Experiment #5: Comparison of different send pin resistances
From research it seems that touch sensors become more sensitive the higher the resistance is between the send pin and the sensor. This experiment puts that to the test.
As noted in experiment #4, only sensors number 3, 6 and 7 will be used from now on, along with the un-covered control sensor. To recap, the sensors’ coverings are:
- 0.5mm thick white plastic card (used in modelling).
- 2 coats of artists acrylic paint over 2 coats of grey car primer.
- 46mm wide brown adhesive parcel tape.
Methodology
It was decided to compare each sensor’s capacitance using three different resistances between the sensor send pin and the sensor: 1MΩ, 2MΩ & 3MΩ. Tests were repeated using 25 and 50 samples per reading.
Since the tests run in experiment #4 used a 1MΩ resistor and included tests at the required sample rates, those results were re-used. New tests were run for all four sensors using 2MΩ and 3MΩ resistors at both 25 and 50 samples per reading. The new tests were identical in every way to experiment #4, except for the change in send pin resistance.
Code
The same code used in experiment #4 was used, in the same way. This was GuidedSensorTouchDataLogger.
Circuit
The circuit used was identical to that used in experiment #4, except that the 1MΩ resistor was swapped for 2MΩ and then 3MΩ resistors. Since no 2MΩ or 3MΩ resistors were available, the test circuits used 2 or 3 1MΩ resistors in series.
The following diagram represents the circuit logically, showing the ability to switch between resistances.
However, this is not what was built. As has been mentioned, test results for the 1MΩ resistor were copied from experiment #4, and the circuit was altered to use either a 2MΩ or 3MΩ resistance as required. The following photo of the actual circuit used for the 3MΩ tests will suffice to illustrate:
Code
GuidedSensorTouchDataLogger was used once again.
Results
The test data was again recorded and analysed in a Libre Office Calc spreadsheet. The results were graphed. Copies of the graphs can be found in the following image carousels, each of which contains a graph for each sensor:
Results at 25 samples per reading
Results at 50 samples per reading
For full details, download the spreadsheet (zipped).
Conclusions
As predicted by the research, the higher the resistance between the send pin and the sensor, the more sensitive the sensors became. The sensitivity appears close to being linearly related to the ratio of the resistances in all cases, i.e. the size of the readings for the 2MΩ resistor is roughly double that for the 1MΩ resistor, while the size of readings for the 3MΩ resistor is approximately triple that for the 1MΩ resistor.
Purely from examining to graphs there would appear to be less variation in results when the sensor is touched for the higher resistance values. However no statistical analysis was performed.
The length of time needed to take a single reading seems to increase with resistance. This is more noticeable at 50 samples per reading than at 25 samples per reading. Consequently there may be a trade off to be made between resistance and speed.