Using the thermistor characterized in the data sheet, and a 555 integrated circuit as a threshold detector, design a circuit to light an LED when the tempreature exceeds 80°C. Draw a circuit diagram with component values marked on it. Estimate the temperature at which the LED would turn off (correct answer attached).

Now what I want to know is:

*How do you know that the supply rail = 9V?

*How do you calculate R = 8K?

I know that you are suppose to use Vout = V.Rth / Rth + R but only knew that Rth ≈ 3.8k when the temperature = 80°C.

 

Do you want the LED to flash above 80oC or to stay lit?

 

Well for example if we assume this circuit:

http://www.fairchildsemi.com/ds/LM/LM555.pdf

And we find in datasheet that supply voltage for 555 must be in the range 4.5V to 16V.
So to to light an LED when the temperature exceeds 80°C
Voltage on pin 2 (trigger node) must be lower then 1/3 of the supply voltage.
Rth for 80°C is equal 3.8KΩ so to get 1/3 voltage on pin 2 R1 mus be large then R2 > V1/Vth = R1/Rth = 2/3/1/3 = 2*Rth (if voltage on Rth is equal 1/3 the voltage on R1 mus be 2/3 of a supply voltages).
Or with voltage divider equation
Vout = Rth / ( R1 + Rth ) * Vcc ---> R1 = (Rth* Vcc - Rth* Vout)/Vout= ( Vcc/Vout - 1) *Rth = (1/0.333 - 1 )*Rth = 2*Rth
So your digram is not quite right.

And real working circuit should look something like this

 

Using the thermistor characterized in the data sheet, and a 555 integrated circuit as a threshold detector, design a circuit to light an LED when the tempreature exceeds 80°C. Draw a circuit diagram with component values marked on it. Estimate the temperature at which the LED would turn off (correct answer attached).

Now what I want to know is:

*How do you know that the supply rail = 9V?

*How do you calculate R = 8K?

I know that you are suppose to use Vout = V.Rth / Rth + R but only knew that Rth ≈ 3.8k when the temperature = 80°C.

Is the answer you posted your attempt at the answer, or is it the actual answer as given by the person or book that posed the problem?

 

Do you want the LED to flash above 80oC or to stay lit?

To say lit.

Is the answer you posted your attempt at the answer, or is it the actual answer as given by the person or book that posed the problem?

Its the actual answer given.