user53525: I want to control a 12 V relay using an Arduino 5 V signal, but due to some unknown reason the relay does not work. I am using a BC548 transistor as a switch for relay.
Please suggest where I go wrong.

user10656: If it's working with 12v on the input but not 5v, it sounds like the base resistor value 
 (which is missing from the diagram) might be too high. A value of around 1k should ensure the transistor is fully switched on.

53525: <p>I want to control a 12 V relay using an Arduino 5 V signal, but due to some unknown reason the relay does not work. I am using a BC548 transistor as a switch for relay.<a href="https://i.stack.imgur.com/PI5Pl.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/PI5Pl.jpg" alt="enter image description here"></a>
Please suggest where I go wrong.</p>
 10656: <p>If it's working with 12v on the input but not 5v, it sounds like the base resistor value 
 (which is missing from the diagram) might be too high. A value of around 1k should ensure the transistor is fully switched on.</p>
