help with circuit plan

dieselboy said:

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

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dieselboy said:

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.

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ericgibbs said:

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

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. good idea

When working with car batteries take care, any connection you make to the battery should be fused. another good idea: noted

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.
i was going to get some water, get it warm and drop the thermistor in there and work out what resistance it was to what temperature = the light to come on.

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aljamri said:

hi,

i suggest to use Red-Green LED ( https://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 )

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