You may use a simple single push button ON/OFF relay circuit to operate a single push button device, such as a motor or light.
Pressing the button causes the relay to flip between its on and off states.
The condition can be reversed once again by pressing the button.
Without the use of microcontrollers or timer integrated circuits, we can construct a simple single push button ON/OFF relay circuit using a few transistors and a capacitor.
Circuit Working:
Parts List:
| Component Type | Description | Quantity | Notes |
|---|---|---|---|
| Resistors | All 1/4 watt unless specified | ||
| 470k | 2 | ||
| 10k | 3 | ||
| 100k | 1 | ||
| 1k | 1 | ||
| 910k | 1 | ||
| 560Ω | 1 | ||
| 10Ω 1W | 1 | 1 watt | |
| Capacitors | |||
| Electrolytic | 1μF 50V | 1 | |
| Semiconductors | |||
| Transistors | |||
| BC547 | 2 | ||
| BC557 | 1 | ||
| LED | green 5mm 20mA | 1 | |
| Diode | 1N4007 | 1 | |
| Miscellaneous | |||
| Push button switch | 1 | ||
| Relay | 12V | 1 |
In this circuit press the ON/OFF push button, which is designed to switch the electric load that is linked to the relay ON and OFF.
This circuit may be built using a standard PCB.
This circuit uses through hole components only.
We chose a 12V relay since this circuit requires a 12V DC power source to work, however you may select a relay based on the operating voltage you apply to the circuit (which ranges from 3V to 12V DC).
When the push button is pressed, capacitor C1 releases the voltage that has passed via resistor R1 and transistor Q1 which are linked between R1 and C1 components.
The voltage stored in C1 travels to the Q3 transistor base through R6 when the push button is pressed turning on Q3.
LED and Relay When Q3s collector terminal is connected, ground supply is obtained and turned on.
When the push button is depressed when the device is in the ON state, the Q2 transistor permits positive voltage to flow via R8 to the LED and relay, turning them off.
With each press of the push button switch, this process is repeated.
This circuit is simple to put together on a dot board or standard PCB board.
Formulas:
The following are fundamental formulas used to create a single push button ON OFF relay circuit with transistors and additional parts:
Bias in Transistors:
Utilizing the following formula, get the base resistor Rbase for the transistors BC547 and BC557:
Rbase = Vin − Vbe / Ib
where,
- The input voltage Vin is usually 12V.
- The base emitter voltage of a transistor is represented as Vbe, for silicon transistors, this voltage is around 0.7V.
- The base current needed to saturate the transistor is denoted by Ib, which is usually a small portion of the collector current.
If applicable, capacitor timing:
For any timing circuit (such as a delay or debounce circuit), get the timing capacitor C value using:
C = T / 1.1 × R
where,
- T is the desired timing interval in seconds.
- R is the resistor value in ohms.
Calculating a Relay Driver:
Make that the transistors BC547 and BC557 have enough power to provide the relay coils needed current.
Determine the necessary base current Ib for saturation by applying the following formula:
Ib = Irelay / hFE
where,
- Relay coil current is represented by Irelay.
- The transistors current gain is expressed as hFE (or β).
Current Limiting Resistor for LEDs:
To restrict the current, calculate the resistor RLED for the green LED.
RLED = Vsupply − VLED / ILED
where,
- The supply voltage is denoted as Vsupply 12V.
- The forward voltage drop of an LED is represented by VLED, which is usually around 2V for a green LED.
- ILED is the intended LED current, which is usually 20mA in LED terms.
Note:
The precise values of the relay coil current and desired switching characteristics, as well as the particular needs of your circuit, will determine the resistor, capacitor and transistor combinations.
Make that all parts, particularly the power supply and relay are rated for the voltage and current levels involved.
These equations offer a place to start when creating the circuit for a single push-button ON/OFF relay.
Specific circuit specifications and component datasheets should be the basis for all computations and adjustments.
How to Build:
To build a Simple Single Push Button ON OFF Relay Circuit follow the below mentioned steps for connections:
- Assemble all the components as mentioned in the diagram above.
- Connect collector of transistor Q1 to positive supply through resistor R2, connect resistor R1 and capacitor C1 to ground through collector of transistor Q1.
- Connect base of transistor Q1 to collector of transistor Q2 through resistor R3, connect emitter of transistor Q1 to ground.
- Connect collector of transistor Q2 to ground through resistor R4 and R5, connect base of transistor Q2 between resistor R7 and R8, connect emitter of transistor Q2 to positive supply.
- Connect collector of transistor Q3 to positive supply through resistor R7 and R8, connect base of transistor Q3 between resistor R1 and capacitor C1 through push button and resistor R6.
- Connect D1 LED1 and resistor R9 from positive supply to between collector of transistor Q3 and resistor R8.
- Connect between collector of transistor Q3 to resistor R8 to positive supply a diode D2 and resistor R10.
- Connect 12V relays coil pins one pin between resistor R10 and D2 and second pin between collector of transistor Q3 and resistor R8.
Safety Notes:
Verify that the circuit is made to handle the voltage and current you want to use.
Leave the high voltage wiring to a licensed electrician specially if you are not experienced in electronics and only manage the circuits low voltage control side.
Select a relay whose contacts can manage the current of your device and whose coil voltage is compatible with the power source of your control circuit.
Conclusion:
In conclusion, this one button circuit provides an easy to use method of turning an electrical item on and off.
Transistors are used in the circuit for control a capacitor for stability and an optional LED for visual cues.
Constructed using easily assembled parts on a common PCB, this circuit offers a flexible solution for a range of low voltage uses.