Hi,
I am trying to save energy on my project by disabling analog sensors when they are not used. I'm using digital output pins for this, but plan to change it later (maybe use transistors).
My problem now is setting a boot time for the sensors. How can I find out how long my app needs to wait before reading from the sensor after turning it on?
Also, if anyone has a suggestion about using transistors to turn sensors on and off, it would also be apretiated.
Check the manufacturer's datasheet for the sensors. You can almost always find it easily with a Google search for the sensor's part number. The datasheet should tell you if there is some required delay between powering the sensor on and reading it.
It will also tell you the maximum current draw of the sensor. As long as that is below the maximum current rating of the Arduino's digital pin (35 mA on the AVR-based Arduino boards like Uno, Nano, Mega, Leonardo, etc.), you are fine to just use a direct connection to the Arduino's digital pin to power it on and off. It's only if the current draw exceeds this (or if the sum of the current draw for all sensors on that port of the Arduino exceeds the port's maximum rating) that you need to use a transistor.
You might also check the quiescent current draw of the sensor to see if it's significant enough for your application to even make it worth turning the sensor off.
Thanks a lot 
You're welcome. I'm glad if I was able to be of assistance. Enjoy!
Per