Have an irrigation control which selectively opens one or two solenoid valves to water two areas. Pressure to the system is via a solar pump. Rather than run the pump continuously I would like to have it turn on when the solenoid valves open and off when they close.The pump controller has an input if open circuit pump runs, closed pump stops.NOTE: Only power available is fro irrigation control 12v DC or solar panels 48v DC.
My idea was to use the current flowing to the solenoid valve to activate a NC relay. When solenoid activates contacts open. The pump control senses open circuit and starts running. Solenoid valve off, relay closes and pump turns off. Fine for one solenoid valve .
Nest step was to fit a second relay parallel to the first. Each activated when a different solenoid valve is powered but sharing a single output connection.

What I forgot was that if only one circuit is active then the other will be in the NC state. The output will be closed circuit and the pump will not run.
Is there some way of using a single relay for this job? Dual coil relay?

 

You can use this. I do on my exhaust fans.

The trick is to select a power NPN transistor that has a large enough base current to run the solenoids and collector current for the pump.
As you have not mentioned currents it is a bit hard to be specific. But a 2N3055 would most likely do.
Edit: The pump may be replaced by a relay if that would work better for your application.
But I think I misunderstood you circuit configuration so this may not be applicable.
Feeding both solinoids from the base of a PNP power transistor that has its emitter to +12v and the collector to a relay coil to 0v may work.
Basically this circuit upside down.
It depends on if the solinoids have a common +Ve or -Ve connection.

 

I think the easiest solution would be to use two relays. You can wire the coil of each relay in parallel with an irrigation valve. If the contact sides of the relays are wired in parallel with each other, then either one will activate the pump (you don't need both on at once.)

The only potential issue I see is if the irrigation control outputs are picky about what load they're driving. It's possible that adding the relay coils in parallel with each valve will draw too much current for the irrigation controls. That risk seems pretty small to me, but without specs I'm only guessing.

I apologize for the messiness of the attached drawing. I sketched it on the bus this morning.

 

Put the NC relay contacts in Series, then when any relay operates the pump will run..

You can do it with one low3V relay put in series with the solenoid supply either pos or neg, that way whichever solenoid is active the pump..

 

Hmmm, I just reread your original post and realized you're talking about using a relay to control the pump controller switch input. My suggestion was based on the idea of leaving the pump controller active at all times, but breaking one of the power lines to the pump itself. I'm not sure if my thinking is wise or not, but figured I should clarify what my original thought was either way.

The easy way to do what you want (again assuming the irrigation control outputs can tolerate the additional load of a relay coil) would be simple diode ORing so that either irrigation valve circuit can trigger the same relay coil without back-feeding into the other valve circuit. Then you can use a single NC relay contact as you described. I'll try to create a simple sketch of that one and upload soon.

 

This drawing is even messier than the first, but hopefully gets the idea across.