relay timer?

Hi All,

I am trying to inject water treatment chemicals into an irrigation system
(mist house/green house application)  The system consists of a typical
irrigation controller that operates several zones and a pump.  The problem
is that the system comes on every 10 mins for only 5 secs.  My chemical inj
pump (pulsafeeder) normally runs off the pump but because the pump is only
on for 5 secs my inj pump needs 7-10secs to come on and thus I cant get any
chems into the system.  Some systems only come on for 2 secs.  These systems
basically provide humidity.

Unless somebody has a better idea and I would appreciate any advice I can
get, I think I need a relay switch with a timer on it.  Something that will
recieve a signal and come on and stay on for 15-20 secs independent of the
water pump.  I can recieve a signal from the controller (24V) or from the
pump start relay switch (115V).   Any advice or contact info is greatly
appreciated.

Thanks,
Joe