I recently installed a rain sensor to my sprinkler system timer to disable the sprinkler valves when it rains. These sensors, depending on their settings and weather conditions, can disable the sprinklers for a number of days. I realized a few days after it had rained, there was no easy way to tell if the sensor was still active or whether I had a failure that was preventing my sprinklers from turning on. The first thing that came to mind was that an indicator light was needed to tell me when the rain sensor was active. Hence, I devised a simple circuit consisting of 2 resistors and 1 Light Emitting Diode (LED) that would illuminate any time the rain sensor was active. The LED was mounted in the cover of my sprinkler timer as shown in Figures 1a & 1b.

Note: This project assumes the rain sensor is wired in series with the sprinkler valve common return line as shown in Figure 2. If your system wires the rain sensor differently this approach may not work.

PARTS LIST:

· LED (Almost any kind will do)
· 1.2 K-Ohm resistor, ¼ watt
· 3.3 K-Ohm resistor, ¼ watt or greater (Note that any value from 2 to 4 K-Ohms should work)
· Hookup wire

Step 1: LED Installation

Locate a convenient place on the front cover of your sprinkler timer to mount your LED indicator such that the LED wires do not interfere with opening/closing the cover. Use a drill the same size as the LED diameter to drill a hole through the cover. Insert the LED from the rear into the hole. Use a rat-tail file to slightly enlarge the hole if the fit is too snug. See Figures 1a & 1b.

Step 2: Wiring

The schematic diagram in Figure 2 shows a typical rain sensor wired in series with the sprinkler valve common return line. Ensure your system is similarly wired. Unplug or turn off AC power to the sprinkler timer (hopefully your system has battery backup to remember your programming). Wire the LED circuit as shown by the red wiring in Figure 3. Since the power is AC, polarity of the LED does not matter. Note that you will have to use trial & error to see which 24 VAC terminal the 3.3 K-Ohm resister needs to be connected to.

Step 3: Test

Reapply AC power to the sprinkler timer. Test your circuit by (1) temporarily disconnecting one of the wires going to the rain sensor or (2) spray water on the rain sensor to cause it to activate. If it doesn’t work, connect the 3.3 K-Ohm resister to the other 24 VAC terminal before troubleshooting any further. If your system does not have battery backup you may have to reprogram the system.