In this blog, we’ll explore how to make a simple OR gate using two BC547 NPN transistors in Tinkercad. This basic digital logic circuit will help you understand how current flows through transistors to control the output — in this case, lighting up an LED.
Components Used
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2 × BC547 NPN transistors
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1 × LED
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1 × 330Ω resistor (for LED)
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2 × 10kΩ resistors (for transistor bases)
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Power supply (3.7V to 5V battery)
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Wires and breadboard (in Tinkercad)
Circuit Connections
1.Power Supply and LED:
• Connect one end of the 330Ω resistor to the positive terminal (Vcc) of the battery.
• Connect the other end of the resistor to the anode of the LED.
• Connect the cathode of the LED to the collectors of both transistors joine together.
2. Transistor Emitters:
• Connect the emitters of both BC547. transistors directly to the negative terminal (ground) of the battery.
3. Base Connections:
• Connect the base of the first transistor to one end of a 10kΩ resistor, and the other end of the resistor to input A (which can be a push button or jumper wire to Vcc).
• Similarly, connect the base of the second transistor to another 10kΩ resistor, then to input B.
How the Circuit Works: Practical Explanation
° This OR gate circuit allows the LED to glow when either input A or input B (or both) is connected to the positive voltage (HIGH).
° When you press input A, current flows through the 10kΩ resistor into the base of the first transistor, turning it ON. This allows current to flow from the LED, through the transistor’s collector-emitter path, to ground, lighting the LED.
° If input A is OFF but input B is ON, the second transistor switches ON and completes the current path, lighting the LED.
° If both inputs are OFF, no base current flows, both transistors are OFF, and the LED stays OFF.
Why Does Current Flow Through One Transistor or the Other?
Think of the LED and resistors as part of a road that connects to two separate gates (the two transistors). When either gate is open (transistor ON), the current chooses that path to complete the circuit.
If transistor 1 is OFF (gate closed), current cannot flow through it.
But if transistor 2 is ON (gate open), current flows through transistor 2 and lights the LED.
This means the LED will glow if any one or both of the transistors are ON — just like how the OR logic gate works.
Summary
Both transistors share the same LED load connected to Vcc.
Emitters are grounded.
Bases are inputs through resistors.
Current chooses the transistor path that is ON to complete the circuit and light the LED.
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