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Engineering LibreTexts

2.4: Procedure

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  • 2.4.1: LDR

    1. Use the DMM to measure the resistance of the LDR. Do this away from any windows and do not block ambient room lighting from hitting the device. Record the resistance value in Table 2.5.1 under “Normal”.

    2. Place a finger over the LDR to block all light, making sure that the leads are not also touched. Record the resulting resistance value under “Dark”.

    3. Shine the pen light directly onto the LDR at a distance of about 25 centimeters. Record the result under “Bright”.

    4. Construct the circuit of Figure 2.3.1 using E = 10 volts and R = 10 k \(\Omega\). Using the voltage divider rule, determine the expected value for the voltage across R under normal lighting and then measure the voltage. Record these values in Table 2.5.2.

    5. Repeat step 4 for the Dark and Bright conditions.

    6. Finally, slowly move the pen light toward and away from the LDR. Note what happens to the voltage, recording the maximum and minimum voltages obtained in Table 2.5.3.

    2.4.2: Thermistor

    7. Use the DMM to measure the resistance of the LDR at room temperature and record the result in Table 2.5.4. Do not handle the device excessively as body heat may affect it.

    8. Build the circuit of Figure 2.3.2 using E = 10 volts and R = 10 k\(\Omega\). Measure the voltage across the thermistor and record it in Table 2.5.5 under “Room Temp”.

    9. Monitor the thermistor voltage while applying heat. Caution: If you are using a standard heat gun, place it on a low setting, use a diffuser or keep the gun at least a half meter away to avoid possibly damaging connecting wires or the protoboard. After 30 to 60 seconds, record the thermistor voltage in Table 2.5.5. Turn off the heat source and note how long it takes the thermistor circuit to recover back to the original reading.

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