Control bulb with inductive proximity sensor
I am using an NPN NC inductive proximity sensor (specs attached) to control a light bulb. The bulb is normally on and goes off when metal is detected.
It is a 12v/21w bulb, powered by 12VDC. I have placed a heat sinked TIP120 NPN High Power Darlington Transistor 60V 5A TO-220 , a 2.2K 0.25W resistor and a 560R 0.25W resistor as shown in the circuit diagram on the right. Please ignore the left diagram for the LED.
voltage readings for sensor with no metal detected:
Between brown and black: 12.06/07V & -12.03/04V.
Between blue and black: 0.0033V & -0.0032V (3.3mV & -3.2mV)
with metal detected:
brown/black: -0.6/7mV & 0.7mV
Blue/Black: 11.96/97V & -11.93V
Although the sensor is powered and detects metal ok, the bulb stays on. The polarity of tIP120 is correct and gives out heat ok. I can't figure out what I'm doing wrong so could you please see an obvious error??