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I'm trying to build an AND gate that takes two inputs from standard 3 wire 24VDC PNP sensors and provides a 24VDC output.

I was thinking using some sort of diode resistor logic (link to circuit: http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/diodgate.html). However, I can't get this to work because when the sensors are off, the output of the sensor does not pull the inputs of the gate to ground.

What is the best / simplest way to to this?

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  • \$\begingroup\$ No pull to ground = RTL. \$\endgroup\$ – Ignacio Vazquez-Abrams Jun 29 '15 at 14:01
  • \$\begingroup\$ Can't you just add pulldowns? \$\endgroup\$ – Nick Johnson Jun 29 '15 at 14:03
  • \$\begingroup\$ I tried to simulate with a pull-down but it doesn't seem to work: <partsim.com/simulator/#33536> \$\endgroup\$ – vg425 Jun 29 '15 at 14:19
  • \$\begingroup\$ What kind of an output is required? PNP also? or NPN? \$\endgroup\$ – Spehro Pefhany Jun 29 '15 at 14:23
  • \$\begingroup\$ The output should be PNP. \$\endgroup\$ – vg425 Jun 29 '15 at 14:31
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One possibility, it could be that you could get away with simply using a very low pull down and a relatively high pullup, but the below will definitely work.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ One issue I have is I have a very small amount of room for the circuit so it would be nice to simply be able to add a pull down resistor. However, when I simulated this, it did not seem to work. \$\endgroup\$ – vg425 Jun 29 '15 at 15:17
  • \$\begingroup\$ Your sim used a high pull down compared to pullup. It has to be opposite, but it may not be reliable. \$\endgroup\$ – Spehro Pefhany Jun 29 '15 at 15:19

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