digital-logicdiodes
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?
This is because of the base current. The current comes from the signal source.
Also, note that the demonstrated schematic is simply wrong. The proper AND gate has to amplify the signal, so with similar technique you can only build NAND element. If you need AND - simply make one NAND and then invert the output signal once more.
simulate this circuit – Schematic created using CircuitLab
The problem is that your bit inputs must be at either a logical 1 (Vcc) or a logical zero, (GND) as they would be if they were connected to the outputs of, say, a counter like an HC191.
You've shown them floating when the switch is OFF, which won't work properly if you're using an R-2R ladder.
This works, and here are the LTspice files so you can play with the circuit if you want to.
Best Answer
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.
simulate this circuit – Schematic created using CircuitLab