I am trying to find a way to manually override the control of some relays.
The netduino uses mux to control 24v which switches the coil voltage of some relays. I want to manually control the relays in case something goes wrong on my control board or with my software.
The manual switches will carry 24v from the same supply that switches the relays on my control boards. Things that I am unsure about include, if the switch is closed supplying 24v to the the relay coil, but the control board is not is closed, how can I ensure current from the switch doesn't travel backward in to the control board mux circuit?
I guess this would be an easy answer with something like diode right? Any help on which one and where would be much appreciated?
Finally, what would happen in the switch was close supplying 24v and the control board was supplying 24v to the relay? Would this be fine as it is the same source or could this create some form of over voltage/over current?
Could this be resolved with a transistor and if so how would I wire it in.
Basically the ciruit has a common ground and the 24v is common to both the ULN2802 which will be used to switch the relay coil and to the switch which will directly supply the same 24v?
There is one final consideration, the relays have a 2.5v voltage drop and a 30ma current drop. How would I handle this in terms of resistors and the 2 possible sources of current (ULN2803 or Switch)?
I would also like a status LED to show which relays are on again 2.5v voltage drop, 30ma current drop.