Nuts and Volts - December 2008

■ FIGURE 23

■ FIGURE 24

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carrier and attach the piezo to the PVC cap as you did before. Next, run the AC adapter cable down through the other hole and then into the coax connector. You can now place the cap on the top of the Dog Detector.

The Program

The Doggy1.txt program is straightforward. The first thing the program does after initializing the I/O ports is to take a set of calibration readings with the docal subroutine. This allows you to place the detector in many locations without having to change the code. The docal subroutine makes several calls to the sensor read routines then sets a couple of variables based on the last reading of each sensor. The value 10 is subtracted from this reading and will be used as the detection point for each sensor.

The Main Loop is where all the work gets done. An A-to-D (Analog-to-Digital) reading is taken every 10 milliseconds and compared against the calibration reading taken earlier. If the current reading is less than the calibration reading, the sensor is considered tripped.

Once tripped, a slight 50 millisecond pause is observed then 30 readings are taken from the upper sensor at 10 millisecond intervals. These readings again are compared against the calibration values and if triggered, it is assumed a human has passed the detector. If all 30 readings are taken without tripping the upper sensor, it is assumed that a dog has passed the detector and the chirp routine is called. After a pause of two seconds, the whole process starts again.

Normally, the RX lead of each sensor is held low to disable them from taking readings. When a reading from a sensor is needed, the RX lead is brought high, then after a 10 millisecond delay an A-to-D reading is taken from the sensors AN lead. After the reading, the RX lead is again taken low.

How Does it Work?

Unlike the early experiments using the IR sensors, the EZ4 sensors have proven a real winner. The small chirp from the piezo is just quiet enough not to annoy or wake you. It is enough to let both you and the dog know that he/she has entered where they should not have.

Going Further

I added a three-pin header that would make it possible to add some sort of negative feedback to the detector.

The height of the lower sensor should be high enough to let small cats pass if you wish, and low enough to catch the dog. You could space the sensors to detect small children roaming where they should not be. NV