Jan 282016
 

Alarm with thermostats

What is a thermostat?

There are two types of thermostats. Mechanical thermostats and digital thermostats. In our alarm with thermostats circuit we will use simple mechanical thermostats.

A thermostat is a device that is intended to control, a heating or a cooling system, such as an air conditioner, to maintain the desired temperature within certain ranges. A thermostat can turn on the heating system when the room temperature is too low or can also activate the cooling or air conditioning system when the temperature is very high.

How the alarm with thermostats works?

This simple alarm with thermostats circuit can be used on places where we must detect overheating. For example, a house, a warehouse, etc. The diagram shows an alarm that uses 3 thermostats, and a one, two (NO) contact, relay. Thermostats are normally open (NO) type and are connected in parallel.

Alarm with thermostats

This group of 3 thermostats are connected in series with one relay contact (1), which, when closed, locks the relay itself, memorizing the alarm signal. The second relay contact (2) connects the warning bell or alarm sound. Normally all contacts of the thermostats are open, the relay is not energized and there is not alarm sound.

When the temperature rises more than usual, one or more thermostats contact get closed, then the relay is energized and the alarm sound is on. Note that there is a switch (SW1) in parallel with thermostats, in order to test the circuit.

When the relay is energised, even momentarily, it locks itself up , the alarm sound is activated and remains on this state. Thus the alert signal is maintained. The circuit has a NC switch (SW2) in series with the contact (2) relay. When this button is pressed the relay is disabled, and the alarm returns to its standby state. This happend, when all contacts of the thermostats are open, otherwise it will continue ringing.

The entire alarm system is connected to a 12VDC battery or a power supply of the same voltage and has no power consumption on its stanby state

List of components for the alarm with thermostats circuit

  • 3 NO common mechanical thermostats (T1, T2, T3)
  • 1 NO switch (SW1)
  • 1 NC switch (SW2)
  • 1  12V dual (NO) contact relay (RL)
  • 1  12 volt siren or alarm bell (S)
  • 1  1K resistor (R)
  • 1 red LED (D)


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