I'm asking for help after spending a couple of hours trying to learn the proper vocabulary. Bottom line is a KISS 24v dc powered NO switch triggered closed by a 3 wire proximity switch. Or do I need a four wire? I don't think PNP or NPN matters for this application.
I'm setting up a Variable Frequency Drive for a small engine lathe. The stop circuit is closed fail-safe. I have a simple series circuit through two emergency stop NC SPST switches. I want to add a proximity switch so the circuit is open unless the chuck key is in the holder.
I know how to use a SPST NO micro switch, but that brings on other design challenges I'd rather avoid. I have 24v DC available to power a relay of "some type." But I have no idea what to look for. As KISS as possible, trying to avoid having to create some kind of an Arduino board etc.
TIA. I volunteer teach bicycle maintenance at a local co-op, was trained in the late 70's as an aircraft mechanic. I generally don't have much trouble with simple circuits. But I don't think I'll ever be much good at electronics.
Ron
Best Answer
The big question is what exactly it means for the chuck key to be in the holder, and how that is sensed. Once you have a electrical signal of some sort, it should be a simple circuit to use it to drive a relay.
A nomally-open (NO) relay sounds like the right thing here. The relay has to be energized as a result of the sensor indicating the chuck key is in the holder for the unit to turn on.
The circuit could be as simple as a low side NPN transistor, emitter to ground, series base resistor, and relay between collector and the power supply. 24 V is a common relay voltage, so finding a relay won't be a problem. Make sure to add a diode across the relay coil in reverse polarity to give the inductive current a place to go when the transistor shuts off.
The above works when the detector signal is high when you want the device enabled. If the other way around, then a high side PNP switch works. It's basically a mirror image of the low side NPN switch circuit.
We can get into details once you have some real specs on the detector.