How to Build a Simple LED Driver Circuit
In this project, we will show how to build a simple LED driver circuit.
A LED driver circuit is a circuit which can power an LED.
We will simply use a transistor and a few resistors to bring about a circuit which, with minimal current, can produce enough current to drive and light an LED.
The point of this circuit is to demonstrate that if you do not have sufficient current powerful enough to drive an LED, you can connect a transistor to the circuit so that it provides enough current amplification to drive it.
This circuit exploits the fact that transistors can function as current amplifiers in circuits. We use the transistor to supply the amplification necessary so that the LED can be driven.
The parts we need to run this circuit are:
- NPN 2N2222 transistor (or similar)
- SPST Switch
- 4 AA Batteries or DC power supply
- 330Ω Resistor
- 30KΩ Resistor
For the transistor, we simply use an NPN transistor. This will be sufficient for providing amplification.
To power the base of the LED and the collector of the transistor, we provide 6 volts of power. This voltage is necessary for the transistor to work. NPN transistors always need
positive voltage applied to the collector in order to work. This is called reverse bias voltage so that the current in the transistor can flow from emitter to collector. This 6 volts can come
either from 4 AA batteries connected in series or from a DC power supply.
LED Driver Circuit
The schematic of the LED driver circuit we will build is shown below:
When the switch of the circuit is open, no current flows to the base of the transistor. Therefore, the transistor cannot turn on. Bipolar junction transistors need base current and sufficient positive voltage to the collector in order to provide amplification. So with the current open, the transistor nor the LED does not turn on. When the switch is closed, base current flows through the transistor, turning it on. The transistor now provides amplification to the current, turning the LED on.
If the LED was just connected straight to the 10KΩ resistor, without any transistor, it would not produce sufficient current to drive the LED. The current through the LED would be I=V/R= 6V/30KΩ= 0.2mA. This is not sufficient to drive an LED with any type of real brightness. With the NPN transistor in the circuit, amplification usually to the degree of about 100 to 150. Therfore, the current will be amplified by 100, which is 0.2mA * 100= 20mA. 20mA is now sufficient to drive and light the LED.
So you see how the transistor allows a circuit to be an LED driver. Without it, there wouldn't be enough power to drive it.
The point of this circuit is that if you don't have too little current in a circuit, you can create a driving, or amplifying, so that you can have sufficient current. LEDs are
current-driven devices. Therefore, an LED driver circuit is one that supplies sufficient current to drive them. This can be done with a simple transistor, as shown above.