# LM335 to control 555 timer

Discussion in 'Homework Help' started by Elbobbo, Jun 3, 2017.

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1. ### ElbobboNew Member

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Hey,
For my project I am trying to use a LM335Z to control the duty cycle of the output of a 555 timer.
So the duty cycle output of the 555 timer is 5 to 95% is this possible?
I had tried to use a thermistor to control the 555 timer to get a 5 to 95% duty cycle, with a 50% duty cycle at 25 degrees C, but to no success.
Is there any other way of gaining a PWM of 5 to 95% with a thermistor or a temperature sensor like the LM335Z?

The end goal is to output this signal to a set comparators at opposite polarity to then drive 4 MOSFETs (2 p and to n channel) in a H-Bridge with a 12vDC motor in the centre.
So the temperature dives the motor to either suck hot air in warming or draw cool are out cooling the room to 25 degrees C
Cheers

2. ### crutschowWell-Known MemberMost Helpful Member

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What temperatures for the 5% and 95% duty-cycles?

3. ### alec_tWell-Known MemberMost Helpful Member

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Controlling the 555 itself won't give you a 5% to 95% range IMO. Better to use the 555 to generate an approximate triangle wave which is then compared with a reference voltage to give the PWM signal.

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5. ### ElbobboNew Member

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5% = 15 degrees C
95% = 35 degrees C
Yeah i am now think of using a LM355 into the circuit.
http://www.kitsrus.com/pdf/k166.pdf

6. ### crutschowWell-Known MemberMost Helpful Member

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Here's a 555 with a comparator to do the PWM as alec suggested.
You replace the pot at the comparator input with an amplified and offset voltage from the LM335 to get the desired variation in duty-cycle with temperature.