Or a Reset Push button can be added to this circuit, to break the Latch state. Now the alarm or device, connected to the Relay, will remain ON until the Power is reset. This in turns keeps the second transistor ON indefinitely and a Latch or Lock is formed instantly. So this Feedback voltage to the base of Transistor BC547, keeps the transistor BC547 ON for indefinite period, even after the input trigger voltage is removed. And when the Transistor BC557 turns ON, current flow in two directions, first to the relay and second to the base of transistor Q1. If you notice, collector of Transistor BC557 is connected to base of Transistor BC547, through a current limiting resistor R4. This is normal behaviour until now, but what makes it a ‘Latch’ circuit. Now when Base of Transistor BC557 gets grounded, it starts conducting and energises the Coil of relay, which activates the Relay and turns ON the device connected to Relay. Resistor R2 and R3 prevents the short circuit in this condition. Now when a small positive voltage is applied to the Base of Transistor BC547, it turns ON the transistor and Base of the transistor Q2 BC557 gets connected to the Ground. Capacitor C1 has been used precautionary, to prevent the accidental and false triggering of circuit. PNP Transistor BC557 base is connected to Positive voltage with current limiting resistor R3, so that it won’t conducts accidently. Initially, both the transistors are in OFF state, and Relay is deactivated. And Transistor BC557 is PNP transistor which Conducts or Turns On, when a negative voltage (or ground) is applied to its base. Purposes of other components have been explained in ‘Working section’ below.īefore going into the explanation, we should note that Transistor Q1 BC547 is a NPN transistor, which conducts or Turns On, when a small positive voltage is applied to its base. Current Limiting resistors must be used at the bases of BJT transistors, otherwise they might burn. Resistor R1 and R4 work as a current limiting resistor for Transistor Q1 and resistors R2 and R3 work as current limiting resistor for Transistor Q2. Resistors- 10k (2), 100k (2), 220 ohm (1)Ĭircuit Diagram of Latching circuit is simple and can be easily built.Today we are going build a very simple and cheap Latch circuit using Transistors, this circuit can be used trigger the AC mains loads and alarms. Fire Alarm System using AVR Microcontroller.We have previously built some alarm circuits: Latch circuit is similar like SCR (Silicon Controlled Rectifier) and can be very useful in alarm circuits, where a small trigger signal will Turn On the alarm for indefinite period, until reset manually. This State will remain indefinitely until the power is reset or some external signal is applied. In Electronics, Latch Circuit is a circuit which locks its output, when a momentarily input trigger signal is applied, and retains that state, even after the input signal is removed. Latch basically means “to become fixed in a particular state”.
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