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Step 11: Testing the Sensor - Determining the Resistor Values

Picture of Testing the Sensor - Determining the Resistor Values

The ribbon cable is connected to the sensor (which is now fixed in position on the washing machine).

The test board is connected to the other end of the ribbon cable.

14 of the conductors carry the signals from the LDRs while the remaining 2 conductors are used for the 0V ground connection.

With the washing machine turned on, the resistance of each LDR is measured with a multimeter as the relevant wash programme LED is on or off. The table of results is shown in the diagram.

During the tests it was clear that two of the LDRs gave very different resistance readings from the others. This could have been because they were of a different type. The LDRs did not have identifying markings to confirm this.

As it would have been difficult to remove and dismantle the sensor, I decided to sidestep the problem by selecting suitable dropper resistors. The values had to result in a voltage at the UNO R3 of less than 3V when the LDR was illuminated, and more than 3V when it was in darkness.

Suitable standard resistor values were found to be 12 x 100k Ohm and 2 x 470k Ohm.