How to Make a Soft Latch Circuit

A latch circuit can "hold" a circuit in an on or off state until any external signal is applied to it. A latch circuit holds its position (on or off) even after the input signal is removed and can store one bit of information as long as the device is powered. For an active high signal, it stores a one, and for an active low signal, it stores a zero.

In this project, we will make a soft latch circuit to turn an electronic device on and off by pressing a button. This circuit is called a soft latch switch. A soft latch circuit is different from a normal latch circuit, in a soft latch, the on and off state can be changed using external means (buttons), but in a normal latch circuit, the circuit can be latched to only one state, and to change the state requires removal of power. Usually shift registers and flip-flops are used in latch circuits, like we used in the Clap-on-Clap-off circuit.

Latch can be compared with a Push-on-Push-off button where the button connects the circuit when pressed once and disconnects the circuit when pressed again. Here we will build a soft latching power switch using BC547 NPN transistor and BC557 PNP transistor with a normal push button. This soft latching circuit does not require any microcontroller or any IC to turn it on and off.

Required Components

Transistors: BC547 (2), BC557

Resistors: 1MΩ, 470KΩ, 220KΩ (2), 100KΩ (2), 10KΩ, 1KΩ, 330Ω

Button

1µF capacitor

Leading

Breadboard

Circuit Schematic

Given above is the circuit diagram of a soft latching power switch circuit. It can be easily constructed on a breadboard or PCB. The components used in this circuit are easily available and very cheap. The resistor is used as a current limiting resistor while the capacitor is used to prevent false triggering of the circuit.

Operation of Soft Latch Switch Circuit

The transistor BC547 is an NPN transistor, while the BC557 is a PNP transistor. The BC547 transistor is turned on by applying a positive voltage to its base; on the other hand, the BC557 can be turned on by applying a negative voltage to its base.

When we first press the button to apply the supply voltage, all three transistors are off and the output voltage is zero; therefore, the circuit remains in the closed or unlocked state. In this case, the capacitor, C1, charges through the R1 and R2 resistors. When we press the push button switch, it causes the capacitor C1 to pass its voltage through the R6 resistor to the base of the transistor Q3. This turns on the Q3 transistor, which in turn turns on the Q2 transistor. When the button is released, the voltage developed across the R4 resistor keeps Q2 in the on state. Q1 is also on during this time, and the circuit is now in the on or locked state, and remains in this state even if S1 is open.

In this state transistor, Q1 is now saturated causing C1 to discharge through R2. When we press the push button switch again, capacitor C1 is in a discharged state and passes zero voltage to transistor Q3 causing the transistor to turn off. As a result, all three transistors are in the off state and the circuit again returns to its off or unlocked state. Since Q1 is now off, capacitor C1 starts charging again through the R1 and R2 resistors. Therefore, each press of the switch follows the same procedure to open and close the circuit.

The capacitor is used to limit the speed of the latching process. Without the capacitor the circuit would turn on and off in a rapid manner. The values ​​of the resistor and capacitor can vary depending on the application.

I built this soft latching switch circuit on a breadboard and a perfboard. After making the connections on the perfboard, my hardware looked like this:

Application of soft latch circuit

The soft latch circuit is ideal for battery powered portable instruments because it has zero voltage in the off state.

The soft latch circuit can be used to automatically shut down an ESP32, ESP8266, Arduino, or any other microcontroller.

Latch circuits are very useful in alarm circuits.