How To Drive A Relay Using The Input To A Piezo Speaker

Thread Starter

M1871

Joined Feb 12, 2022
6
I have a very simple basic consumer level home alarm system.
It uses a single piezo speaker to issue voice commands and a siren signal with the alarm is activated. The problem is the siren is not very loud and there's no way to connect another siren and there is not any output to control an external relay

I can turn off the voice commands and just have the siren only when needed. So I'd think I could use the driver (voltage ?) going to the speaker to control an external relay

In the past on a similar application I have constructed a very basic one transistor circuit that would monitor a conventional speakers input and convert that to a usable voltage that would control a relay. From that relay I could control anything;another siren, lights etc.

I have briefly tested this transistor ckt device connected to the alarm system's piezo speaker.

It does seem to work but the volume of the piezo speakers greatly reduced. Which might indicate that this approach might be to much of a load . The reduced volume in it'self is not a problem since I would be using other sounders. Actually it would be OK to not have the original speaker connected.

My knowledge is limited on how the Piezo driver works tioproduce sound on the piezo speaker.
I know there's a big difference in the input to piezo speaker as compared to a conventional speaker.
I know one is a Resistive load and the latter is a Capacitive load.

So I'm not sure if this one transistor circuit I have is exactly what I need to use. I have a feeling that it is perhaps not the best solution. Plus..... I'm worried that it might damage the Piezo driver ckt.

I have researched this question and not been able to find any examples how to accomplish what I would like to do.

I'm open to any suggestions and I'm capable of building basic electronic circuits.
 

Thread Starter

M1871

Joined Feb 12, 2022
6
No apparent connection to ground. Alhough I assume there is some connection through a logic circuit.
When I put my DVM accros the speaker while the siren is sounding I read aprox. 130vdc
Where as on a conventional speaker input I would read less that 3 volts.
 

Thread Starter

M1871

Joined Feb 12, 2022
6
OK, I just double checked my readings and I was not completely correct..

On the AC range of the DVM I get 130 vac Same if I reverse the DVM leads

On the DC range of the DVM I get...... 0.003 vdc
With the DVM leads reversed I get 2.6 vdc

Also no reference to ground on the speaker.

Oh... the alarm sounder is powered with 6 volts. (4 x AA batts)
 

Thread Starter

M1871

Joined Feb 12, 2022
6
On a public forum I'm not going to reveal the manufacturer. There are no openly published schematics of the system.
If we could PM I will give you more info.
 

Eddie1974

Joined Nov 26, 2023
1
On a public forum I'm not going to reveal the manufacturer. There are no openly published schematics of the system.
If we could PM I will give you more info.
Hi, I was just wondering if you managed to resolve this, I have just been reading your post and I would like to do the same thing but using the piezo output from a wireless siren unit that is linked to my alarm system. Thanks Eddie
 

Audioguru again

Joined Oct 21, 2019
6,604
A piezo transducer uses a fairly high voltage with a VERY low current. It produces only high frequency squeaking sounds.
A conventional speaker uses a fairly high current with a fairly low voltage. It produces most audio frequency sounds.
 

MisterBill2

Joined Jan 23, 2018
17,736
A VERY INTERESTING CONVERSATION. I have been playing with a slightly similar challenge with a digital alarm clock that delivers a quiet beeping when the alarm sounds. Possibly loud enough to rouse a cat sleeping near the alarm, maybe. so not nearly loud enough.
The measured voltage across the piezo sounder is about 2 volts DC (quiet) and about 9 volts AC+DC with the alarm sounding. As measured with a Simson 260 VOM on the ten volt range. And the alarm is not as loud with the meter connected.
My intention is to drive a transistor to operate a relay to flash a light from a totally isolated power source..

For the TS application I suggest a similar scheme, but using a capacitor so that the transistor base only sees AC peaks, and having the transistor operate a magnetic relay to switch 12 volt power to an automotive alarm speaker. LOTS OF NOISE and totally isolated.
The circuit in post #3 should work for both of us, although I might use a 6 volt relay and a 5.5 volt supply, because of convenience.
 

jiggermole

Joined Jul 29, 2016
161
yeah the input impedance of the amplifier would make a huge difference. The peizo is effectively infinite impedance so the driving circuitry just has to provide a high voltage ac to drive the piezo.
 

MisterBill2

Joined Jan 23, 2018
17,736
The diode in post #3 causes the transistor circuit to have a very low input resistance. Do it like this:
The relay probably does not need that big capacitor. The current from the collapsing magnetic field, because of the diode, will keep it engaged. I discovered that back in 1966, stopping power contactors from buzzing. But nobody believed me. It did take two rectifiers to do it, back to back across the AC line, with the contactor coil connected across one of them. You can try it , it still works. Of course the diodes need to have adequate voltage and current ratings.
 
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