Wiring a Solid-State Buzzer to Trip a Relay

Can you access the square wave signal as a voltage or can you only get to the audio?

If you can get to the votage a simple resistor, capacitor and transistor will be enough to pull in a relay which can control your fans. You'll need to add an extra battery as youll have trouble getting a relay that will reliably pull in from 2AAs... a PP3 9v batter will do the job and last ages.
I'll do you a pic' later.

Del

Something like this should do...excuse the scruffy pic...I havn't got my graphics tablet here, it's 'paint' and a mousie for me

Thanks Del!

That cicruit is easy enough and not at all expensive. Are you assuming that I simply remove the buzzer (assuming I can do that) and connecting the two terminals to your circuit?

Or do I cut one of the leads to the buzzer and wire your circuit in place in series?

I am a bit confused. There are five leads in your design and I'm unclear which one goes where

Thanks

L.J.

Ok.

The two leads on the left connect across the buzzer (you don't need to remove it).

I'm assuming one terminal of the buzzer it at 0volts, it will probably have a black wire or you can see if one terminal connects back to the negative side of your 2AA batteries.

Tell you what it'll be quicker if I re draw it....

Del

Is this clearer?

Fantastic!

The buzzer isn't on stead but pulses. Will this circuit pulse too?

Does it matter?

Thanks

L.J.

It depends on how slow the pulses are...the 1uF capacitor is there to hold the transistor on while the pulse is off... you may need to make it bigger, say 10uF.

The circuit values are not critical, ideally you need a scope to see whats happening, but if you put a DVM across the buzzer when it's on you will see which side of the buzzer is + and which - . On my diagram the + is the top connection.
You can build the circuit and test it without connecting it to your meter, just use 1 or 2 AA batteries connected as a 'pretend buzzer' and it should pull in the relay and do it's funky thing.

Del

(Please take out KrisDel^TM accident insurance before proceeding)

The alarm function is triggering on a relative humdity set point and not a dew point. To prevent dew from forming you would need to have the fans/heater turn on when the Dew point conditions are reached which is both a function of RH and temperature Many times the RH stay the same or even drops during the night but dew forms when the temperature drops low enough. So controlling on RH is not the correct parameter to use to turn on your fans/heater.

- DG

Yes, the alarm goes on when the relative humidity setting is reached. I will set the thresholds so the alarm goes off when the RH is just below 100%. That should be sufficient.

For less than $50 (that includes Del's mods), I do not expect to challenge anything out of NASA.

Thanks

L.J.

Lt,

Your missing the point, dew doesn't have to form when the humidity is just below or at 100% RH. It can form at just about any humdity level. You can have nights were the at humidity is 40% as measured by your unit and if the temperaure is low enough dew will form. Dew forms as a function of BOTH humdity and temperature, that is why the unit display the Dew point temperature. If you control on humdity there will be many nights if not most nights that dew will form on your optics and your fans/ heaters will not turn on since the humdity in the air will not be 100% or just below it. If you look at the picture of the unit you posted it show a humidity reading of 70% and dew point temperature of 62.9. When the air temperature reaches 62.9F the humidity reading will not read 100%,it will still be at 70%, so setting the alarm to go off at or near 100% RH will do you no good. To have your system work you need to control on the dew point temperature and have the fans turn on when the air temperature reaches the dew point point temperature.

DG

"Your missing the point, dew doesn't have to form when the humidity is just below or at 100% RH."

I just realized something. Embarrassing too!

I am more used to monitoring the spread between dew point and ambient temp in flight planning as a predictor of instrument conditions.

Some how I got the symptoms and the criteria jumbled up in the grey matter, badly too!

Del, I've copied your schematic and saved it to my workstation. I'm going to try the device first and see what it does and how. I apologize: my error. Thanks for your generous help.

Thanks . . . . . everyone!

L.J.

Del, your circuit is almost perfect but I'd replace the 1k resistor with a diode...

Good point, but I'd go for both.

Add a diode in series with the 1K to act as a peak detector, the cathode should point toward the capacitor. This will keep the circuit on for longer (few hundred mS) once the buzzer stops.

Just had a thought...
It may be the buzzer is driven the other way up, in which case the whole circuit needs drawing up the other way.
Easiest way to check...
Take the batteries out of the meter, take a multimeter set on ohms and test from each end of the buzzer to each end of the battery connector. You should find one end of the buzzer connects to one end of the battery connector.
If one end of the buzzer connects to the -ve battery connection, then it's like in my diagram.
If one end connects to the +ve, then you will need a PNP transistor and I'll re-draw the circuit for you.

Let me know what you find.

(Alternatively you could connect across the LED...does that flash or stay on solid?)

Del

"the LED...does that flash or stay on solid?"

It flashes in sync with the buzzer.

I've not yet opened up this device so I'm not sure how accessible things are.

Please give me a few days and I'll get back to you.

Thanks Del.

L.J.

LJ, without playing down Del's practical approach to getting a relay contact type output from your sensor, I think Guest DG has a very valid point. The humidity alone doesn't tell you whether you're going to get fogged up.

Maybe something like the condensation sensor from these people would be more appropriate. No ideas what they charge - and I suspect they'll be piling it on for the relay output module thing you need to get a usable output from the sensor. If they could be persuaded to reveal the output characteristics of their sensor, I don't suppose the trip comparator/relay drive circuit would be hard to cobble up.

They do mention Astronomical Telescope Fogging in their applications section. (BTW, they're just the first people I came across on a Googlisation).

Another idea could be checking out some blogs on astronomy enthusiast sites - you can't be the first stargazer to have had this problem (my specs get fogged up often enough - never mind using a telescope).

Timer relay. Your signal to continuously reset the relay timer. The time set to last thru the pulse width of the signal.