Next step I’ll define the all the functions that we need for our system, list out the the possible options for each function and the pros and cons utilizing each of them.

I/A way to control the oven with low voltage digital signal (the switch):

In order to control the behavior of our oven so that it follows our set-up temperature profile, we need to put a device which helps us control our circuit with low-power signal, or in other word we need something like a switch within our circuit. There are several electrical switches that fulfills these requirements:

1. AC Solid-state relay (SSR): a solid state relay  is an electronic switching device that switches states when an external voltage is applied along its n-type and p-type junctions. SSR has a small control signal that controls a larger load current or voltage. It consists of a sensor which responds to an appropriate input (control signal), a solid-state electronic switching device which switches power to the load circuitry, and some coupling mechanism to enable the control signal to activate this switch without mechanical parts. Compared to the mechanical relay, it has certain advantages such as longer lifetime, slimmer profile, totally silent operation, etc. but it also has some disadvantages such as higher transient reverse recovery time, possibility of spurious switching, etc…

Resources: http://en.wikipedia.org/wiki/Solid-state_relay

2. Electromechanical relay (EMR): provides a similar function to the solid-state relay, but it has the mechanical part which is the moving components, which results in a shorter life-time reliability. On the other hand, EMR does have some other advantages compared to the SSR. Therefore, we can only tell which one is more suitable for us by looking up at the specific applicati0n requirements of our project.

Resources: http://electronicdesign.com/components/electromechanical-relays-versus-solid-state-each-has-its-place

3. Thyrister: The thyristor is a four-layered, three terminal semiconductor device, with each layer consisting of alternately N-type or P-type material. Thyristors act exclusively as bistable (flip-flop) switches, conducting when their gates receives a current trigger, and continue to conduct while they are forward biased. Compared to the SSR and EMR, the thyrister is more consistent but also has some other disadvantages.

Resources: http://en.wikipedia.org/wiki/Thyristor#Switching_characteristics

II/A way to measure the temperature:

There are many possible tools that we can use for our system to measure the temperature. Nevertheless, based on the following requirements and elements, we’ll pick the one that is most appropriate and suitable for our project:

• Accuracy: Plus or minus how many degrees
• Temperature range: Over what range of temperature can we measure with that accuracy
• Calibration and linearity: Whether if the measurement is repeatable and if the measurement values are linear over the temperature range
• Sensor response: How quickly does the sensor value change with temperature change?
• Complexity of sensor use: How hard is it to implement into our system?
• Cost: A cost-effective device would be an ideal option.

Most people from the past project used the k-type thermocouple or the thermistor as they are generally inexpensive, easy to use and also interchangeable. But we also have many other options such as thermowell, thermometer, resistance temperature detector, etc… that can possibly be implemented for our project.

 

III/A way for the user to interact and control the system:

Generally, there are two main ways that we can make an interface for the user to interact and control the system, either by connecting our system to a PC or to a set of separated LCD display and keypad.

1. Connecting to a PC: Our solder toaster oven if successfully constructed would mostly be used in our engineering lab and for the student projects purposes only. Therefore our oven doesn’t need to be portable and it’s a good option if we can use a lab PC as a user interface for inputs and data. Using a PC, we can easily check the output data and analyze how the oven is functioning and performing. But according to professor Hedrick, the problem with using a PC is that it would be more difficult to do things in real time and the programming would also be more challenging.

2. A LCD display and keypad: By using a small LCD display and keypad as our user interface, it’s possible that we can move the oven around, even though it might not be necessary. Also, this method if works out would be more user-friendly than using a PC because it provides a simplified controlling system and and an easy to understand and informative LCD screen.