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Control bulb with inductive proximity sensor Answered

Please see attached diagrams in my other message below.
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??
thank you



3 years ago

Forget the 560R. Just put a 1K resistor from Brown to Black and then to the transistor base. Maximum current in the sensor will be 12mA. Base current will be roughly the same, which should switch your transistor nicely.


3 years ago

All sensors I worked with have two wires for the power supply and one or two wires for the signal at the same level as the input voltage.
This would mean a 12V sensor is directly connected to 12V+ and ground with wtwo wires.
The output wire drives the transistor, depending on the required bas voltages with a resistor divider.


Reply 3 years ago

That's what I thought.
Brown is 12V positive for the sensor, Blue is negative for the sensor.
Between Brown and Black you have the 12V signal from the sensor.
You can explain those shematics for sensors in "noob terms" like this:
+ and - are always the supply for the sensor - check colors in shematics.
The remaining wire always provides the signal - for NPN between this wire and positive, for PNP between this wire and negative.
With every recognised signal the output state changes from the default state to the other and back when the signal is gone.


Reply 3 years ago

so is there something wrong in the way I have connected the components in the images below? I think everything is right.