The project we have chosen to implement is the "Digital Thermostat including Temperature Control". The operation of said circuit is defined as follows:
"Design and construct a digital thermostat which will operate in two modes. The system will display the temperature of a Peltier effect cooler over a prespecified range and given a temperature will control a Peltier effect cooler appropriately. "
To achieve this result, we have decided to build the circuit pictured in the block diagram below. As you can see, we plan to use a series of switches as our main inputs. These switches will allow us to select a specific current level, which when applied to the Peltier Apparatus will correspond to a given temperature. We will be making use of a thermistor to measure the temperature being output by the Peltier Apparatus, and a comparison circuit to determine whether the Peltier Apparatus needs to be activated or deactivated, in order to match the specified temperature. The main output of our system will be a pair of seven-segment displays, which will display the current temperature being generated by the Peltier Apparatus.
Our circuit will function as follows: The user will enter a temperature via the series of switches. The signal from the switches will be fed into a comparison circuit. This comparison circuit will take the current temperature of the Peltier Apparatus as measured by the Thermistor, and compare it to the temperature specified by the switches. If the temperature of the Peltier Apparatus is equal to, or less than the specified temperature, the device remains off, or is turned off if it is on. If the temperature of the Peltier Apparatus is greater than the specified temperature, the Apparatus is activated through the use of a transistor relay (noted as BJT on the block diagram). The transistor relay is necessary since the Peltier Apparatus requires a very high current level (about 10 Amps) to be activated, were this level of current applied directly to the rest of the circuit, the TTL gates would break down. The seven segment displays are activated by taking the signal from the thermistor after it has been converted from analog to digital, and then running these 8 inputs to a CPLD. The use of a CPLD will make driving the seven segment displays much more efficient then simply hardwiring their specifications into the circuit.