if i have a variable input voltage (for example from solar) and I want the output voltage will be stable , what I can use?

 

Voltage regulator if powering load directly.

 

If you aren't worried about efficiency, you can use a linear regulator.
If you are, you could use a switching buck regulator.

 

if i have a variable input voltage (for example from solar) and I want the output voltage will be stable , what I can use?

The devil is in the details. A buck-boost regulator will give you the widest range of input variability, since the solar voltage could be below or above the load voltage. In a simple (cheap) arrangement, you may just use a linear or buck regulator to convert the higher panel voltage down t the load voltage. A solar panel makes so much less power when poorly lit, it's not worth trying to regulate the low end of its voltage range.

 

if i have a variable input voltage (for example from solar) and I want the output voltage will be stable , what I can use?

If you aren't talking about a lot of power, another option is to use a battery at the desired voltage and use a diode so that the solar cells dump charge into the battery whenever their lit strongly enough to do so. If you are concerned about potential overcharging, you could but a zener bypass circuit to dump the charge if the battery rises above a certain voltage. That would be a simple solution using only a couple of passive components.

 

I have a 5 watt 18 volt solar panel on my pavilion roof. It's used to power an ultrasonic bark controller. During the day when the dogs barking is a nuisance the 18 volts is regulated down through a buck converter to give me a regulated 10 volts. The ultrasonic bark controller operates on a 9 volt battery. As the sun diminishes in strength eventually there's not enough power to run the bark controller. Face it, at night if a dog needs to bark I don't want to inhibit that. My arrangement works nicely. Prior to this I used to have to stuff a new 9 volt battery in the device every two weeks. The cost of the small panel and the buck converter was less than the cost of continued replacement of batteries. And far more convenient too. With the battery, the controller would be active day and night. I would have had to build a light sensing circuit to switch it off at dusk, so I would have been spending money there too. But I'm happy with the arrangement I went with. Wasn't expensive, it's convenient, works during the day and silent during the night.

I used a solar panel and a buck converter. Don't know what you're doing but it might work for you.