Using two 100 ohm 1/4W resistors in parallel would be another approach. Power through the that resistor is P = 60mA * 3V = 180mW so a 1/4W resistor will work, though a 1/2W would be a better choice for long-term use. The example assumes an LED voltage drop of 2V, so R = (5V – Vled) / 60mA = 50 ohms. This value may vary a bit depending on what LEDs are being used. The 50 ohm resistor sets the overall current for the 3 LEDs. The 4.7K resistor on the base is there to limit the current out of the uC pin to safe levels and the value is not overly critical. This simple circuit allows you to drive all three LEDs off the same uC pin. You could use 3 pins on the uC to drive the 3 LEDs, but that is a waste of pins unless you have pins to spare. The indicator consists of 3 LEDs which each draw 20mA of current.Ī uC pin can easily drive an LED at 20mA, but this circuit will require 60mA which is outside the range of the typical uC drive capability. We have an LED indicator that we want to drive. NPN transistors are generally used in low-side switching applications where they are connected between the load and ground. These can be very handy when you need to boost the output of a uC pin to drive something that it can not drive directly. The 2N3904 are a good inexpensive general purpose NPN transistor for low power amplification and switching and should be part of every parts bin. Up to 40V collector-emitter and 60V collector-base.KEY FEATURES OF 2N3904 NPN GENERAL PURPOSE TRANSISTOR: Qty 10 – 2N3904 NPN General Purpose Transistors.Darlington Pairs - It can be used in Darlington Pairs.The 2N3904 are a good general purpose NPN transistor for low power amplification and switching.Sensor Circuits - It can be used in sensor circuits.Home Appliances -It is suitable for home appliances e.g television.Fast Switching - It can be used as fast switching.Automatic Switching - It can be used where automatic switching is required.Amplifier Modules - It can be used as amplifier modules such as Audio amplifiers, signal amplifiers etc.Load Switch - It can be used as a minimum load switch having less saturation voltage and maximum gain.When it is acting as an amplifier, the DC gain of the transistor can be calculated by the following formula ĭC Current Gain = Collector Current (IC) / Base Current (IB) The configurations used in the amplifier circuit are It acts as amplifier while operating in an active region. The transistor can amplify the power, current and voltage at different configurations. This is why a resistor is connected in series with the base pin. The current at the base pin should me 5mA maximum more than it will destroy the transistor. When we provide the required amount of current at the base pin, the transistor will be an open switch when it is forward biased and it will e a closed switch when it is reverse biased. When a transistor is working as a switch, it works on two modes saturation region and cut off the region as explained earlier. Operating Temperature (To) -55 to 155 C.Maximum Collector-Base Voltage (Vcb) 60V.
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