Relay Trigger Type Is Short
It could mean:


"Short-circuit", as in connected to ground as you have found by experimentation. This is not normal terminology though.
"Short-duration", as in "a brief connection of the trigger to GND will start the timer. Normally a hundredth of a second would be adequate for this so I don't know why they're looking for 1 - 3 s.


You'll have to do more experimentation. I suggest:


Set up a 10 s delay.
See what happens with a 0.5 s trigger (to GND).
See what happens with a 3 s trigger.
See what happens with a 5 s trigger.
See what happens if you hold the trigger on indefinitely.


You should be able to figure out from these tests:


Whether the trigger duration matters.
Whether the load switches immediately on trigger.
Does the load switch off when trigger is removed.
Does the load stay on if the trigger is held or does it switch off until the trigger turns off and on again.





  Voltage does change with the sensor, I have 4.85v with the light on
  and 2.15v with light off but that doesn't trigger.


The problem is that your sensor is giving an analog output. Your trigger requires a digital signal, on / off.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Potentiometer trigger test. Figure 2. Diode voltage dropper.

It seems as though your sensor output voltage swing is not adequate to trigger. You need to do some experiments:


If you have a potentiometer you could test to see how low the voltage has to go before the trigger works. Hook it up as shown in Figure 1 and monitor the voltage on the input.
Then, if you have a few diodes, you could try the circuit in Figure 2. Each diode will subtract 0.6 to 0.7 V from the sensor signal and, hopefully, drop it below the threshold voltage for the trigger. Again use your multimeter to check what's going on.