Relay triggering circuit using SCR
Created: May 18, 2015
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Have you ever had a dilemma with your employees casually using the fire exit for short cuts? Then this simple circuit is your solution to that problem. It acts as a switch that causes the fire alarm to activate whenever a fire exit door is opened. Achieving that would be too easy to overcome by simply closing the door. Thus, the introduction to the usage of a Silicon Controlled Rectifier (SCR).
The second switch (S2) is a ball switch which is normally closed due to the pressure applied by the fire exit door. An open circuit will be produced if a fire exit door opens, triggering the SCR to switch the relay to it's on state. This would then complete the connection from the AC source and the buzzer. The only way to cease the alarm is through the first switch (S1). This would be switched off by using a key from an authorised personnel. The purpose of the diode is simply to avoid the current to flow back to the source. The circuit is powered by a single 12V or two 6V in series battery while the buzzer is powered by an AC source.
As you may have already observed, There are many possible ways to use this circuit design. One of which is to use it as a burglar alarm system wherein instead of placing it at the fire exit, it will be used at the main door or windows.
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