I have a 9v solenoid valve and an ESP32 development board. I want to control it by using a relay. I tried to do that using a 12v relay but nothing happens (and it got hot). I have read online that you need a flyback diode and a transistor to control the relay and the valve. I'm not sure what to buy. My goal is to create an IoT device that turns on and off a solenoid valve. What do I need? (I don't have an electric diagram)
What got hot? The relay? The valve? The ESP32?
Please post a schematic and links to the specs of all 3 of those things you mentioned.
The relay got hot but that's not important. It doesn't work when it is working fine and cool. When I try to change the relay state, it doesn't make a click sound and the valve doesn't turn on either.
The specified relay coil voltage is independent from its contact voltage and current. You have to feed the relay by its specified coil voltage. Add flyback diodes to both the relay and load.
What to buy depends on for example relay coil current & voltage. Those will help you determine which transistor to drive your relay with. What relay exactly? Next, many visitors are calling a complete relay module just a relay. A Relay Module looks like this, some have a single relay and some multiple relays. Modules like this come in a variety of voltages as to relay coils. Next these are plain relays and like the complete modules come in assorted coil voltages. The latter require a flyback diode. In the forum General Electronics section I suggest you read a little about Flyback Diodes and why you need them.
Now what exactly do you have and what are you trying to do?
Ron
Why not drive the valve directly by the transistor?
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Now that's a good question. I never thought to ask. 
Ron
Assuming the valve is 9 volt DC, how much current (Amps) does it draw?
Ohm's law will tell you: I=U/R
Measure the coil Resistance and you'll know...
What is the value of R?
This was answered in another post, he had the battery across the relay contacts. Here is the link: Relay got really hot.
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