sound detector

[Pappy427]

First, I'm not a dummy to circuits but I am 85 so please go easy on the depth. I am in need of a simple way to detect the “click or snap” of my thermostat whenever it activates. Would prefer a BEEP instead of a light. Needs to be battery powered.
Not looking for a permanent installation.

The thermostat is a standard wall mount, however, I cannot simply go inside the unit and pick up the voltage nor can I get to the backside.    Thank You

[Kim Christensen]

A device like that could pick up other noises in the house and then give a false "beep".
Have you thought of monitoring the heat produced by the device it controls? That might give a more consistent, though delayed, beep.
May I ask why you need to know when the thermostat cycles on? It'll go on/off many times during the day/night, so that could be a lot of beeping.

[bostonman]

I was typing a reply similar, but you beat me to it.

Without designing/buying something that's specifically tuned to the sound you want to detect, as stated, it will provide false triggering due to other sounds.

You could monitor the voltage on the wire that triggers the heat to turn on/off, or, as already stated, monitor the heat. I did something similar a few years ago to determine how many times my dehumidifier turned on/off.

I took all the temperature data and used Excel to first detect when warmer air was leaving the unit. This told me when it came on, but required I manually counted each turn on. After I took that data and used Excel to determine when the system turned off. Finally, I did a count on each chunk.

It was a bit more involved trying to get Excel to tell me what I wanted, but that's the summarized version.

[berke]

Place a small coil nearby to detect the 60 Hz magnetic field produced by the current that flows when the thermostat is on.  This way you won't have false alarms and you won't have trouble distinguishing the on-click from the off-click.  Amplifier, rectifier, low-pass RC filter, comparator with a pot to set the threshold, a status LED and a 556 to produce a beep.

[Terry Bites]

Its going to be hard to get the detector to ignore other noise sources in the area.
But it's unlikely that anything under about 2KHz or above 10KHz matters here.
You could use a phone app to get a snapshot of its audio spectrum. Tune up the amp characteristics to suit.
I'd be thinking about a 20mm piezo disk transducer as a mic. They're absolutely crap at low frequencies so ideal for this.
An amp, a comparator a 555 monostable of, say a few 100mS and a piezo buzzer.
I've attached an idea. Tweak R6 for pickup gain. I hope you dont think I'm trying to teach you how to suck eggs here!

Could also be used with berke's coil pickup idea.

[BILLPOD]

How many wires connect to the thermostat, from the furnace?, (mine has 3: red, white and green).   Most furnaces send 24 volts AC to the thermostat, (with mine it is through the white and red).   When the thermostat calls for heat, it connects the red and white together.   Measure  your voltage going to the thermostat, coming from the furnace, and make note of the colors of the wires).  Then we can go further.

[richnormand]

"The thermostat is a standard wall mount, however, I cannot simply go inside the unit and pick up the voltage nor can I get to the backside."


If the thermostat has an audible click when calling for heat it is most likely actuating a relay (as opposed to a SCR or MOSFET).
In that case the relay coil should give you a EM signal when it actuates.

"A few moments later"....
Just tried it. I used a open air coil with about 200 turns on 5cm diameter near my Honeywell programmable thermostat.
The small DSO138 (ebay cheapy "scope") could pick it up on the 0.1V scale. Just a blip going both ways. I assume the relay is DC actuated.
Similar signals when actuating the fan air circulation relay too.

This might be more robust than an acoustic detector? Still would require a bit of filtering and such as shown in the previous post by Terry Bites.

If his thermostat does not yield a useful signal but his system is forced air a simple flap over the air register could activate a microswitch or opto sensor?