Simple circuit to detect a beeping thermo sensor

Thread Starter

skedem

Joined Oct 13, 2023
3
Hello I'm new to this forum hope I landed in the correct forum

I have a thermo sensor which monitors several freezers temperature
When temperature is out of bounds it emmits a continuous fixed frequency beep. Its not smart home sensor but old style one.

The beep is not loud (I guess some small piazo buzzer) and cannot be heard if I'm not in vicinity or not at home

What I would want is to attach externally to the thermo sensor a small circuit with a tiny microphone that can trigger ,based on the beeping sound, a small dry contact relay that I can connect to my smart home system as input trigger. This circuit should somehow avoid environmental noise if possible

If anyone can help how to design such a circuit or built something simiar and can share

Thanks
 

Thread Starter

skedem

Joined Oct 13, 2023
3
A simple solution would be to feed the sound signal to a LM567 (or NE567) tone decoder IC that will detect just the frequency you need
Thanks
Can you suggest a circuit with LM567 ,and does it connect with small microphone
 

AnalogKid

Joined Aug 1, 2013
10,944
A simple solution would be to feed the sound signal to a LM567 (or NE567) tone decoder IC that will detect just the frequency you need
That is a seriously old-school suggestion. Gotta love it.

GREAT find. The output can sink 100 mA, way more than enough to drive a small reed relay. Don't forget a suppression diode across the coil. 1N914, 1N4148, etc.

ak
 
Last edited:

AnalogKid

Joined Aug 1, 2013
10,944
Do you have the relay already, or are you looking for suggestions? If the latter, do you know the open-circuit voltage and closed-circuit current (or input impedance, or whatever) of what is being switched?

a
k
 

Dodgydave

Joined Jun 22, 2012
11,237
Hello I'm new to this forum hope I landed in the correct forum

I have a thermo sensor which monitors several freezers temperature
When temperature is out of bounds it emmits a continuous fixed frequency beep. Its not smart home sensor but old style one.

The beep is not loud (I guess some small piazo buzzer) and cannot be heard if I'm not in vicinity or not at home

What I would want is to attach externally to the thermo sensor a small circuit with a tiny microphone that can trigger ,based on the beeping sound, a small dry contact relay that I can connect to my smart home system as input trigger. This circuit should somehow avoid environmental noise if possible

If anyone can help how to design such a circuit or built something simiar and can share

Thanks
Can you not tap across the buzzer output driver , and use it to drive a slave transistor relay?

Any pictures of the circuit board .
 
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