Mosfet switching

[mp90]

Goodmorning,


I'm trying to light up a led with an old light barrier I still have around. I want the led to light up when someone passed the barrier. Unfortunately I have a 24v signal when the barrier is safe and when someone walks through it, the voltage drops to 0.


I still have some mosfets around but I’m not sure how to tackle this problem. I know I could use relay’s but I do not really have space for one. I have some mosfets and resistors so that shouldn’t be much of a problem.


Long story short: I’d like to make a 24v circuit with mosfets. When there is no signal my led should light up.


Is there a simple way to do it?

[sanwal209]

If you have a mosfet with VGS above 24V you can apply that signal to its gate via series resistor and turn on LED.

Check the attached Image which shows the basic configuration of Mosfet switch

[alsetalokin4017]

But the requirement is that the LED light up when there is NO signal, that is, the voltage level from the barrier strip is zero. And I think we are to assume that no microprocessor is used or desired.

What then will power the LED? There has to be a power source for the LED that is separate from the barrier strip signal, that is, stays ON when the barrier strip signal goes low. Then a P-channel mosfet in a highside switch arrangement can be used.

[danadak]

Simple method, attached.

Vz must be chosen based on Vth of MOSFET and system tolerances.

If you don't like fact when LED is off power is being used, add another
MOSFET or bipolar to invert signal and place LED in drain lead.


Regards, Dana.

[radiogeek381]

Assuming you want to turn on an LED when the output from your sensor is low,
and that you only have a 24V supply,  Assume further that the sensor output
can sink 10mA or so:

What you need is not a MOSFET,  You need a resistor.

If the diode drops 2V then you need to take care of the remaining 24V.
Most LEDs are happy to shine with about 10mA of current.  So, we need
10mA to cause a 22V drop:

R = E/I = 22 / 0.01 = 2200 ohms. 

do this:

24V supply --- 2200 ohm resistor -- LED -- > output of sensor.