The article explains how to build a tiny electrical switch with two ON/OFF settings using transistors.
Specially made switches called SR latches are used in several kinds of devices to preserve data even when the power is switched off and back on.
Two common transistors used in this kind of circuit are the BC547 and the BC557.
What is a Set Reset Latch Circuit:
Transistors are used in a transistor Set Reset SR latch circuit with two pushbuttons to create a memory element with two stable states often known as Set and Reset.
The latches ON and OFF modes are manually switched by the user because of to the push buttons.
Circuit Working:
Parts List:
| Component Type | Component Details | Quantity |
|---|---|---|
| Resistors | 4.7k 1/4 W CFR | 2 |
| 100k 1/4 W CFR | 2 | |
| Semiconductors | Transistor BC547 | 1 |
| Transistor BC557 | 1 | |
| Diode 1N4148 | 1 | |
| Relay 12V | 1 | |
| Push Buttons | 2 |
In the beginning nothing happens when the circuit is powered by a 12V DC source.
The BC547 base receives a low ground signal when the Set push button connected to it is pressed.
When BC547 is turned on current can go from the positive 12V to the collector through its 4.7k collector resistor.
This collector current activates the BC557 by passing through its base and another 4.7k resistor.
When the relay connected to BC557s collector is turned on a feedback loop is created between BC557s collector and BC547s base through a 100k resistor.
This is the latching state of the circuit until the reset button is pressed.
The BC547 base receives a low ground signal when the Reset push button attached to it is pressed.
By doing this BC547 is turned off the latch fails and BC557s base current is shut off.
The circuit is in the reset state when the relay deactivates.
The cycle must be restarted by pressing the set push button one more to set the latch.
Formulas and Calculations:
Make a Set-Reset (SR) latch circuit using a standard bistable multivibrator configuration.
The SR latch which stores a single bit of data and has two stable states Set and Reset is constructed from two cross coupled transistors.
Resistors and other components are chosen based on practical considerations and usual values while the base current and other parameters are calculated using the following formula:
Calculating the Base Resistor:
Rbase = Vsource − VBE / Ibase
where,
- The supply voltage is represents by Vsource (12V).
- The base emitter voltage drop VBE is measured in volts which is 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 required:
Rbase = 12V − 0.7V / 1mA = 11.3kΩ
Using a 100kΩ resistor to give enough base drive seems logical.
How to Build:
To build a Simple Set Reset Latch Circuit using Transistors follow the below steps for connections:
Place Transistors:
- Place the transistors BC547 and BC557 onto the printed circuit board.
Connect Collector Resistors:
- From the BC547s collector connect a 4.7k resistor to the positive supply.
- Connect the BC557s collector to the positive supply via an additional 4.7k resistor.
Connect Base Resistors:
- A 100k resistor should be connected between the BC547s base and the positive supply.
- From the BC547s base connect a 100k resistor to the desired push button.
- Attach the set push button to the ground on the opposite side.
Connect Set Push Button:
- Connect the Set push buttons opposite side to the power source.
Connect Relay:
- On one side of the relay coil connect the BC557s collector.
- Connect the positive supply to the relay coils opposite side.
- Join the BC557 emitter to the positive power source.
Create Feedback Loop:
- From BC557s collector to BC547s base connect a 100k resistor.
Connect Reset Push Button:
- Connect the reset push button to the base of the BC547.
- Reset the push button by connecting its opposite side to the ground.
Connect Relay Switch:
- The relay switches common COM terminal should be connected to the positive supply.
- The generally open NO terminal should be connected to the load such as a light or other gadget.
- Connect the ground to the generally closed NC terminal.
- The power supplies positive terminal should be connected to the positive supply.
- The power supplies negative terminal should be connected to ground.
- Supply the circuit with 12V DC electricity.
- To check if the relay activates and the latch sets press the set push button.
- Try pressing the reset push button to check whether the relay deactivates and the latch resets.
Conclusion:
Make sure the connections are tight and the transistor pinouts are set up correctly.
Transistors and push buttons are used in this simple set reset latch circuit to show the basics of an SR latch.
Depending on the needs and features of certain components adjustments can be required.
Before switching on power always make sure all connections and polarity are correct to avoid component damage.