RELAY question for Christmas lights using 2n3904 NPN

Thread Starter


Joined Nov 30, 2012
Hey guys,

so I have this "" schematic from a DIY project and SChematic1 represents that. but I was wondering...

Exactly what is biasing this transistor? (im pretty sure its Emitter Biased)

And Should there be a 1k transistor in the Area of Question?

** I didn't anticipate my computer would take a revers picture of it. but you get the idea. 10k on the base, led with 330 on it. im wondering about the circle. should there be a resistor on it and why?

Edit: I attached another picture. will this work for my relay board? the female socket is going to be old dead Christmas lights. I didn't quite understand why all the loads would be connected. does this make the difference of potential needed


Last edited:


Joined Jul 18, 2013
Looks OK to me, just that they are showing the cathode of the LED connected to the same point as the emitter of the transistor, maybe makes it misleading?
But they are sharing the same P.S. common point.


Joined Jun 26, 2012
The catch diode across the relay coil is backwards but the rest seems OK. As Max says, the way its drawn is kind of non-standard so takes a closer look to see what's going on.