Question

design a heat sensing circuit,consisting of transistor and a comparator

Answer 1

SYSTEM DESIGN AND IMPLEMENTATION:

1. CIRCUIT DESIGN AND ANALYSIS:

The circuit is designed based on the mode of operation and functions of each component provided by the datasheets. The whole component manufacturers' specifications were carefully put into consideration. The circuit is aimed at minimum or a limited number of components for simplicity. The modular design of the system is shown in the block diagram below.

The power supply unit as shown in Fig. 2 is designed to provide 5V supply to the circuit. The voltage from an A.C mains supply is stepped down to 12V using a step-down transformer and converted to D.C using the bridge system of rectification. This is done by connecting the A.C. supply across one diagonal AB, and D.C output is taken from the other CD line. The bridge rectifier's system output still has a lot of ripples that have to be smoothed out in order to generate genuine D.C. Capacitors resist changes of voltage across them. Hence they are used to provide the desired smoothing action.

Hence a 2200uF capacitor is used since it's close to 1684uF. A switch is used for opening and closing the complete circuit. A 5V regulator (7805) is incorporated into the circuit to provide regulated 5V supply from the 12V. A light-emitting diode is used to indicate the presence of electrical current through the circuit. A resistor is connected in series with the light-emitting diode (2.7V) in the forward bias mode to limit electric current. Assuming electric current through the diode is 3.2mA.  The 12V and 5V power supplies through the power unit are for the purpose of the integrated circuits and alarm output respectively.

2. THE TEMPERATURE UNIT:

As shown in Fig. 3 above, the temperature sensor module consists of two LM 335 temperature sensors. Each provides temperature measurement in Kelvin with respect to the voltage. The LM 335 provides a linear relationship between temperature and voltage which is 10mV/°K. The temperature sensors receive input from an external device and they in turn act as input to the rest of the circuit. In other words, the other parts of the circuit respond to the signal from the temperature sensors. The LM 335 measures temperature in Kelvin hence its output voltage is relatively large. For example, at a temperature of 25°C i.e. 298.2K, it has a corresponding voltage of 298.2mV

3.The Operational Amplifier Unit:

Due to the relatively large value of the output voltage of the temperature sensor, the output is connected to an operational amplifier in the subtraction mode. The LM 324 operational amplifier is used to effect subtraction as s

The circuit of Fig. 4 is specified by the component datasheet, RR (10kΩ) is a potential divider for applying 273mV at the negative input of the operational amplifier. The voltage of the positive input of the device is subtracted by 273mV. The output of the LM 324 signifies temperature measurement in degrees Celsius. Two of such operational amplifiers are used in the circuit as shown in Fig. 4.

4. Comparator Unit:

The outputs of the operational amplifiers are connected to their corresponding comparators as shown in Fig. 5.

The comparator module as shown in Fig. 5, involves two comparators (LM 339). The outputs of the corresponding operational amplifiers are connected to the non-inverting input of the comparator. The inverting inputs Vin (-) are referenced to a particular voltage through a 10kΩ variable resistor. The resistor circuit that is related to the inverting input Vin (-) is used for adjusting a particular circuit's response. Assuming the inverting input of a particular comparator is adjusted to 300mV and the non-inverting input is 200mV. The initial output condition of the corresponding comparator is logic 0 as shown in the relationship below:

Vin(+) > Vin(-) = logical 1 output

Vin(+) < Vin(-) = logical 0 output

Hence if the input voltage at the non-inverting pin becomes greater (temperature increases) than that of the voltage at the inverting pin, the output of the comparator changes from logical 0 to logical 1 [1]. By adjusting the variable resistor, different responses are achieved by the circuit.

5. The Alarm Unit:

The Tone Generator Unit is built on the 555 Timer IC but in the Astable mode. Unlike in the Monostable state, the Astable mode has no stable state; the output is continually changing between 'low' and 'high’.

CALCULATIONS:

TON = 0.7 x( R1 + R2) x C2

And TOFF = 0.7 x R3 x C3

Where, T = time period in seconds (s) = TON + TOFF With R1 = R2 = 68KΩ and C2 = luF 9 TON = 0.7 x (2 x 68 x103 ) x 10-6 TON = 0.0952 seconds.

With R3 = 8.2KΩ and C3 = 47uF, TOFF = 0.7 x (8.2 x103 ) x 47 x10-6 TOFF = 0.26978 seconds, and TOFF ≈ 0.27seconds.

Therefore, T = 0.0952+ 0.27 T = 0.3652 seconds.

Since frequency, F = 1/T, it then implies that F = 1/0.3652 F = 2.74 Hz. The tone generator unit is controlled by the timing unit via a transistor (BC 108), which acts as a switch to trigger the generator unit ON. Hence, once the timing period is elapsed, the generated tone also ceases.

LM335:

The LM 335 is a convenient 2-terminal temperature sensor that behaves like a Zener diode with a voltage of+10mV/°K (i.e. It develops an output voltage proportional to absolute temperature). For example, at 25°C (298.2°K), the LM 335 acts like a 2.982 volts Zener. It comes with an initial accuracy as good as ±1oC and it can be externally trimmed. A single point calibration can typically improve its accuracy to ±0.5°C max over a -55°C to 125°C range. These features of the LM 335 are used in the system for precise response to a particular temperature. The LM 335 was chosen for this system because of its linearity, accuracy, features and ease of designing the necessary support circuitry. 10 3.1.6.2. Comparator (LM 393) The LM393 series consists of two independent precision voltage comparators with an offset voltage specification as low as 2.0mV max for two comparators, which were designed specifically to operate from a single power supply over a wide range of voltages. Operation from split power supplies is also possible and the low power supply current drain is independent of the magnitude of the power supply voltage. These comparators also have a unique characteristic in that the input common-mode voltage range includes ground, even though operated from a single power supply voltage. The comparator has a wide supply (Voltage range: 2.0V to 36V, Single or dual supplies: ±1.0V to ±18V), Very low supply current drain (0.4 mA) independent of supply voltage, input common-mode voltage range includes ground, Differential input voltage range equal to the power supply voltage. Application areas include limit comparators, simple analogue to digital converters; pulse, square wave and time delay generators; wide range VCO; MOS clock timers; multivibrator and high voltage digital logic gates.