Simple Set Reset Latch Circuit using Transistors

This post shows you how to build a tiny electronic switch with two settings on or off using transistors.

These special switches called SR latches, are used in many electronics to remember things even when you turn the power off and back on.

This one uses two common transistors a BC547 and a BC557.

What is a Set Reset Latch Circuit:

A transistor Set Reset SR latch circuit with two push buttons is a digital circuit that uses transistors to create a memory element with two stable states often referred to as “Set” and “Reset.”

The addition of push buttons allows a user to manually toggle the ON or OFF state of the latch.

Circuit Operation:

Parts List:

Powering Up:

When the circuit is powered with a 12V DC supply initially nothing happens.

Set Operation:

Pressing the Set push button connected to the base of BC547 provides a low ground signal to the base of BC547.

BC547 turns on, allowing current to flow from the positive 12V through its collector resistor 4.7k to the collector.

This collector current flows through the base of BC557 through another 4.7k resistor, turning it on.

The relay connected to the collector of BC557 is activated and the feedback loop is formed through the 100k resistor from the collector of BC557 to the base of BC547.

The circuit is latched in this state until the reset button is pressed.

Reset Operation:

Pressing the Reset push button connected to the base of BC547 provides a low ground signal to the base of BC547.

This turns off BC547 breaking the latch and cutting off the base current to BC557.

The relay deactivates and the circuit is in the reset state.

Latch Reset and Set Again:

To set the latch again the set push button must be pressed again, starting the cycle anew.

Formulas and Calculations:

Use a traditional bistable multivibrator setup to create a Set-Reset (SR) latch circuit.

Two cross coupled transistors are used to build the SR latch, which has two stable states Set and Reset and is used to store a single bit of data.

The following formula can be used to compute the base current and other parameters, while resistor and other component values are selected based on typical values and practical considerations:

Calculating the Base Resistor:

R_base​ = V_source​ − V_BE​​ / I_base​

where,

• The supply voltage is denoted by Vsource (12V).
• The base emitter voltage drop VBE, is measured in volts (around 0.7V for silicon transistors).
• The base current needed to turn on the transistor is called Ibase.

For example, if a 1mA base current is desired:

R_base​ = 12V − 0.7V​ / 1mA = 11.3kΩ

It is sensible to use the 100kΩ resistor to provide adequate base drive.

How to Build:

Building the set reset SR latch circuit described would involve necessary connections.

Place Transistors:

• Insert the BC547 and BC557 transistors onto the PCB.

Connect Collector Resistors:

• Connect a 4.7k resistor from the collector of BC547 to the positive supply.
• Connect another 4.7k resistor from the collector of BC557 to the positive supply.

Connect Base Resistors:

• Connect a 100k resistor from the base of BC547 to the positive supply.
• Connect a 100k resistor from the base of BC547 to the set push button.
• Connect the other side of the set push button to the ground.

Connect Set Push Button:

• Connect the other side of the Set push button to the positive supply.

Connect Relay:

• Connect the collector of BC557 to one side of the relay coil.
• Connect the other side of the relay coil to the positive supply.
• Connect the emitter of BC557 to the positive supply.

Create Feedback Loop:

• Connect a 100k resistor from the collector of BC557 to the base of BC547.

Connect Reset Push Button:

• Connect the base of BC547 to the reset push button.
• Connect the other side of the reset push button to the ground.

Connect Relay Switch:

• Connect the common COM terminal of the relay switch to the positive supply.
• Connect the normally open NO terminal to the load e.g. a light or another device.
• Connect the normally closed NC terminal to the ground.

Power Supply:

• Connect the positive terminal of the power supply to the positive supply.
• Connect the negative terminal of the power supply to the ground.

Testing:

• Apply power 12V DC to the circuit.
• Press the set push button to see if the latch sets and the relay activates.
• Press the reset push button to see if the latch resets and the relay deactivates.

Conclusion

Ensure that the transistor pinouts are correctly oriented and the connections are secure.

This simple set reset latch circuit provides a basic understanding of an SR latch using transistors and push buttons.

Adjustments might be necessary based on specific component characteristics and requirements.

Always double check connections and polarities before applying power to avoid damaging the components.

References

Set-Reset (electronics)