My supervisor Andrew McPherson visited from London last week and recommended building a differential amplifier which could (as I understand it) read signals down to DC. This would give us much greater sensitivity around the lower frequency range where much of the information from the piezo film is being created. This in turn would give us greater ability to read subtle gestures like brushing or stroking the paper. There's a circuit diagram below, increasing the difference in resistance between Rf and Ri gives us more gain.
The second image shows it prototyped on the breadboard. It is built using two TLO72 opamp chips. Andrew recommended using an OPA4317 (soic) surface mount chip for a more developed prototype.
We didn't connect it up to an ADC yet, but the results we saw on the oscilloscope were encouraging. We tested it with both 10V and 5V. It was less successful at 3.3V, but a rail to rail opamp should help that.
The next step is to connect the output to an ADC and perhaps also an Arduino to see what values we get.
We didn't connect it up to an ADC yet, but the results we saw on the oscilloscope were encouraging. We tested it with both 10V and 5V. It was less successful at 3.3V, but a rail to rail opamp should help that.
The next step is to connect the output to an ADC and perhaps also an Arduino to see what values we get.