After reading this in the blog from “KL”, I too went to Harbor Freight and purchased several units. To offer some further information, each box contains one sender and one receiver. Each box has a small oval sticker on the front identifying the channel. All of my units utilize the same tone. So even if I purchase units with different channels, they all sound the same when they go off. I wish the manufacture offered different choices of alerts per unit. The receiver can utilize an optional 6 volt, 200 mA power adapter, but the manufacture doesn’t offer it for sale. Oh yeah, there is a warning on the instructions on page 3 “Do not set up the Alert System if under the influence of alcohol or drugs”. Thank you China for your concern. All that being said, I still think this is a good buy. My concern is being alerted when someone enters an area when I’m normally in bed, so I will locate the receivers in our bedroom and write in big letters (that I can read without my glasses) which location each is. This will hopefully give me a heads up when someone is approaching the house. One final note: The receiver unit has a visible red indicator light. If you are concerned about someone seeing this while they are “casing the joint” you may want to cover it with black tape. – A.C.