This Set Reset Latch Circuit using Transistors tell us how to make an easy electric switch with two ON/OFF using two transistors.
Some special switch like SR latch is used in many devices for keep data safe even when power goes OFF and come ON again.
Transistors like BC547 and BC557 mostly used in this type circuit.
Transistors used in SR latch circuit with two push buttons to make memory part with two stable states called Set and Reset.
User press push buttons to change latch ON and OFF by hand.
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 |
During start nothing happens when circuit get power from 12V DC.
When Set push button is pressed BC547 base get low ground signal.
Then BC547 turn ON and current goes from 12V to collector through 4.7k resistor.
This current goes to BC557 base through another 4.7k resistor and BC557 also turn ON.
Relay connect to BC557 collector now turn ON and 100k resistor send feedback from BC557 collector to BC547 base.
This make latch stay ON until Reset button is pressed.
When Reset push button is pressed the BC547 base get ground again.
This turn OFF the BC547 and so the latch stops working and BC557 base current also stops.
Relay turn OFF so circuit now is in reset state.
To start again user must press set button one more time to turn latch ON again.
Formulas and Calculations:
Make a Set-Reset (SR) latch circuit using normal bistable multivibrator setup.
SR latch can store 1 bit data and it has two stable states which is Set and Reset.
It is made using two transistors connect to each other in cross coupled.
Resistors and other parts are picked with normal values and for practical use.
Base current and other things are calculated using this formula:
How to calculate base resistor:
Rbase = (Vsource − VBE) / Ibase
where:
- Vsource is power supply of 12V
- VBE is base emitter voltage drop which is around 0.7V for silicon transistor
- Ibase is base current needed to turn ON the transistor
Example:
If base current is 1mA:
Rbase = (12V − 0.7V) / 1mA = 11.3kΩ
So using 100kΩ resistor for base is okay which gives enough current to turn ON the transistor..
How to Build:
To build a Set Reset Latch Circuit using Transistors follow the below steps for connections:
- Put BC547 and BC557 transistors on the PCB board.
- From BC547 collector connect 4.7k resistor to positive power.
- Also connect BC557 collector to positive power using one more 4.7k resistor.
- Put 100k resistor between BC547 base and positive power.
- From same BC547 base connect another 100k resistor to push button.
- Connect other side of Set push button to ground.
- One side of Set button goes to BC547 base other side go to ground.
- One side of relay coil connect to BC557 collector.
- Other side of coil connect to positive power.
- BC557 emitter also goes to positive power.
- Connect 100k resistor from BC557 collector to BC547 base.
- Reset button connect to BC547 base.
- Other side of Reset button connects to ground.
- COM terminal of relay switch goes to positive power.
- NO (normally open) terminal goes to load like light or fan.
- NC (normally closed) terminal goes to ground.
- Connect power supply positive (+) to positive line.
- Connect power supply negative (–) to ground.
- Give 12V DC to circuit.
- Press Set button and relay should turn ON.
- Press Reset button and relay should turn OFF.
Conclusion:
This Set Reset Latch Circuit using Transistors and push buttons show how latch work.
Change parts if needed based on what circuit need.
Be sure all wires are tight and transistor pins are correct.
Before giving power always check connections and correct polarity to avoid damage.