Project overview:The built temperature control circuit is an integration of electronic components that function in a proper manner. It was designed for the intention of monitoring room temperature using a thermistor, and limiting its highest value to 25˚C. This is achieved by turning an indicator lamp and fan ON, through a transistor switch, when the comparator (LM339) identifies a temperature greater than 25˚C.

As mentioned above three main tasks are needed to be accomplished in order to carry out the desired outcome.

Measuring Temperature (T)

An NTC thermistor, whose resistance RT decreases as temperature increase, is utilized to sense the room temperature. A voltage divider is then used to obtain an electrical signal associated with the measured temperature. As a result an output voltage Vs is obtained ready to serve as one of the two inputs required by the comparator.

Circuit Analysis:

Aim:

- Choosing an appropriate reference resistor RC based on an assumed value of Vs

Components:

- Power supply providing an input voltage (Vin) of 12v- NTC Thermistor ttc-502- Resistor RC

Figure 1 (voltage divider circuit of the thermistor)

Using the data sheet of the thermistor we found that for a temperature T=25˚C the equivalent resistance is RT= 5000 Ω.

Using voltage divider: Vs = RT

RC+RT * Vin

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Assume Vs= 6;

6= 5000

RC+5000 * 12 → RC =5000 Ω

Since a 5000 K Ω resistor is not available with us, we chose two make a parallel connection of two 10kΩ resistors which gives the intended 5000 Ω resistanceRC.

(110

− 110

¿−1= 5 k Ω

Select RC =5 K Ω

Check if T > 25˚C

An Op-Amp is employed to compare actual temperature with the required 25˚C temperature. The comparator is provided by two inputs Vs, output of thermistor connected to the non inverting input pin (pin 6) and VR , a fixed reference output of a voltage divider(In our case) connected to the inverting input pin (pin7). Note that the location of the input, output, ground and Vcc are provided by the op-amp data sheet. The output voltage Vo is then obtained at pin 1.

Figure 2

Circuit Analysis:

Aim:

- Choosing appropriate V+, V- values based on the chosen comparator specifications. - Choosing appropriate values for the resistances R1∧R2 which compose the voltage divider

at VR.

Components:

- Power supply providing an input voltage (Vin) of 12v (same supply used above)- Comparator LM339- Resistors R1∧R2

From the data sheet of the comparator, it is shown that V+, V- ranges between (±1 to ±18) or

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(±2 to ±36)

choose 0 to 12 volt.

Making reference temperature 25˚C

With Vs=6V at 25˚C → VR=6 V at 25˚C

Figure 3

Finding R2 and R3:

VR = R2

R1+R2 * VIN

(R1+R2) VR = VIN *R2

R1 * VR = (VIN - VR)*R2

(V∈−V R)V R

=R1R2

(12 –6)6

=R1R2

→ R1R2

=1

Select R1 =100 k Ω & R2 =100 k Ω

Turn ON a Motor and a LED

A resistor RB is connected at output of the comparator, according to the output of the Op-Amp and resistance RB, current IB will flow forming the input of transistor. If this current is greater than IBs the transistor is saturated and acts as switch hence providing the motor with required current. The motor in turn drives the fan.

Motor specifications:

RM=9.5 k Ω measured by DMM

VM= 12v

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IM=V mRm =

129.5∗103

= 1.26 *10−3A

IM=1.26mA

We chose a green diode having Vturn ON=1.65V, RD(typical)=19Ω Vin =12 V Rlimitting = 220 Ω

Rlimitting is formed from a combination of series and parallel connections of resistances.

(1220

+ 1220

¿−1= 110 Ω

Note that the LED was connected in parallel with the motor.

Component table:

Table 1. Specifications of components employed in the system

Part Description Part # and manufacturer Specifications

1 Thermistor ttc-502 NTC type; β=3950 Ω;At 25°C RT =5000 Ω

2 Comparator LM 339N Vcc =12 V

3 Transistor 2N3904 npn-BJT type; 40 V, 200 mA,20-200 gain

4 DC motor MM1-6S2R13 July 9851 VM = 12V; IM = 1.26 mA; RM = 9.5 KΩ

5 Resistor R=100 kΩ; PR =3 W; 5% 6 Resistor R=5 kΩ; PR =3 W; 5%7 Resistor R= 220 Ω8 Green LED Vturn ON=1.65V, RD(typical)=19Ω

Possible Applications of the system

This system can be also used for parked car ventilation. Where inside temperature of car is monitored and fans are turned on when a parked car temperature go above 25˚C. Solar panels can be used as a power source in this case.

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Reference

www.alldatasheets.com

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