When using resistive sensors, the designer must keep in mind some important questions.
1. How much does the resistance change under the desired operating conditions. The resistance range of a sensor is important in the design or choice of the circuit. If a sensor changes by only few Ω resistance when excited, the circuit must have special characteristics to react to this small change. LDRs change resistance over a large range of Values, so they are easy to use. NTCs, depending on the type, also have a wide resistance range, but others (e.g., pressure sensors) do not.
2. Is the current provided by the sensor enough to drive a Circuit? If the sensor has a large resistance drop when excited, the current can be enough to drive any of our circuits. In general, to drive a block with a bipolar transistor, the resistances in the low-value condition must be less than 50,000 ohm. If Darlington transistors, CMOS ICS, or two-stage circuits are used, this resistance can be higher in the range of 200,000 to 1,000,000 ohm. As a general rule, if your sensor doesn”t trigger the circuit, try another block With higher sensitivity.
