amplify the beeping signal to an scr ?
i am not sure how to wire this up. the fewer components the better.
appreciate help.

 

Hey Dave. Just how are you detecting the beep?

 

i was intending to take the signal from the +ve of the piezo buzzer,is that what you mean?

 

I investigated a similar scheme to trigger an SSR from the piezo beeper in a digital alarm clock. I found several volts of DC across the piezo sounder constantly, and about 20 volts of AC across it when it was sounding. So amplifying the signal will require avoiding the effect of the constant DC level. I gave up at the time and used a mechanical digital clock with actual switches, and a Veeder-Style digital display. Easily able to switch 5 volts DC to operate a standard SSR and switch any mains powered light I chose.

An adequate capacitor coupling could deliver the AC to either an op-amp or a darlington transistor to trigger the SCR. It might even provide enough current to trigger the SCR, if it were a sensitive one. BUT there is the issue of isolation between the SCR circuit and whatever powers the PIEZO buzzer sound.
You could also close-couple a small CRYSTAL microphone to the sounder and use that signal for the trigger.

 

i was intending to take the signal from the +ve of the piezo buzzer,is that what you mean?

Do you have the schematic of how the buzzer is connected in the circuit?
Or measure the DC voltage on both pins of the buzzer when it's ON and OFF.
Is the LED or relay to stay on continuously when the buzzer sounds?

 

its just a basic kitchen timer powered by a 1.5v battery
so i guess there will not be any more than 1.5v across the piezo, but i havnt measured it.
would connecting the +ve of the piezo to the base of a darlington transitor powered by 9v and feed the emitter to the scr work?
(connecting the -ves of the buzzer and battery together)
yes it should stay on once the scr is triggered

 

Not sure that's why I need voltage readings on both sides of the buzzer.

 

i was intending to take the signal from the +ve of the piezo buzzer,is that what you mean?

Wasn't sure if you were thinking about using a microphone as a detector.

Since you are taking whatever output there is to the piezo you could just take both sides of the output as a source for detection. Rectify it and send it to a darlington or even just a simple BJT (Base Junction Transistor). You're only powering an LED, and chances are its power requirements may easily not exceed what the piezo signal is surpassing. Not sure if 1.5V will be enough on its own, so perhaps a higher DC voltage is needed.

That's why I need voltage readings on both sides of the buzzer.

And I agree. Each lead of the piezo to ground and I'd be curious about the piezo voltage lead to lead. Since piezo's can also be used as an energy source the oscillation of the crystal might even produce enough voltage to light the LED directly. Since none of us have it in hand for testing - we have to rely upon you for the test and diagnostic work.

 

ok,
so there is ~0.05v max across the piezo when its beeping.
does that help.
i thought if i could trigger an scr i could drive an led or even a relay

 

So it's a beep not a continuous tone?
If it's a short beep probably won't get an accurate reading but still a chance it can be made to work.
Again try and get a DC voltage reading from the negative side of the battery to both sides of the buzzer when on and off.

 

Make sure your meter is set to AC. If it's set to DC it will not get a good sampling of the voltage. Whereas if it's set to AC it will read the changing signal. And this is why I'm thinking you should test for voltage across the piezo. @sghioto is a sharp member of AAC and I would trust him. In this case reading voltages from the negative battery terminal might not produce a good reading. That's why I'd look for voltage (AC) across the piezo itself.

Sghiggy, sorry if I disagree with you on this one.

 

The buzzer may be have an internal driver which runs on DC voltage is what I'm trying to find out.

 

ok
so with the meter set to measure ac, the voltage accross the piezo fluctuates between ~0v and 0.1v when its beeping.

 

Can you get the DC voltage readings asked in post#10 please ?

 

answer to post 10.....
off no beeps on beeping
-ve battery to +of piezo 1.28v 1.21v
-ve battery to -ve of piezo 1.28v 1.21v

 

Wasn't sure if you were thinking about using a microphone as a detector.

Like this -- The sound active control circuit.

 

not really

 

If you don't know how to design the circuit and draw the schematic, maybe you can draw a block diagram to show what you want to do.

 

This could go on for a while. Please post photos of the circuit board or whatever the piezo is mounted to. In particular, how thick is the beeper?

There are two types of small piezo beepers. One is a bare element that requires and AC signal, and the other requires only DC power because it has an oscillator circuit built-in. photos might tell us which one you have.

Separate from that, 1.5 V is not enough to energize most visible LEDs, so that would need its own oscillator circuit, probably a simple charge pump. Also, I don't know of a commercially available relay that will activate reliably on 1.5 V.

What is it you are trying to achieve?
How much power do you want to control with the relay contacts?

ak

 

Hey Dave. Just how are you detecting the beep?

i was intending to take the signal from the +ve of the piezo buzzer,is that what you mean?

Like this -- The sound active control circuit.

I'm still not sure what would be the best way to detect the beep signal. Much of the ambiguity depends on determining the actual piezo voltage. I have no idea what voltage they typically work on. But even if it IS a 1.5V source, it should be possible to amplify that signal enough to switch on a transistor or MOSFET, which in turn turns on the LED.

Why the LED? Is the piezo not loud enough? Is this for a deaf person?