Change closing transistor to opening in a Microwave Radar Switch?
I traced the output section of your RADAR module from the product photo. It appears to look like this:-







simulate this circuit &ndash; Schematic created using CircuitLab

The BISS0001 is designed for use in PIR motion detectors, and performs a similar function in the RADAR detector. When a signal is detected pin 12 goes high, turning on Q3 through R11. This then connects R13 to ground, drawing current out of Q2's Base to turn it on. As each transistor inverts the signal it comes out on module pin 'O' in the same phase as the IC output, ie. Q2 pulls up when the RADAR detector output is high.

To make the output pull up when the RADAR output is off you need an extra stage of inversion. The easiest way to do this is add another PNP bipolar transistor or P-Channel MOSFET, with a pull-down resistor to ensure that it gets sufficient bias when turned on. When the module output is on it shorts across the Base-Emitter or Gate-Source junction to make the transistor turn off. 

The additional circuit would look like this:-







simulate this circuit

In the PNP bipolar version the pull-down resistor has to supply enough current to keep the transistor fully turned on ('saturated') at maximum output current. Most of the time the resistor will have 5V across it, so it will dissipate significant power (~110 mW for 220 &ohm;, so a 1/4 watt or higher rated resistor should be used).   

The MOSFET version is more efficient because a FET is turned on by voltage, so the pull-down resistor just has to draw enough current to charge the Gate-Source capacitance quickly and soak up any EMI or leakage current. However finding a suitable P-Channel FET may be more difficult. It should have RDSON &lt; 0.1 &ohm; at 4.5 V.