I have a project currently that I need some assistance with. The project to sum it up is a front panel with 18 switches that control different loads. When the selection switch is in manual mode it provides 120VAC to all 17 switches then when any switch is turned on the load(mostly small motors) is supplied 120V for that circuit. Now when the selection switch is in Auto mode it disconnects 120VAC from the 17 switches and instead supply's it to one leg of the relay output. Using the relays the CPU board is able to then control the loads with a 5V source.

The problem is the way it's currently connected if in Auto mode and two load switches are turned on one being a switch that isn't supposed to be on that load will then be supplied 120VAC.

In my example circuit provided the main 120V switch is in Auto mode. When VR1 is energized node T17 is backfed 120VAC if someone forgot to open sw1 and sw2 now the load on t44 is running through the voltage backfeed.
I am looking for some suggestions to fix this issue as I assume it's a problem with switch placement that I'm not seeing. I don't really want to add 17 relays to isolate each switch.

 

Is there a way to use two position DPDT switches? Have one pole act as your normal switch. Connect the second poles of each switch in series in a way that auto mode can only be enabled if all the switches are turned off?

Can you reprogram the CPU and if so is there a way to connect the switches to the CPU instead of to 120V? This is a problem that could easily be solved in program.

 

I don't really want to add 17 relays to isolate each switch.

This arrangement will work using DPDT relays.
Any load activated by a relay would disconnect the feedback through the manual switch.

 

Is there a way to use two position DPDT switches? Have one pole act as your normal switch. Connect the second poles of each switch in series in a way that auto mode can only be enabled if all the switches are turned off?

Can you reprogram the CPU and if so is there a way to connect the switches to the CPU instead of to 120V? This is a problem that could easily be solved in program.

I like the DPDT switch idea easier than my extra relays. As for software fix that was my thoughts as well. If it wasn't easily solved on the hardware side that was my next move. It will be tricky, the project currently uses 48 IO ports as it is. Like you said it should be easy to run a check to see if the switches are all closed and only let the operator use the touch screen to run the loads in auto mode when they are. I suppose I would just need one or two IO ports to see if the disconnected buss is high at any time,

 

This arrangement will work using DPDT relays.
Any load activated by a relay would disconnect the feedback through the manual switch.
View attachment 299736

I am going to dig into this solution tomorrow this seems like the best idea just replace there SPST relays with the DPDT in that configuration. Thanks!

 

Here's another representation of sghioto's schematic.


Nandu.

 

Here's another representation of sghioto's schematic.

View attachment 299758
Nandu.

Problem was solved with adding another G4IAC5 to sense when the manual bus goes high and is not supposed to. The CPU then pauses the program and alerts the user a switch is left on.
Thanks everyone!