This article is a practical work that requires less effort which you can try by yourself. It is a simple LED dimmer circuit using single Bipolar Junction Transistor (BJT). This is an interesting article especially for beginners in electronics which don’t have much idea on electronics or using transistors. Remember, transistor can be used as a switch or an amplifier.
We use the transistor here as a switch but in a special way that is, with a variable output. However, to better understand how transistor switches read Transistor acting as a switch. Also, you may want to know how transistor amplifies, read Transistor acting as an amplifier.
The two circuits are the same thing; the first uses an NPN Bipolar Junction Transistor (BJT) while the second circuit uses the PNP Bipolar Junction Transistor (BJT) type.
The circuit is used to vary the brightness of a white Light Emitting Diode (LED) or a Laser Light Emitting Diode. A potential difference (PD) is applied across the circuit via +V and 0V. The variable resistor (VR) is used in the circuit as a variable voltage divider. The wiper that is, the second (middle) terminal of the variable resistor is connected to the base of the transistor as shown in the circuit.
The region of the transistor either saturation or cut off is determined by the resistance level pointed by the wiper so that the current entering to the base of the transistor is made variable. The variation is between the saturation region and cutoff region of the BJT. The R1 which is in series to the LED is to limit the current that will pass through the LED.
Simple LED dimmer circuit using NPN BJT and PNP BJT