System that depends on thermostat's state

Discussion in 'The Projects Forum' started by Bluerocket, Dec 14, 2013.

  1. Bluerocket

    Thread Starter New Member

    Dec 10, 2013
    Hello. First of all I want to say that I'm very happy to find this electronics international forum.

    I'm working hard on a circuit that depends on the thermostat's state (on/off). The thermostat gets in the off state when it's case reaches 160 degrees Celsius.

    When the thermostat is ON a green LED lights.
    When the thermostat is OFF the green LED get's OFF and a red LED get's ON. Among with this red LED there's an oscillator that I want to beep just for 5 seconds when the thermostat get's OFF (here is the big problem).

    I figured out a circuit (I tried to made it by myself), I attached it. Please tell me where is the problem. In the right part of the circuit there's the oscillator which depends on the relays state. The NC means normally closed and NO, normally open (relay is in normal state when it's not energised). The circle with the T is the thermostat. I hope you can understand the circuit.

    THANK YOU! :)

  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    The transistor on the right will never turn on, also the relays should be in the Collector side of the transistors, also where is the oscillator?

    why dont you use an op amp thermostat like this, and use a 555 timer oscillator?
  3. Bluerocket

    Thread Starter New Member

    Dec 10, 2013
    Yes, the transistor will turn on when the thermostat gets off so the NO will get closed and the capacitor will discharge from the base to the emittor.
    The relays are on the collector side, I don't understand what's the problem here?
    The oscillator is one the right side of the page, the side where it is not clear. The oscillator is switched by the relay on the right. It is already made and works great. Yes, I used the 555 timer. The oscillator is not a problem.

    You don't understand. If I use the op amp circuit then when the thermistor has a certain heat the oscillator will beep continuosly. Annoying, right? I need it to beep just for 5 seconds, a warning, that's all.
  4. shortbus

    AAC Fanatic!

    Sep 30, 2009
    You need a second 555 set up in the monostable mode to give you your 5 second time. The 555 you used to make the "beep" tone is in the astable mode. The monostable will give a time period and the astable the tone.
  5. Bluerocket

    Thread Starter New Member

    Dec 10, 2013
    Yes, I know that something can be realised triggering a 555 timer in monostable mode.
    But here's the big problem. The trigger will recieve a continous current (not a short pulse) and the timer will be triggered again,again and again . I don't need that. If there's a way to make the timer to consider just the first trigger pulse and ignore the others then it would be great.
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    Feed the trigger input via a capacitor. The input then only sees a pulse; not a continuous voltage.