I know what's wrong. I did almost the same thing. I used my HS200 to replace an existing light switch with two wires - a black wire and a white wire. I connected the two HS-200 black wires to the black wire and the HS200 white wire to the white wire. The HS200 seemed to be working fine. However, an incandescent bulb wouldn't come on at all and an LED bulb flashed on and off. After a lengthy call with a Tech Support person and further web research, I discovered what was wrong. The HS200 is NOT like a dimmer switch or wall timer where you just disconnect the two wires from the existing switch and connect them to the dimmer switch or timer. You have to think of the HS200 like a timer that you plug into an outlet and then you plug a lamp into it. I found a FAQ on the TP-Link website ( http://www.tp-link.com/ca/faq-1262.html) that explains this - "... not only is our HS200 a switch, but also it is a powerline appliance.". You have to connect one of the HS200 black wires (it doesn't matter which one) to one of the two wires on the existing switch and the other HS200 black wire to the other wire on the existing switch. AND you need to connect the HS200 white wire to Neutral (white wire). This is shown schematically in the FAQ. BTW, the HS200 green wire is Ground and should be connected to one of the screws on the electrical box (aka "powerline socket") itself. The HS200 white and green wires should NOT be connected together. Bottom line - you can NOT use the HS200 to replace the switch in an electrical box that has only the two wires going to the existing switch (Line and Load), and no accessible Neutral wire. I ended up using my HS200 to replace the switch for a ceiling fan where the electrical box had an accessible Neutral wire. And it works perfectly. Note that all of this applies to lights operated by a SINGLE switch and not by two (or more) 3-way switches.