Originally Posted by KeithB
Just to be clear, instead of grounding the in-cab SPST switch, I connect to the reverse circuit, right. So when switch is on, lights are on as long as the key is in ACC position or truck is running. When switch is off, I should get lights only when truck is in reverse. Would a SPDT switch be better?
up = lights on
neutral = lights off
down = lights on in reverse only
up = lights on
down = off except when in reverse
No, the spst switch will not work if you just hook the negative on the switch to a reverse circuit fuse tap.
The switch needs to be SPDT, both of the switches you described would be considered SPDT, but the first one is a 3 position (on-off-on) and the second one is a 2 position SPDT (on-on in rev).
The "negative" on an SPST switch is not for the circuit, it will only be present on the switch if there is an indicator light, and the negative is to complete the indicator light's circuit.
"Pole" refers to the number of separate circuits that a switch controls, "throw" refers to the number of positions the switch can have to complete the circuits.
A simple SPST switch, with no indicator lights would have 2 pins, one source (incoming + or -) and one load (outgoing, same polarity as source). In the "off" position, the first pin is disconnected from the second, turning the switch "on" connects the 2 pins' contacts. If it has an indicator light, it will have a 3rd pin, that gets grounded, and completes the circuit for the light when switch is in the on position.
A simple SPDT switch with no indicator lights, would have three pins. SPDT can have 2 or 3 positions. If it has to positions, both would be "on", so if pin 2 is your common pin, on a 2 position SPDT switch, then in position one, pin 1 would connect to pin 2 (p2 is disconnected from p3), and in position 2, pin 3 would connect to pin 2 (p2 is disconnected from p1).
If you have a 3 position SPDT, one of the positions would be off.
Position 1: P2 connects to P1
Position 2 (center/off): P2 is Disconnected from either P1 or P3.
Position 3: P2 is connected to P3.