This Set Reset Latch Circuit using Transistors tell us how to make an easy electric switch with two ON/OFF using two transistors.
Also, many devices are used in special switching circuits like an SR latch to keep data even when the power turns OFF and then ON again.
Furthermore, this type of circuit mostly uses transistors like BC547 and BC557.
In an SR latch circuit, two transistors and two push buttons create a memory unit with two stable states called Set and Reset; the user presses the push buttons to switch the latch ON and OFF manually.
Circuit Working:
Parts List:
| Components | Values | 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 |
First, nothing happens when the circuit receives 12V DC power; however, when we press the Set push button, the BC547 base receives a low ground signal.
Then BC547 turn ON and current goes from 12V to collector through 4.7k resistor and this current goes to BC557 base through another 4.7k resistor and BC557 also turn ON.
Also, relay connected to BC557 collector now turn ON and 100k resistor send feedback from BC557 collector to BC547 base.
Hence, this latch stays ON until we press the Reset button and when we press the Reset push button, it grounds the base of the BC547 again.
Then this turn OFF the BC547 and so the latch stops working and BC557 base current also stops.
After that, relay turn OFF so circuit now is in reset state and to start again user must press set button one more time to turn latch ON again.
Formulas and Calculations:
Use a normal bistable multivibrator setup to make a Set-Reset (SR) latch circuit; the SR latch stores 1 bit of data and has two stable states: Set and Reset.
Also, two transistors connect to each other in a cross-coupled configuration to form the circuit.
Choose resistors and other components with standard values for practical use and calculate the base current and other parameters 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 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:
- First, put BC547 and BC557 transistors on the PCB board.
- After that, from BC547 collector connect 4.7k resistor to positive power.
- Also connect BC557 collector to positive power using one more 4.7k resistor.
- Then put 100k resistor between BC547 base and positive power.
- From same BC547 base connect another 100k resistor to push button.
- After that, connect other side of set push button to ground.
- Join, one side of set button to BC547 base and other side go to ground.
- Also, one side of relay coil connect to BC557 collector and other side of coil connect to positive power.
- Next, BC557 emitter also goes to positive power.
- Then connect 100k resistor from BC557 collector to BC547 base
- After that, reset button connect to BC547 base and 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 and NC (normally closed) terminal goes to ground.
- Next, connect power supply positive (+) to positive line and connect power supply negative (–) to ground.
- Lastly, give 12V DC to circuit and press set button and relay should turn ON and also press reset button and relay should turn OFF.
Conclusion:
To conclude, this Set Reset Latch Circuit using Transistors and push buttons show how latch work.
Also, we can change parts if needed based on what circuit need, but be sure all wires are tight and transistor pins are correct.
Finally, before giving power always check connections and correct polarity to avoid damage.