I'm trying to drive a 6-28VDC siren from an Arduino Nano output pin. The latter provides 5V (max 50mA).
I've connected Arduino output to base of NPN through a 100k resistor, with emitter to ground and collector to the negative terminal of my sounder. Positive terminal of my sounder to 19.5V. (I'm using an old Dell charger to power the whole project, and a separate 5V regulator for the Arduino).
The NPN transistor is a BC548. I've checked the order of pins as follows: reading from left to right, with flat side of the transistor facing me: collector, base, emitter.
When I bring the Arduino output pin high, the siren sounds like a dead cat. The current through it is about 3mA. The siren does not pulsate.
When I connect the siren directly to the Dell charger, it pulsates very loudly, as it should.
What am I doing wrong here?