Relay Latching Circuit

Relay Latching Circuit

Let say you have a circuit assembled like below. When you press the push button, the buzzer turns on and when you released it, the buzzer turns off ( spring return type push button switch ).

Now what if you want the buzzer to stay on after you released the push button switch ? Change the push button switch type ? Yeah, that’s one way you can do it.

Another way is to add a relay. The circuit is known as a latching circuit. But you also have to add another switch to reset the circuit. Study below diagram, this is very-very useful especially if you want to apply it as a bugler alarm circuit. Anyway, I hope you can read and understand a ladder diagram. It is easier to understand compare to schematic.

PBNO : PUSH BUTTON NORMALLY OPENED

PBNC : PUSH BUTTON NORMALLY CLOSED

R : RELAY

RNO1 : RELAY CONTACT NORMALLY OPENED 1

RNO2 : RELAY CONTACT NORMALLY OPENED 2

There is a set (or more than one set ) of contacts in relay. One set of contact consist of C (common), NO(Normally opened) and NC(Normally closed). Above circuit using relay with 2 sets of contact( NO1 and NO2). Can we maintain the same function in above circuit using relay with only one set of contact ? Think about it, the answer is, yes we can……

Sample application like I already mentioned is in bugler alarm. Refer to above circuit, PBNO is the one that you will place at the door or window. Normally I will use a roller or lever microswitch. The rest of the circuit or the 'control box' you isolate and place it far away from the window, at a hidden place better.

What happen is when the thief force open the window, the buzzer will turn on. Immediately the reaction is he will close back the window hoping that the buzzer will turn off, but no way, the relay already turns on, the buzzer keep on making noise.  So he quickly open the window and search for the sensor or switch, found it, he quickly smash the microswitch or cut the wire. Unfortunately for the thief the buzzer keep on giving alarm because the only way to turn it of is by pressing the PBNC switch at the control box.  Hopefully when the thief realized that finding and shutting off the alarm is time consuming, he will abort his mission.

Another useful application is as a safety feature at any machines.  You can maintain the buzzer or make a direct connection to shut off the machine for example when someone opened the safety doors during operation.

Well, relay is indeed a very useful  device. You can actually wired up many relays together to make a complicated controller with input and output devices. But of course with technology that we have now,  if you want to build a controller level circuit, it is better and easier to use a PLC (programmable logic controller) or microcontrollers. But then you have to master another skill.....programming.