# Why do you need a resistor for a PNP-sensor

Discussion in 'General Electronics Chat' started by Martijn Kortenhoeven, Apr 16, 2015.

1. ### Martijn Kortenhoeven Thread Starter New Member

Apr 16, 2015
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0
Currently my school project group and I are working on a robot. We received a pnp optical sensor. We've been told that we have to use a 120Ohm resistor between the ground and the sensor output signal. Well, the robot worked and right now we are writing our report. In it, we have to specify why the resistor was needed. Unfortunately we have no idea and are having trouble finding it on the internet. Does anyone know why we need a resistor between the ground and the sensor output? Thanks in advance!

2. ### ericgibbs Moderator

Jan 29, 2010
7,539
1,520
hi,
Look up Open Collector output.
Without knowing the sensor type its difficult to give a definite answer
E

3. ### DickCappels Moderator

Aug 21, 2008
5,395
1,673
It would be a great help to see your circuit, but here goes anyway.

I assume your phototransistor has two leads. One goes to the power supply and the other goes to the resistor, the other end of the resistor is grounded.

The voltage across the resistor is the current from the phototransistor times the value of the resistance to ground. If the resistance were infinite (not there) then the voltage at the phototransistor output would drift toward the power supply.

4. ### Martijn Kortenhoeven Thread Starter New Member

Apr 16, 2015
3
0
Here is the circuit we made. It is put into a MyRIO module from National Instruments. On the left of the module you can see the sensors.

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5. ### Martijn Kortenhoeven Thread Starter New Member

Apr 16, 2015
3
0
Problem is solved thanks! We've found out that the resistor is used to let the final load from the sensor output through to the ground. This is so the MyRIO module won't get a signal from the sensor anymore.

Jul 18, 2013
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These components generally have open collector output so require a load pull up/pull-down resistor depending on NPN/PNP.
Max.

7. ### crutschow Expert

Mar 14, 2008
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The sensor controls the current through it. To get a signal voltage from that current it needs to go through a resistor.

Last edited: Apr 16, 2015

Jul 18, 2013
17,756
5,403
See three wire devices.
Max.

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