Nuts and Volts - October 2015

Our prototype of the circuit appears in Figure 5. The piezoelectric flapper is mounted directly on the board in Figure 5 for debug purposes. Later, it can be mounted to a door so that its tab will be bent by the door when the door is opened. The sensor is connected to the circuit board using twisted pair.

Note that a voltage limit to CFE is introduced by T0. P2 will activate the gate of T0 to turn it on. As CFE charges up during a read operation, it will stop 0.7 volts below the gate voltage. This will not be a problem in our recommended circuit as the Arduino Uno I/O pins operate at five volts and the Type AD capacitor saturates at 2.1 volts. It does become a problem for thicker ferroelectric capacitors like the Type AB. The voltage drop across the transistor disappears when CFE is being written down by P1, while P0 is ground.

Programming

The program to operate the event detector from the Arduino is similar to that explained in the February article, but more automatic. Only one routine is needed. The routine always executes a read followed by a write DOWN

1. Set P2 high to turn on T0.

2. Leave P1 as an input to detect the voltage across CSense.

3. Set P0 high.

4. Read the digital state of P1. a. If CF = DOWN, P1 will show Logic 1 indicating that no event occurred. b. If CF = UP, P1 will show Logic 0 indicating that the event occurred.

5. Set P0 low. CFE and CSense will both be at zero volts.

6. Set P1 high to write CFE DOWN to prepare for the next detection period.

7. Set P1 low. CFE and CSense will again both be at zero volts.

8. Set P2 low to turn off T0 and isolate P1.

The next power-on detection period now starts. If power is to be turned off, first set both P0 and P1 as inputs to ensure no voltage spikes are applied to CFE during power-down when the logic inside the Arduino goes unstable as voltage drops below the minimum specification.

The Arduino can be set up to execute the read-reset routine from one of three situations:

1. Subroutine call from a master program.

2. Upon reset by the reset button on the Uno.

3. Upon power-up.

The program to operate the Uno and the event detector is located at the article link. The program turns on the LED on the Arduino board attached to pin 13 to indicate if an event occurred.

The Event Detector in Operation

It is important to remember when mounting the LDT0-028K flapper on the door that it can generate either a positive or negative voltage. The direction can be determined with an oscilloscope connected to the event detector circuit at point A.