Homebrew PCBs

I have lost the schematic. I tried to find it when the question came
up a few days ago but can't find it!
There was even a guy in Brazil(?) which duplicated it and made a
webpage, can't find that either.

Anyway you make a voltage divider with a potentiometer and thermistor,
connected between the supply and ground.
The base of the transistor goes in the middle node (output of voltage
divider). Emitter is ground and collector is output (drive relay like
you do).

Depending on wheter you have a PTC or NTC thermistor you need to put
it towards ground or positive supply, the potentiometer vice-versa. If
the potentiometer is towards positive you want to add a series
resistor in case it is adjusted to zero.

When the voltage on the base reaches ~0.7V the transistor turns on.
You can even add hysteresis  by putting a resistor from collector to
base (high value), same as you use R4.

Of course this all depends on supply voltage stability and transistor
junction temperature and... BUT all this seems no problem it works
just fine for years now.

ST


On Thu, Dec 24, 2009 at 6:42 PM, saposoft <sapoliste@...> wrote:
> Well Well this is fantastic ... care to share the schematic ?
>
> --- In Homebrew_PCBs@yahoogroups.com, Stefan Trethan <stefan_trethan@...> wrote:
>>
>> I used a thermistor, and a BC547 transisor in place of the ua741.
>> ICs are way too expensive to waste on a thermostat ;-)
>>
>> ST
>>
>> On Thu, Dec 24, 2009 at 6:31 PM, saposoft <sapoliste@...> wrote:
>> > I forgot the most important part !!
>> > Where it is !
>> > http://sites.google.com/site/elettronicaarduinoesperimenti/Home/thermostat-with-ua-741
>> >
>> >
>> >
>>
>
>
>
>
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