The circuit uses a movement sensor with an output voltage of circa 3.3 V to turn on/off a lamp/load through a relay with a supply voltage of 9 V.
The movement sensor is simulated in this circuit using a voltage pulse. (lower left). Because the output voltage of the sensor might not high enough to drive the relay, I use a NPN transistor as a solid state switch. When the output of the sensor is high, the gate of the transistor opens and the current flows through the "switch", drives the relay on. Wenn the output is low, the transistor transits into cut-off mode and switches off the relay. Any NPN will serve the purpose, for example 2N3904.
The value of R(Load) is critical to make sure the transistor is operating in saturated and cut-off mode. The driving current is negligible (here 2.6 mA).
