We can make Single Push Button ON OFF Relay Circuit with one push button to control motor or light.
Press button and relay will turn ON and then press again and relay will turn OFF; hence, there is no need of microcontroller or timer IC, so use only some transistors and one capacitor.
Circuit Working:
Parts List:
| Components | Values | Quantity |
|---|---|---|
| Resistors (All resistors are 1/4 watt unless specified) | 470k | 2 |
| 10k | 3 | |
| 100k | 1 | |
| 1k | 1 | |
| 910k | 1 | |
| 560Ω | 1 | |
| 10Ω 1W | 1 | |
| Capacitors | Electrolytic 1μF 50V | 1 |
| Semiconductors | Transistors BC547 | 2 |
| Transistor BC557 | 1 | |
| LED green 5mm 20mA | 1 | |
| Diode 1N4007 | 1 | |
| Push button switch | 1 | |
| Relay 12V | 1 |
Press ON/OFF push button and it turns into load like light or motor ON or OFF by relay, also the circuit can build on normal PCB; so use only through hole parts.
Use 12V relay because circuit need 12V DC; but we can use 3V to 12V relay which depends on our power supply.
Pressing the button causes resistor R1 and transistor Q1 to charge capacitor C1, next, capacitor C1 supplies base current to transistor Q3 through resistor R6, which turns on Q3.
When the collector of Q3 goes to ground, it activates the relay and turns ON the LED.
Then, if the user presses the button again while the circuit is on, transistor Q2 sends a positive voltage through resistor R8, which turns off both the LED and the relay.
Each button press changes the state with ON and then OFF and repeats; hence, this circuit is easy to build on dot board or normal PCB.
Formulas:
Basic Formulas for One Push Button ON/OFF Relay Circuit with Transistors:
1. Transistor Bias (Base Resistor):
Use this to find base resistor Rbase for BC547 or BC557:
Rbase = (Vin − Vbe) / Ib
where,
- Vin is the input voltage which is usually 12V
- Vbe is the base-emitter voltage to about 0.7V for silicon transistor
- Ib is the base current with small part of collector current
2. Capacitor Timing if needed:
For delay or debounce use this formula:
C = T / (1.1 × R)
where,
- T is the time in seconds
- R is the resistor value in ohms
3. Relay Driver Transistor (Base Current):
Transistor must give enough current to relay coil and then use this formula:
Ib = Irelay / hFE
where,
- Irelay is the relay coil current
- The hFE value represents the current gain of a transistor and is also known as β (beta).
4. LED Current Limiting Resistor:
To protect LED we need to calculate:
RLED = (Vsupply − VLED) / ILED
where,
- Vsupply is the power voltage which is usually 12V
- VLED is the LED forward voltage to around 2V for green LED
- ILED is the LED current with normally 20mA
Note:
Final values depend on relay coil current, switching needs and other parts, so check power supply and relay match voltage/current for our circuit.
Hence, use these formulas as starting point and always double check with real part datasheets and circuit needs.
How to Build:
To build a Single Push Button ON OFF Relay Circuit follow the below mentioned steps for connections:
- First, connect Q1 collector to positive through R2, then connect R1 and C1 to ground from Q1 collector, connect Q1 base to Q2 collector through R3 and then Q1 emitter goes to ground.
- Next, connect Q2 collector to ground through R4 and R5, connect Q2 base between R7 and R8 and Q2 emitter goes to positive.
- Then Q3 collector connect to positive through R7 and R8, Q3 base connect between R1 and C1 through push button and R6.
- Now connect LED1 D1 and R9 from positive to point between Q3 collector and R8.
- Lastly, connect D2 diode and R10 from Q3 collector and R8 to positive.
Relay coil:
- One pin goes between R10 and D2 and other pin between Q3 collector and R8.
Safety Tips:
- First, check all parts can handle the voltage and current and if circuit has high voltage then let trained electrician handle it.
- Then pick relay that matches our device current and control circuit voltage.
Conclusion:
To conclude, this Single Push Button ON OFF Relay Circuit is simple way to turn device ON and OFF, as it uses transistors for control, capacitor for stability and LED for visual signal.
Finally, easy to build with basic parts on normal PCB and is good for small low voltage projects.