COMPLETED TASKS DURING THE PERIOD

1.Circuit Design Model

A constant temperature circuit regulates the load temperature at a specific set point. By having a temperature sensor at the load, the algorithm of the temperature controller will adjust the output control so that the load temperature is constantly sustained. Irrespective of external factors, the load temperature will remain the same. So we need a constant temperature control circuit as our amplifier circuit.

Choosing a suitable Thermistor Sensor

As our approach, We needed a good sensible and suitable thermistor sensor to detect urine drops. According to our drop detecting algorithm, we decided to choose 100 type thermistor sensor. Although it a temperature sensor the sensor could be also used as flow measurement sensor to measure low isothermal air and water velocities.

Bridge Circuit with Thermistor Sensor

The bridge circuit is provided input voltage drop signal (Voltage difference across the temperature sensor) for the first stage amplifier(INA128 instrumentation amplifier). At the beginning, temperature of the temperature sensor should be higher than room temperature (about 60 Celsius ). In this situation bridge circuit is in balanced mode. Hence input voltage difference of the INA128 amplifier should be near to zero. Therefore the bridge is in balance condition.

According to above figure, assume J1 is the temperature sensor socket and its resistance in room temperature is 100 Ohm. We should choose R23 resistor with near resistance for the Thermistor resistance. Also if we choose 10k POT for as RV1, then we can adjust 10k POT until bridge circuit become balance at normal condition.

Amplifier Stages of the circuit

The amplifier circuit was made to amplify the voltage drop across the thermistor and get clear drop signal with a reasonable gain. INA128 Instrumentation amplifier and UA741 general purpose amplifiers are used amplifiers. TIP30 Silicon power transistor is the used transistor. Here the thermistor is in a Wheatstone bridge and the output of the thermistor is amplified using three amplifiers. The thermistor is connected as removable sensor. From that this device can be used for several patients. The patients need only the removable sensor to keep with him.

The circuit consists of an INA128 with low noise and high CMRR to minimize output noise and measurement error and gain the input drop signal. Output voltage of the INA128 can be in any voltage range within -6V/+6V. The output of the INA128 can be saturated for large flow rates. The op amp supply voltage (-6V/+6V) was not sufficient and that stable operation could not be achieved using op-amps that were rated for use with larger supply voltages. Also we wanted to flow higher current through the Thermistor sensor. Because Thermistor must be in higher temperature for getting clear drop signal. To satisfy above essentials, TIP30 power transistor was used. Also this configuration had no detrimental effects on the stability of the circuit and Supply voltage of the sensor had be symmetrical around the ground. The offset voltage injection circuit part into the feedback loop of the amplifier circuit was used to improve frequency response of the system and to get better drop signal with better amplification. Output voltage of the constant temperature feedback control circuit can be in voltage range of -6V/+6V. But input voltage range of the Atmega328p Microcontroller is 0-5V. Hence second signal conditioning circuit part with offset voltage injection was used to overcome above reason with small signal amplification. All the output nodes were connected with 1uF or 100nF capacitors to reduce noise effects.

Signal Processing Circuit Part

A Atmega328P microcontroller was used to signal processing. 16MHz crystal oscillator was used as the oscillator of the microcontroller. One of 10bit ADC channel was used to convert AC signal to DC signal (If we want to increase bit resolution of the microcontroller, we can use ADS1115 16bit ADC module). Operating voltage of the Atmega328P microcontroller is 0V to 5.6V.

Network Unit

This device can be uploaded real time processed data to web server page through ESP8266 wifi module. Operating voltage range of the module is 3.0V to 3.6V. So we need to provide 3.3V supply voltage as the best input voltage of the module. The best way to generate 3.3V voltage is using LM317 regulator.

Power Circuit

230V AC to 12V DC Switch Mode Power Supply(SMPS) was used. Because SMPS is the more efficient and more secure switching power supply. Power Circuit of the above circuit diagram gives +6V/-6V output pole with a circuit ground pole. Circuit ground was made by UA741 buffering circuit. Output waveform of the power circuit was smoothed by using 4.7uF capacitors.