Alarm silence circuit

Discussion in 'The Projects Forum' started by ipanos, Apr 4, 2016.

  1. ipanos

    Thread Starter Member

    May 7, 2010
    12
    0
    Hello guys,
    I am trying to build a circuit that seamed simple at first (and I am pretty sure it is) but my very basic knowledge in electronics betrayed me midway. Any help will be greatly appreciated!

    Here it is:
    I have a AC monitor which among other functions also has an alarm for exceeding an adjustable power threshold. When too much power is drawn it flashes the LCD screen backlight. That is alright but I wanted to add an audible alarm to it as well, so on the monitor's board I found the alarm output pin on the IC, which is oscillating 3VDC at 2Hz. I also located the output of an LM7812 at the very beginning of the circuit, so I thought, lets hook up a 12 volt buzzer with a PNP and bias it from the oscillating alarm output. It worked nicely but...

    I want the alarm to buzz at a threshold lower than the absolute max possible, which means that sometimes the alarm buzzes while I still have some margin, which is irritating. I would like to add a silence/acknowledge momentary switch, which will mute the buzzer until the alarm is off and re-triggered by the monitor's circuitry (or even better if it is not too complicated, for 5-10 minutes). And that's where I am stuck.

    Thank you in advance,
    Pete
     
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,386
    1,605
    Hi ipasos, welcome to the forums!

    Without a schematic of some sort it is near impossible to give you any help. Could you sketch something up?
     
  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,530
    1,248
    There have been other threads here and on other fora that discuss this kind of self-resetting disable circuit. One of the challenges is the long-duration timer. Multiple-minutes is a lot to ask of a simple resistor-capacitor timing circuit like a NE555 or CD4093. Semi-accurate and repeatable timing of several minutes usually requires an oscillator and a counter. Not a significant increase in complexity above a basic timer, but controlling it can be tricky depending on the rest of the information you have not yet supplied. Schematic - ?

    ak
     
  4. ipanos

    Thread Starter Member

    May 7, 2010
    12
    0
    Thank you for your interest in my project.

    Here you go...

    This is what I have so far.
    As far as the timer goes, accuracy is not important (+/- 10 seconds wont hurt) and I think I can live with the maximum delay a 555 can provide.
     
    • 01.jpg
      01.jpg
      File size:
      374.1 KB
      Views:
      11
  5. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,530
    1,248
    With 12 Vdc available, you can use a CD4093 to make a timing monostable and the necessary gating to combine it with the alarm output. The problem is that the capacitor size for a 300 to 600 second delay is large, to the point that its leakage current might be greater than the timing current. For example, for 10 minutes the approximate circuit values would be 1 megohm and 600 uF, or 1000 uF and 620K. Easy components to get, but even if the capacitor tolerance is +/-10% (considered tight tolerance for that size cap), that is +/- 1 minute, without other circuit errors. These same issues hold when using a CMOS 555.

    One alternative is PIC or other small microcontroller. 8-pin part, under $1, no other external gating. But you need a small voltage regulator to run it on 12 V, and of course there is the programmer, software development software, learning time and writing and debugging the application, after which you still have to wire up a circuit.

    Another alternative is a counter/oscillator like the CD4060. The timing capacitor is much smaller, more accurate, and more stable. The circuit is simple, but it powers up in the muting condition. If that is ok, this is the fastest path to a working solution. If you need for it to power up in the active (unmuted) state, that is fixed with a 2nd chip like a CD4001 to make a flipflop and gating to control the beeper.

    OR - buy a 12 V timer module on ebay and simply wire it into what you already have.

    ak
     
  6. ipanos

    Thread Starter Member

    May 7, 2010
    12
    0
    OK, how about forgetting the timer all together and make the mute reset when the alarm signal disappears.
     
  7. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,530
    1,248
    Easier. It is called a missing pulse detector, driving a flipflop. Pushing a button sets the FF and turns off the alarm. Assuming you are not holding down the button continuously, then about two cycle times after the output quits cycling, the missing pulse detector detects the missing pulses going to the alarm and resets the FF. All of this fits within one CD4093 gate package.

    1 - SPST switch
    1 - CD4093 (not CD4011)
    3 - resistors (3 x 10 K)
    3 - capacitors (1 x 10 uF, 2 x 100 nF)
    1 - NPN transistor (same type as driving the beeper) or 2N7000/7002 MOSFET
    1 - diode (1N914, 1N4148, ETC.)

    Is this within your skill set?

    ak
     
  8. ipanos

    Thread Starter Member

    May 7, 2010
    12
    0
    You are a star!
    Care to draw a schematic maybe?
     
  9. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,530
    1,248
    Things got slightly more complex because the driving signal is only 3 V, too low to drive a logic gate running on 12 V, so there is an extra transistor and an unused inverter.
    R5 and Q1 are the original beeper drive parts.
    U1A-U1B form a Set-Reset FF with active-low inputs.
    SW1 pulls U1B pin 6 low to set the FF.
    When set, U1B pin 4 is high. This turns on Q2, which removes the alarm signal drive from the base of Q1, which silences the alarm.
    R2-C2-Q3 convert the driving square wave to a logic 0 when present. When the waveform stops, R2 charges up C2 to a logic 1 after 5 seconds.
    U1C inverts this level to a logic 0 reset the FF.

    With this circuit, assuming R5 is in fact 1.0 K, the worst case load on the alarm output is 3.3 mA. An alternate output has Q1 and Q2 in series to form a 2-input NAND gate beeper driver. This presents less of a load on the monitor output.

    ak
    Alarm-Reset-2-c.gif
     
  10. ipanos

    Thread Starter Member

    May 7, 2010
    12
    0
    Thanks for that, I am making it tonight and coming back with my impressions!!

    PS: What about pins 14 and 7? Do I power it from 12V? Shall I add the diode across the piezo?
     
  11. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,530
    1,248
    A diode across the piezo would be safe, but it must be the kind with a transistor oscillator circuit built-in so it is not as likely to present a spike.

    C3 is the decoupling cap for the 4093. 14 and 7 are the power pins.

    ak
     
  12. ipanos

    Thread Starter Member

    May 7, 2010
    12
    0
    Am I also supposed to connect 7 to ground and 14 to 12VDC?
     
  13. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,530
    1,248
    Yes, just like on the datasheet.

    ak
     
  14. ipanos

    Thread Starter Member

    May 7, 2010
    12
    0
    I built this last night and tested it this morning. Here are some observations:

    I am having two issues (I am pretty sure I have no errors in my circuitry):

    1. The alarm output also gives a pulse every 10 seconds while the alarm is not activated. This is something I had not noticed before and causes a beep every 10 seconds even when the alarm is off. Any way around this?

    2. When the alarm activates, the output to the buzzer oscillates (once per second). This is excellent, but...
    When the mute switch is pressed, the alarm mutes for as long as the switch is held down. When the switch is released, the output will continue oscillating.

    Any thoughts on these?
     
  15. ipanos

    Thread Starter Member

    May 7, 2010
    12
    0
    Triple-checked the circuit just now, everything is as of your schematic.
    Something I should note, although it should not be an issue, I have substituted all three transistors with BC547 because I had them sitting around and added a 4004 diode at the output just in case I want to add a relay in the future.
    I wouldn't think any of those changes is interfering with the functionality of the circuit, but thought I'd mention them.
    When idle the circuit draws 2.5mA.
     
  16. ipanos

    Thread Starter Member

    May 7, 2010
    12
    0
    Anybody cares to contribute a little more?
    I've built this and almost works...
     
Loading...