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\label{sec:Presence}  \subsubsection{Breakbeam Sensors}  \subsubsection{Reflective Ultrasonic}  Ultrasonic sensors [INSERT FIGURE] use ultrasonic waves to detect a wide variety of targets in many environmental conditions, and are particularly useful for detecting transparent objects\cite{Tektron}. However, they must be aligned very precisely with the desired surface and have a minimum sensing distance as there is often a blind spot directly in front of the device, which is an issue for the small scale on which the candlelight’s presence will need to be determined. Furthermore, the sensors must be placed a minimum distance (sometimes up to 1.5 m) away from each other to prevent interference between sensors\cite{RockwellAutomation}. This is also problematic given that the design calls for nine sensors spaced close together. As such, reflective ultrasonic sensors are impractical for for the detection of candlelights and/or the tray in this application.  \subsubsection{Microswitches}  Microswitches [INSERT FIGURE] are used for a variety of purposes including contact/touch sensing. Microswitches are electric switches actuated by little physical force, which use a series of springs, levers and/or hinges to bring circuit components into contact\cite{FutureElectronics}. Once activated, these switches require a large movement in the opposite direction to reverse the electrical contact. This helps to smooth out the signals received from the switch. Microswitches can currently be found in door latches for microwaves, as well as in safety switches for elevators. They also have many other applications in diverse fields. As we wish to ensure both the presence of the tray and its correct positioning, microswitches are an appropriate sensing tool for this machine.