In this block, the load is powered on when the resistance of the sensor is reduced. For an LDR, the load triggers on when the light falls on the sensor. This circuit is indicated for low-resistance sensors (below 10 kΩ). The circuit, with indications of the blocks that can be triggered, is given in Figure 1. The potentiometer must have a value between 3 and 10 times the resistance of the sensor under normal conditions.
Figure 1 Basic block using PNP transistor II.
