# My project needs YOUR help

#### brendyboy

Joined Dec 21, 2005
27

Im working on a project which needs a temperature control circuit to activate a relay. The project is due in next week so i desperately desperately need someones help!!!! please please please!!

http://www.electronic-kits-and-projects.co...s/1000/1048.pdf is a link to the info sheet of the circuit i have purchased. However the temperature range of this circuit is 20-80 degrees and i need it within 2-8.
I heard that i only need to change the variable resistor (is that also called a potentiometer?) to a 4.7k. Would that work?
Basically i need the relay to switch when the temperature gets to about 8 degrees and then switch back when it gets around 4.

Please also understand that I do not wish to over complecate it as I know nothing about electronics (wish i listened in class now) so im after the simplest solution!!

Would really appreciate any help!!
Many thanks
Brendan

#### brendyboy

Joined Dec 21, 2005
27
Forgot to mention that the thermistor is negative coefficient

at room temp its 1k
at 0 degrees its 2.7k

if that helps

#### mozikluv

Joined Jan 22, 2004
1,435
Originally posted by brendyboy@Apr 19 2006, 11:21 AM
Forgot to mention that the thermistor is negative coefficient

at room temp its 1k
at 0 degrees its 2.7k

if that helps
[post=16322]Quoted post[/post]​

hi

why don't you try it, just use a linear pot.

moz

#### brendyboy

Joined Dec 21, 2005
27
Originally posted by mozikluv@Apr 22 2006, 08:13 AM
hi

why don't you try it, just use a linear pot.

moz
[post=16405]Quoted post[/post]​

Ive soldered the circuit up with a 4.7k variable resistor and it seems to be doing something (the relay clicks when i turn the variable rsistor. But there are three pins connected to the relay, and im not sure which to connec the the fan to as the fans polarised.

#### haider

Joined Apr 22, 2006
1

Thanks.

#### mozikluv

Joined Jan 22, 2004
1,435
hi,

you said your fan is polarized, so just connect the negative side to ground. as to the relay based on the schem, connect your supply (positive) to pin 4 and the positive side of the fan to pin 5. pin 3 is the Normally Closed Terminal and pin 5 is the Normally Open terminal. pin 1 is connected to the collector of Q3, and pin 2 is connected to ground.

if you want to know w/c is pin 1 & 2, use an analog multimeter. set you meter to x10 & be sure to set it first to zero ohms. between any two terminal, you will have these kind of readings;

pin3 & 5 - infinite or no deflection at all
pin1 & 2 - ohmic reading of 60ohms to 120ohms (coil resistance)
pin 4 & 3 - 0 ohm
pin 4 & 5 - no deflection
pin 4 & 1 or 2 - no deflection

hope this helps

moz