hi everyone
im trying to get into doing some more tinkering with electronics like i used to a few years ago. i would like to build a circuit but need some assistance and would be greatful for any help or a finger point where i can read up what i need to learn.

basically i want to build a circuit that measures a temperature and then once a cirtain temperature is reach, an LED will come on. this is simple but ive not done anything like this since school/college
so, the temperature sensor is 12v, and all power that is available is 12v,
i would like the point that the led comes one to be adjustable.
the temperature reader is an auto one but still a thermistor right? more heat = less resistance?

once i have done this and tested it working, i would like to place a test button for the led, and hopefully improve the design so that the led starts flashing slowly at a lower temp and more quickly as the temp raises.

the problem i can see is that the temperature sensor will start to earth the 12v feed as it heats up (making a temperature gauge show an analagoue reading. i didnt want to power this circuit off this 12v source anyhow.

thanks for any help

 

hi,
If you dont want to use +12V as a supply, what supply do you want to use?

Normally the thermistor is connected in series with a suitable resistor.
The voltage drop across the thermistor or resistor is connected into a transistor [amplifier], the output of the transistor drives the LED.
If you use a flashing LED [FLED] its easier.

Does this help?

__________________
Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/

 

ah, a flashing led will be a lot of help. forgot about those. may just use them from the start.

sorry, i meant i didnt want to use the 12v feeding the temperature sensor to power the led as well. i can wire a wire straight to the car battery, and i can wire an earth straight to the battery as well, or chassis.
the thing is, because the thermister earths through the engine, im not going to be able to measure that. but i could measure the voltage drop from the wire before it as it starts to heat up?
i tried doing this on a simulator program and didnt get anything good. the simulator isnt all that though, it will let me run a standard LED of a 240v AC supply and say all is ok.

 

hi,
Are you saying the thermistor is mounted in a car engine, with one side of the thermistor connected to the car chassis?

One way is to connect a suitable resistor is series with input [non chassis] side of the thermistor and the car battery and to measure the voltage drop across the resistor. The voltage drop will a function of the thermistors temperature.

When working with car batteries take care, any connection you make to the battery should be fused.

Use a ohm meter to carry out resistance measurements of the thermistor when its NOT connected to the battery.
When you have an idea when the resistance range is over a range of engine temperatures you can calculate the value of the series resistor you require.

__________________
Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/

 

do you suggest any simulator or somwhere i can go to get help with a plan of circuit design?

 

hi,

i suggest to use Red-Green LED ( http://futurlec.com/LED/LED5RG.shtml ) and using a simple comparator circuit you may set it to give you GREEN for Cold Temp, ORNANGE for Normal and RED for hot. ( ORANGE when both RED and GREEN let )

__________________
Its what your friend in Your mind, what you in your friends mind

 

nice idea, but what im after is something to shock the person in to noticing. so this is going to mounted right in eye view. if a led is on all the time it wont have the same effect. and will blind at night! lol

 

hi,

do you suggest any simulator or somwhere i can go to get help with a plan of circuit design?

Measure and let us know the hot and cold resistance of the thermistor.

Let us know what you measure.

__________________
Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/

 

ok i will get this done at tthe weekend
thank you for your help today