Basic premise: I have 7 free pins (A0-5 + D8) and am looking to run 4 or more relay circuits (110v AC, 10-30A) and 6-12 sensors. I also have some 74HC595's run up on a buss, and have one free pin on that buss for adding in another one. It would take one additional pin per additional 74HC595, and programming may become more complex.
I could run a Shift Register for my relay circuits, but that would limit the number of relays to either 4 and a mux chip for sensors, or all 8 and chewing up extra pins for muxing sensors.
I was thinking of using the shift register with 2 16 channel mux chips, one for relays and one for sensors. Problem with muxing is keeping multiple relays open concurrently. I wouldn't mind a latching relay, but I want the circuit to default off on a power fail condition.
I was thinking of using 3 relays, 2 NO and a NC, one of the NO being the load (VAC) relay. My idea is putting the 2 relays (NO & NC) in series feeding into the load Relay, with the current back-fed into the coil of the NO switch. Would this give me the ability of turning on the load from powering up the NO coil but keeping it on until I juice up the NC coil? See pic below, pretend the LED is my Load Relay.
Would it be possible to toss in a diode on each of the coils so I could turn it off/on from source/sinking current from a single pin? Could the same thing be done easier (cheaper) using a MOSFET and a JFET? Is there a relay type out there that does what I want and I just can't find them?
Thanks!