Hi Nourtamer,
There is a project on the web that measures the moisure of a potted plant's soil. It is supposed to dim its LED when the soil is moist then brighten the LED gradually as it dries. Many people complained that it didn't dim, its LED was on dimly or off. They said it didn't work with some ICs with a 3V battery. With an IC that worked at 3V, it stopped when the battery voltage dropped a little as it was used. Its battery didn't last very long.
I fixed its lack of dimming by adding a capacitor and a resistor so it dimmed using PWM. I fixed its problems of a low battery voltage by using a 74HCxx IC. I made its LED much brighter when it wasn't dimmed by adding two transistors. I added a blinking feature so a smaller battery lasts a very long time.
You could find a measuring project on the web that doesn't work, then fix it like I did.