Electronic – Simplest LED-transistor switch LED being on in switch’s off-state


I'm very, very new to EE, so this might be a really basic question. I am trying to make a very basic LED-switch using a bipolar NPN-transistor. The circuit I use can be seen below.


simulate this circuit – Schematic created using CircuitLab

Now, when I prepare this circuit on the breadboard, the LED will dimly emit some light even when the switch is open. When the switch is closed, the LED turns fully on, as desired.

Does anyone know why the LED is on, even though the switch is open?

Best Answer

The QED 123 is a infrared LED. Light is not visible. The 1k is to high. Calculate the correct value. The QED 123 = 1.7V 100mA says the datasheet. So 1.3V (3V-1.7V) over the resistor (when the transistor is fully open). The (If) 100mA 1.3/0.1= 13 ohm. 15ohm is ok. The switch circuit should work. But best to connect the base to ground with a resistor if the transitor opens slightly a wierd way when the switch is open.