Vehicle coolant temperature gauge

[tblo163]

I've been off the forum for a while,been that busy fixing computers for others,that I've been neglecting my own projects,ok, here's what I want to do:-build a circuit board to operatate like traffic lights,using leds, eg.,green as the system warms up,amber as the system reaches the normal operating temp., red when it overheats.The temerature sender on the car simply varies the resistance which would normally control the usual temerature gauge,it starts at 25K when cold and decreases as the engine warms up.Initial tests using multimeter and thermometer shows that around 90 degrees C the reading is 4K and at 101 degrees it shows 2K. So I need the green to change to amber at 4K & then change to red at 2K. Any ideas?

 

[Dx3]

Use a window comparator.

 

[Sceadwian]

Use a second resistor to act as a voltage divider and use comparators to trigger the LED's at the specific voltages.
Using a 4k resistor with the 4k value for 'warm' you'd have half supply voltage, with a 4k resistor and the 2k value for 'hot' you'd get 1/4 the supply voltage. Feed a comparator a 4k/4k resistor divider and a 4k/2k resistor divider for the hot comparator reference and you'd get the desired results regardless of the supply voltage as it's ratiometric, you'd only have to keep the system voltage in the range the comparator works in and prevent the LED from burning out under worst case conditions with a current limit resistor.

Does that make sense to you?

 

[tblo163]

Yes,I can understand the principle and will see if I find a data sheet for the IC,(I love wiring diagrams) I was also thinking of a simpler circuit,green stays on all the time,to indicate that the "gauge" was on,blue lights up at 4K and stays on and then red lights up at 2k and possibly trim pots to callibrate. Thanks for the reply.

 

[crutschow]

You could also use the **broken link removed** Dot/Bar Display Driver. The input could be from a voltage divider as suggested by Sceadwian. Just use only the LED outputs you need to get the desired indications. You can parallel the outputs to one LED if you want it to be lit over a range of voltage.

 

[tblo163]

OK.Thanks,I'll download the data sheet for the LM3914 and look at the prospects.

 

[Reloadron]

Something you want to keep in mind is if you work with an automotive application you need regulation. Especially if you go with a comparator design. Your 12 volts can be between 8 volts when cranking (not that it matters) and 14.5 volts running.

Ron

 

[tblo163]

Yes,I would use a volage regulator or stabliser.and am also considering using 12v.1.2w dash board bulbs,they have less chance of failing.