Optex is a company who’s products I have used for years. Yes, there are less expensive brands that by description do the same thing, but a favorite saying of mine is, “There is no skill in being the cheapest”. Optex builds good stuff at reasonable prices.
The Optex RCTD-20U is a wireless motion sensor kit that includes a motion sensor (TD-20U) that transmits to a base station (RC-20U) when motion is detected. When the sensor detects motion the base station makes a sound. The motion sensor is rated for outdoor installations, operates on a 9V battery and has two selectable coverage areas. You can select either a 50 foot long range or 17 foot wide detection area.
In addition to motion sensors you can add wireless door contacts (TC-10U) or a push button transmitter (TS-10U). The range on the transmitters ranges from 700 to 2000 feet. Real world usage is less, but the range is still impressive. If you need increased distance, Optex sells a hardwired repeater (TR-20U) to increase the range upto 1000 feet (depending on device). I have used these products for years and have never needed or used a repeater. The repeater can only extend the range of four devices so plan accordingly.
You can use up to three separate sensor per zone and with three available zones, up to nine sensors can be added to the system. Each zone can have a unique tone to help identify the activated sensor. By combining motion sensor, door sensor and push button (door bell) transmitter you can build a complete alert system.
For more advanced installations the receiver base features a class C relay. This relay can be used to trigger alarm system inputs, additional notification equipment, external bells (1 Amp max) or many other uses.