# Calculating voltage divider bias, using a thermistor for transistor circuit

#### Cretin

Joined Dec 13, 2012
69
hey all, i have been working on this problem for days now...and it should not be this hard but i can not get it to work.

I need to design a simple circuit so that when a thermistor is heated with a lighter, its resistance drops and creates a bias voltage of at least .7V volts through an npn transistor, and then this transistor lights an LED.

Could someone please explain this circuit, how it works, and perhaps even draw a simple schematic? I can only thank you so so much.

I can't even get the right resistor value, but here are the values you have

Resistance of thermistor when at room temperature: 4500kohms,
Resistance of thermistor when heated up: 3500 kohms.

Resistance of collector: 1000 ohms

VCC: 5 volts

I need to find R2, please help lead me through a voltage divider calculation and please help me draw out the schematic. I've spent way too much time on this

#### Dodgydave

Joined Jun 22, 2012
10,319

#### Cretin

Joined Dec 13, 2012
69
No Dave, it's part of a side project im working on

#### Jony130

Joined Feb 17, 2009
5,337
Hmm

4500Komh = 4.5Mohm ??

Or maybe 4500ohm = 4.5Kohm ?

#### Cretin

Joined Dec 13, 2012
69
sorry jony, you are correct, 4500ohms = 4.5khm not megaohm...my apologies

#### Jony130

Joined Feb 17, 2009
5,337
Hi,
First what you need to understand is that BJT transistor don't have "trigger" voltage which turn-ON the transistor instantly .
In reality BJT start to conduct the current from about 0.5V voltages present between base and emitter (Vbe). And BJT is "Full-ON" when Vbe > 0.6V.

But I would encourage you to try to build this circuit on the breadboard.

Where RT is yours NTC thermistor.
And we can find R1 value quite easily. We simply can use a voltage divider equation.
You want for 4.5K BJT to be OFF. So we have

0.5V = 5V * (R1)/(R1 + 4.5K)

0.5 = (5 * R1)/(R1 + 4.5K)

0.5*(R1 + 4.5K) = 5R1

5R1 = 0.5*(R1 + 4.5K)

5R1 = 0.5R1 + 2.25K

4.5R1 = 2.25K

R1 = 2.25K/4.5 = 500Ω

So we need to use a 510Ω resistor.

But in real life we a force to use a potentiometer instead of a R1 resistor.
Why we need a POT? Well the answer is simple.
Even if you just have a bucket full of transistors of the same type (2N2222). Each of them will have a slightly different turn-ON voltage.
So we need to use a 4.7K POT and then on the on the bench trim the POT.
Until we get the desired result.

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