I have a 3 way light switch which works fine with a regular light switch. That is, I can control on/off without issue from either switch.

I moved one of the switches to a timer, the Intermatic ST01, which according to my research, should work fine. Many people who have issues with flickering lights are advised to use this switch as it's switching mechanism works with low loads. I following the existing wire (common to common, traveler to traveler etc).

When the timer is hooked up I have the following:

Turn Intermatic On with Regular Switch Off: Lights turn on correctly Turn Intermatic Off with Regular Switch Off: Lights turn off correctly Turn Regular Switch On with Intermatic Off: Lights turn on Turn Regular Switch Off with Intermatic Off: Lights turn off (flicker)

The flickering is at a high rate, probably once or twice a second. I can hear the intermatic's switch clickign on/off during the flicker.

Any ideas on what's going on? Is it possible something is wrong with how the 3 way switches were wired before and I need to modify the timer switch/regular switch for the timer to work?

  • I don't think "regular switch" means what you think it means. I think it's time for photos of all switches, pulled out of the box, including tge one that was eliminated. Also, what exactly is the nature of the flickering, and what kind of lights are we talking about? Photos (of the light, not the flickering) or model numbers would be helpful. Commented Nov 5, 2019 at 14:47
  • What is tge root problem you are trying to solve? Stop flickering? Add timer control? Commented Nov 5, 2019 at 14:48
  • I just reread your post after answering, if you can can you check and verify your common and travelers. I don’t think it should matter since everything looks to be switching correctly but I do remember a caution that the unit may not work with a contactor because of the counter emf on release or off. but have never had any problems, and I do have a pair of these set up in 3 way so operations can override and the program will take back over at the next setting.
    – Ed Beal
    Commented Nov 5, 2019 at 16:04
  • Can you post photos of the insides of the boxes involved here? Commented Nov 6, 2019 at 0:01

2 Answers 2


Thanks for the tips.

I had removed all the wires, taken pictures and was going to test the configuration again to double check that everything was correct. I was emptying the timer box and noticed a black wire there. I thought it was odd to see a wire there.. I re-read the instructions and it turns out I missed a step! As you can see below, there is a step "E" - After installing that wire the switches are working correctly.

ST01 Manual


I am not sure what would cause your flicker. I use these same switches at my plant and because of the mechanical sound have always thought they are mechanical contacts. I drive contactors with them to power much larger lighting loads and love the astrometric timer feature. The switch contacts are not connected to the power so I don’t know why your lights would now be flickering, flickering is usually caused on led lighting when the switch uses a small amount of power from the circuit to power the device. the ST01 uses its cr2 battery for the power. At this point I might call Intermatic Or verify the other switch is not a lighted type switch, this can also cause the lights to flicker. I wonder if swapping the red and blue will make a difference? These are the travelers ( It should not matter with a standard non lighted snap switch).

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