Initiating image capture in response to a trigger was the next step, my plan is to use a button, or a proximity sensor like the passive infrared (PIR) module in the second image to trigger a photo.

For my test rig (in addition to a RaspberryPI & generic USB Web camera) I’m using some Seeedstudio gear

• Grove Base Hat for Raspberry PI USD9.90
• Grove – Button USD1.90
• Grove – PIR Motion Sensor USD7.90
• Grove – Touch Sensor USD3.90

The first step was to write an interrupt handler for the digital input, I figured triggering on the button push rather than release would make device more responsive.

/*
    Copyright ® 2019 Feb devMobile Software, All Rights Reserved
 
    MIT License
...
*/
namespace devMobile.Windows10IotCore.IoT.DigitalInputTrigger
{
	using System;
	using System.Diagnostics;
	using Windows.ApplicationModel.Background;
	using Windows.Devices.Gpio;

	public sealed class StartupTask : IBackgroundTask
	{
		private BackgroundTaskDeferral backgroundTaskDeferral = null;
		private GpioPin InterruptGpioPin = null;
		private const int InterruptPinNumber = 5;

		public void Run(IBackgroundTaskInstance taskInstance)
		{
			Debug.WriteLine("Application startup");

			try
			{
				GpioController gpioController = GpioController.GetDefault();

				InterruptGpioPin = gpioController.OpenPin(InterruptPinNumber);
				InterruptGpioPin.SetDriveMode(GpioPinDriveMode.InputPullUp);
				InterruptGpioPin.ValueChanged += InterruptGpioPin_ValueChanged;

				Debug.WriteLine("Digital Input Interrupt configuration success");
			}
			catch (Exception ex)
			{
				Debug.WriteLine($"Digital Input Interrupt configuration failed " + ex.Message);
				return;
			}

			//enable task to continue running in background
			backgroundTaskDeferral = taskInstance.GetDeferral();
		}

		private void InterruptGpioPin_ValueChanged(GpioPin sender, GpioPinValueChangedEventArgs args)
		{
			Debug.WriteLine($"{DateTime.UtcNow.ToLongTimeString()} Digital Input Interrupt {sender.PinNumber} triggered {args.Edge}");
		}
	}
}

Then I added in the camera functionality and made the interrupt handler async and await the camera and file system calls.

/*
    Copyright ® 2019 Feb devMobile Software, All Rights Reserved
 
    MIT License
...
*/
namespace devMobile.Windows10IotCore.IoT.PhotoDigitalInputTrigger
{
	using System;
	using System.Diagnostics;
	using Windows.ApplicationModel.Background;
	using Windows.Devices.Gpio;
	using Windows.Foundation.Diagnostics;
	using Windows.Media.Capture;
	using Windows.Media.MediaProperties;
	using Windows.Storage;

	public sealed class StartupTask : IBackgroundTask
	{
		private readonly LoggingChannel logging = new LoggingChannel("devMobile Photo Digital Input Trigger demo", null, new Guid("4bd2826e-54a1-4ba9-bf63-92b73ea1ac4a"));
		private BackgroundTaskDeferral backgroundTaskDeferral = null;
		private GpioPin InterruptGpioPin = null;
		private const int InterruptPinNumber = 5;
		private MediaCapture mediaCapture;
		private const string ImageFilenameFormat = "Image{0:yyMMddhhmmss}.jpg";
		private volatile bool CameraBusy = false;

		public void Run(IBackgroundTaskInstance taskInstance)
		{
			LoggingFields startupInformation = new LoggingFields();

			this.logging.LogEvent("Application starting");

			try
			{
				mediaCapture = new MediaCapture();
				mediaCapture.InitializeAsync().AsTask().Wait();
				Debug.WriteLine("Camera configuration success");

				GpioController gpioController = GpioController.GetDefault();

				InterruptGpioPin = gpioController.OpenPin(InterruptPinNumber);
				InterruptGpioPin.SetDriveMode(GpioPinDriveMode.InputPullUp);
				InterruptGpioPin.ValueChanged += InterruptGpioPin_ValueChanged;
				Debug.WriteLine("Digital Input Interrupt configuration success");
			}
			catch (Exception ex)
			{
				this.logging.LogMessage("Camera or digital input configuration failed " + ex.Message, LoggingLevel.Error);
				return;
			}

			startupInformation.AddString("PrimaryUse", mediaCapture.VideoDeviceController.PrimaryUse.ToString());
			startupInformation.AddInt32("Interrupt pin", InterruptPinNumber);

			this.logging.LogEvent("Application started", startupInformation);

			//enable task to continue running in background
			backgroundTaskDeferral = taskInstance.GetDeferral();
		}

		private async void InterruptGpioPin_ValueChanged(GpioPin sender, GpioPinValueChangedEventArgs args)
		{
			DateTime currentTime = DateTime.UtcNow;
			Debug.WriteLine($"{DateTime.UtcNow.ToLongTimeString()} Digital Input Interrupt {sender.PinNumber} triggered {args.Edge}");

			if (args.Edge == GpioPinEdge.RisingEdge)
			{
				return;
			}

			// Just incase - stop code being called while photo already in progress
			if (CameraBusy)
			{
				return;
			}
			CameraBusy = true;

			try
			{
				string filename = string.Format(ImageFilenameFormat, currentTime);

				IStorageFile photoFile = await KnownFolders.PicturesLibrary.CreateFileAsync(filename, CreationCollisionOption.ReplaceExisting);
				ImageEncodingProperties imageProperties = ImageEncodingProperties.CreateJpeg();
				await mediaCapture.CapturePhotoToStorageFileAsync(imageProperties, photoFile);

				LoggingFields imageInformation = new LoggingFields();

				imageInformation.AddDateTime("TakenAtUTC", currentTime);
				imageInformation.AddString("Filename", filename);
				imageInformation.AddString("Path", photoFile.Path);

				this.logging.LogEvent("Captured image saved to storage", imageInformation);
			}
			catch (Exception ex)
			{
				this.logging.LogMessage("Camera photo or save failed " + ex.Message, LoggingLevel.Error);
			}
			CameraBusy = false;
		}
	}
}

I found that contactor bounce was an issue (Grove- Touch Sensor OK) with larger mechanical buttons so I added the CameraBusy boolean flag to try and prevent re-entrancy problems. I’ll trial some other types of proximity and beam based on real-world student projects.

The code is available on GitHub and is a bit of a work in progress.