thermostat buzzer temporary silence?

Discussion in 'The Projects Forum' started by mvmacd, Jan 5, 2015.

  1. mvmacd

    Thread Starter New Member

    Jan 5, 2015
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    Ok here is my project.

    My house uses wood heat so a digital thermostat was just installed near the stove. When the temp drops too low it opens a relay, and that closes a circuit to a 1.5v buzzer (Right now it's on a AA battery).

    The idea is, when the buzzer rings, we know it's time to put wood into the stove (which is in the basement).

    It works, however it continues to buzz until the heat surpasses the set thermostat temp which could be 5 minutes or more after it starts buzzing.

    So here is the circuit I'm requesting.
    When a 555 timer receives power (relay on from thermostat), it should activate the buzzer. When a "silence" button is pressed, the buzzer should be silenced for about 10 minutes. If at this point the power hasn't cut off to the whole circuit, it should start buzzing again. If the power has cut off it should still work the next time the relay closes.

    I have searched bistable circuits and that is almost close to what I think I need, but I have a feeling 2 timers will be needed.
    Anybody with the expertise is free to weigh in :)
    thanks for reading my long post
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Welcome to the forum!
    Ten minutes is asking a bit much for a consistent delay from a 555-based circuit. It would involve a capacitor charging current very similar to a self-discharge leakage current. You might be better off using a clocked counter IC for the timing. Only one timer would be needed.
     
  3. mvmacd

    Thread Starter New Member

    Jan 5, 2015
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    Okay so let's say, 5 minutes then?
    I'd like to start somewhere and then increase the delay until unstable, then I would know what is too high of a delay.

    Do you have an idea how I could do it with 5 or less minutes?
     
  4. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    I think you could accomplish what you want with one 555 one-shot. Here's the sequence I imagine.
    Temperature falls and power is switched on through a normally closed relay to buzzer, and buzzer sounds; 555 circuit is constantly powered, but not triggered.
    Button is pushed, which triggers the 555 timer which operates a relay for x minutes, which opens the power to the buzzer.
    555 times out, which releases the relay, which allows buzzer to sound if power is still supplied by the thermostat.
     
  5. mvmacd

    Thread Starter New Member

    Jan 5, 2015
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    I see what you're saying, and I think you've got something. But actually the thermostat relay is N.O. and is only closed when it gets cold, and opens again when it warms up.

    But it's a little round black plastic buzzer, about the diameter of a dime, so I think it would be more efficient to use a transistor (NPN?), and upon initial on, the timer supplies power to the gate. Then when the button presses, turn off the transistor for 5 minutes, then turn back on.

    Can someone suggest a way that I would wire this and what parts I would need? A rough schematic would be very helpful.
     
  6. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    For a schematic, you have to provide the voltages involved. What voltage operates the relay, and what voltage operates the buzzer?
     
  7. mvmacd

    Thread Starter New Member

    Jan 5, 2015
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    Okay well, the relay, as I have described it, is not really relevant. It is in a separate circuit entirely, and when it closes, that's when my 555 circuit activates and needs to buzz, and silence temporarily on the button press.

    Right now the N.O. relay is wired to a 1.5v AA battery, but I know that won't be enough to run the timer, so I am considering a 3v coin battery (CR2032).

    If that is not enough I could use 2 to make 6v. Thanks
     
  8. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Your problem seems to be the following: The temperature drops. The low temperature causes a Buzzer to sound. You push a button to silence the alarm for at least 10min, put wood in the stove, and go back to bed. It takes time for the wood to catch fire and the temperature to rise as the heat permeates the house, so this is the time during which the buzzer is to be silenced. The fire again burns low, the heat output from the stove decreases, the house cools, sets off the alarm again. If the new log failed to catch fire, the timing cycle would elapse, the temperature would still be low, so the buzzer would sound.

    The operating current of a standard 555 will kill two coin cells in a day or so. Even a CMOS 555C would require larger C or D cells to get any reasonable battery life.

    Here is a circuit that draws only 60uA from a 6V battery (4 AAs) when the house is cold and the alarm is silenced. It draws whatever the buzzer draws while the buzzer is sounding. It draws zero current from the battery if the house is warm. I am assuming that the thermostat is a make on temperature drop type. I am assuming that the push-button used to silence the buzzer is momentary, normally open.

    The time during which the buzzer is silenced is set by R1 and C1. C1 should be a low-leakage 10V electrolytic. The values shown give a delay of ~ 700s.


    117.gif
     
  9. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    If your buzzer/horn is an electromagnetic type you will need a reverse-biased diode connected across it in Mike's circuit, to protect M2 from any voltage spikes.
     
  10. MikeML

    AAC Fanatic!

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    Since he talks about powering it with coin cells, I was assuming it was a piezo sounder, like a Sonealert.
     
  11. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    The reason I asked about the coil voltage of the existing relay is to know what voltage is being supplied by the thermostat output. When you say, "the N.O. relay is wired to a 1.5v AA battery," do you mean the relay contacts are switching 1.5V to the buzzer, or that the 1.5V is powering the relay coil? I still would like to know what voltage operates the relay, and what voltage operates the buzzer?
     
  12. MikeML

    AAC Fanatic!

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    No relays required in my circuit. How long would a coin cell operate a relay, any relay?
     
  13. MikeML

    AAC Fanatic!

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    I agree that the description of his present setup is very confusing. His confusion is catching...
     
  14. tracecom

    AAC Fanatic!

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    I agree, and would like to better understand the current setup.
     
  15. mvmacd

    Thread Starter New Member

    Jan 5, 2015
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    Thank you Mike for the schematic! (i think your summary nailed it too)
    I will hunt these parts down in my spare parts bin and test it on a bread board. I didn't know I could do it without a 555 timer.

    Sorry everybody for the confusion, let me try to clear it up.

    The thermostat, is a digital device connected to mains for power, with a lead with the actual temperature sensor going down to the stove, while the thermostat sits upstairs. In can be programmed (a led clock-like display) to come on when the temperature drops below the threshold (it's supposed to turn on a furnace control), so that is the relay it closes. There are 2 wire sockets coming out the back, and those 2 wires are in series with the AA battery and peizeo-type buzzer (self-oscillating).

    Thanks everyone for the help, I am glad there is a forum like this with helpful and friendly people. :)
     
  16. mvmacd

    Thread Starter New Member

    Jan 5, 2015
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    It won't let me edit my post, so I just wanted to say also, that the reason for batteries was simplicity. The buzzer uses not even a mA at 3V, so it should last a long time before a battery change.
    If needed I could run a 5V usb charger, but I'll see how Mike's circuit works then decide.
     
  17. mvmacd

    Thread Starter New Member

    Jan 5, 2015
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    Sigh, it looks like I don't have the MOSFET transistor, I initially thought they were normal transistors so I'll have to order some on ebay.
     
  18. MikeML

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    The secret to getting the long time delay is the high input impedance of the NFET.
     
  19. mvmacd

    Thread Starter New Member

    Jan 5, 2015
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    Hello, I wanted to again say thanks for the schematic.
    The mosfets arrived and I built the circuit. I ended up using 5V power (Chinese usb wall charger), I dropped R4 (because the time varied depending how long the momentary button was pressed), increased R1 to 20M ohm, and decreased C1 to 22uF.

    The result was a 9m 30s delay. very consistently the times that I tested it with a timer.
    I was expecting it to reset when the power was cut, but it remembers how long is left because the capacitor discharges like nothing happened.

    I soldered it on a PCB and added a 555 circuit I already had to give it a time beep similar to a time bomb (lol) instead of a dull solid "beeeeeeeeeep..." It beeps shortly.

    Matt
     
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