While most cars have a temperature gauge alerting you of overheating, they fail to warn you of the issue in time to resolve it before the likelyhood of a broken engine. Today I will show you how to tell when your car is experiencing coolant loss by building and installing a low coolant indicator in the coolant reservoir. The idea is that the coolant reservoir will likely go empty before the radiator does and while the coolant level in the reservoir changes as you drive, an empty reservoir is a good indicator of a problem.
Coolant loss could be the result of any number of things including a bad water pump, bad gasket, or broken hose. This instructable does not cover troubleshooting the source of the problem however.
The indicator uses 2 LED's. A green LED flashes when there is coolant in the reservoir. Otherwise, a red LED flashes if it's low or empty on coolant. By choosing the right fuse in your car, the indicator is turned on only when the car is running or the key is in the "ON" position. The indicator is microcontroller based and it incorporates built-in hardware features called "sleep mode" and "watchdog timer" to periodically wake up, take a reading, flash the corresponding LED, and go back to sleep. This way, it's most efficient in power consumption while it's turned on.
Technically, this project uses an equivalent circuit to my "Electronic Diaper" project in that it uses a voltage divider connected to an ADC (analog to digital converter) pin.