user43674: I know very little about Arduino and even less about programming one. Before I purchase a board, and learn how to program it, I was wondering if my idea is feasible.
I want to program the Arduino to sequentially open one after another irrigation valve. There would be approximately eight 24 volt solenoid valves. Each one would need to be open for approximately 15 minutes, then open the next valve in sequence and finally close the previous valve. Similar to a multi-zone irrigation controller.
I see that I may need to use a transistor in the power circuit of the solenoid. But it is not clear to me if the Arduino can control eight solenoids and if so, does each solenoid require its own transistor?

user42539: Yes, Arduino can control eight transistors.
Each solenoid will need a transistor.
Ideal would be to use logic level N-channel MOSFETs that the Arduino 0/5V output can drive directly.  IRFL540 is mentioned a lot, you can get them at digikey.com.
Drive the gate thru a 150 ohm resistor, and have a 10K resistor from the gate to Gnd so the transistor is off while the Arduino is reset (and the IO pins all revert to Inputs).
Depending on the current draw, you may able to use a shift register with Open Drain outputs. TPIC6C595, 100mA. TPIC6B595, 150mA. TPIC6595, and TPIC6A595 are even higher current capable parts.
Add a diode with the same current rating across the solenoid coil, cathode to +24V, so the burst of current generated by the coil when the transistor closes has somewhere to dissipate. (current in a coil wants to keep flowing as the magnetic field in the coil collapses, it can't go thru the turned-off transistor, so it goes thru the coil and dissipates in the coil's wire-resistance).

user29270: An alternative method is to use an I2c port expander such as a PCF8574.  These are available as break out boards.  By selecting a different I2C address on each break out board up to 64 relays can be controlled using opto isolated relay modules.  Thus 64 0utputs or inputs with just two Arduino pins.

user42135: An Arduino board is merely a microcontroller with the stuff around it that is necessary to keep it running. By stufff, I mean thinks such as the voltage regulator that provides the power, the oscillator that provides the frequency for the controller, the GPIO pin connectors, and so on.
This is a very simple application and could easily be done by an Arduino. The programming doesn't have to run quickly. You are only doing one thing at a time.
I found an 8-channel relay module that should do what you need. You would need four of them, but they are only about $11 apiece. I have never used this device before, so I don't know how well it works, but the specifications seem right. They can be controlled by almost any computer with output pins that have 5 volts. So you'd need to use an Arduino with 5V output pins.
You will probably need more output pins than a standard Arduino, but that is easy enough to do with the Mega version.

user43684: Yes, you will need something to drive 24 volt solenoids, because the Arduino cannot do it alone.
My preferred method would be to use a ULN2803 device, which will allow you to drive all those eight solenoids (assuming they take less than 500mA or current) and has inbuilt protection against the voltage spike when the solenoid switches off.
http://www.ti.com/lit/ds/symlink/uln2803a.pdf
These devices are available on eBay.
The software will be quite simple for this application and should be well within the capabilities of anyone who reads an Arduino tutorial.

user43703: There are 2 wires to a solenoid, and 2 ways to operate them:
1- Closing a relay sends 24V out, with return wires going to 0 volts. 
2- 24V Always goes out, and a relay can short the return wire to 0 volts. 
Either way, test 2 by running the wires to a control board, and manually activating solenoids. When that works, add Arduino relay control as other suggest. 
Keeping the switches will help, if you want to operate a solenoid manually OR you suspect you have a broken relay / solenoid.

user10147: Most irrigation valves are AC operated, typically using 24VAC at about 250mA (Double that to 500mA for the inrush current).  The best way to drive these is via a triac.  If you want isolation between your controller and the valve voltage an opto coupler with a triac output is the best way to go.  A good choice given the typical irrigation valve is the Vishay VO3023:  5mA LED input current and 1A output current and only about $0.50 per unit.  Triacs are pretty tough devices and have no moving parts like relays so they are quite reliable.

user31105: An Arduino can control (at least) 8 devices. Instead of a transistor, it might be much easier to use an 8 way relay controller, something like this:
8 relay module
Just connect all 8 valves to the relays, you power the relay with a separate source for the valves, and connect the pins to the Arduino.
These ones have opto couplers, which give an extra security between the valves and the Arduino. 
8 relay module with opto couplers
(note I just put the first entry I found, you might find better/cheaper similar items, e.g. at AliExpress for $4).
As Andre said in a comment below, you can use an Arduino Mega if you need more outputs. Or if you want a challenge you can use use a multiplexer IC like 74HC595 (plenty of examples to be found with Arduino).
For controlling one relay with an Arduino enough examples can be found, for 8 it's similar (just use 8 GPIOs), or a multiplexer IC.

43674: <p>I know very little about Arduino and even less about programming one. Before I purchase a board, and learn how to program it, I was wondering if my idea is feasible.</p>

<p>I want to program the Arduino to sequentially open one after another irrigation valve. There would be approximately eight 24 volt solenoid valves. Each one would need to be open for approximately 15 minutes, then open the next valve in sequence and finally close the previous valve. Similar to a multi-zone irrigation controller.</p>

<p>I see that I may need to use a transistor in the power circuit of the solenoid. But it is not clear to me if the Arduino can control eight solenoids and if so, does each solenoid require its own transistor?</p>
 42539: <p>Yes, Arduino can control eight transistors.</p>

<p>Each solenoid will need a transistor.</p>

<p>Ideal would be to use logic level N-channel MOSFETs that the Arduino 0/5V output can drive directly.  IRFL540 is mentioned a lot, you can get them at digikey.com.
Drive the gate thru a 150 ohm resistor, and have a 10K resistor from the gate to Gnd so the transistor is off while the Arduino is reset (and the IO pins all revert to Inputs).</p>

<p>Depending on the current draw, you may able to use a shift register with Open Drain outputs. TPIC6C595, 100mA. TPIC6B595, 150mA. TPIC6595, and TPIC6A595 are even higher current capable parts.</p>

<p>Add a diode with the same current rating across the solenoid coil, cathode to +24V, so the burst of current generated by the coil when the transistor closes has somewhere to dissipate. (current in a coil wants to keep flowing as the magnetic field in the coil collapses, it can't go thru the turned-off transistor, so it goes thru the coil and dissipates in the coil's wire-resistance).</p>
 29270: <p>An alternative method is to use an I2c port expander such as a PCF8574.  These are available as break out boards.  By selecting a different I2C address on each break out board up to 64 relays can be controlled using opto isolated relay modules.  Thus 64 0utputs or inputs with just two Arduino pins.</p>
 42135: <p>An Arduino board is merely a microcontroller with the stuff around it that is necessary to keep it running. By stufff, I mean thinks such as the voltage regulator that provides the power, the oscillator that provides the frequency for the controller, the GPIO pin connectors, and so on.</p>

<p>This is a very simple application and could easily be done by an Arduino. The programming doesn't have to run quickly. You are only doing one thing at a time.</p>

<p>I found an <a href="https://rads.stackoverflow.com/amzn/click/B01ARS8OVQ" rel="nofollow noreferrer">8-channel relay module</a> that should do what you need. You would need four of them, but they are only about $11 apiece. I have never used this device before, so I don't know how well it works, but the specifications seem right. They can be controlled by almost any computer with output pins that have 5 volts. So you'd need to use an Arduino with 5V output pins.</p>

<p>You will probably need more output pins than a standard Arduino, but that is easy enough to do with the Mega version.</p>
 43684: <p>Yes, you will need something to drive 24 volt solenoids, because the Arduino cannot do it alone.</p>

<p>My preferred method would be to use a ULN2803 device, which will allow you to drive all those eight solenoids (assuming they take less than 500mA or current) and has inbuilt protection against the voltage spike when the solenoid switches off.</p>

<p><a href="http://www.ti.com/lit/ds/symlink/uln2803a.pdf" rel="nofollow noreferrer">http://www.ti.com/lit/ds/symlink/uln2803a.pdf</a></p>

<p>These devices are available on eBay.</p>

<p>The software will be quite simple for this application and should be well within the capabilities of anyone who reads an Arduino tutorial.</p>
 43703: <p>There are 2 wires to a solenoid, and 2 ways to operate them:</p>

<p>1- Closing a relay sends 24V out, with return wires going to 0 volts. </p>

<p>2- 24V Always goes out, and a relay can short the return wire to 0 volts. </p>

<p>Either way, test 2 by running the wires to a control board, and manually activating solenoids. When that works, add Arduino relay control as other suggest. </p>

<p>Keeping the switches will help, if you want to operate a solenoid manually OR you suspect you have a broken relay / solenoid.</p>
 10147: <p>Most irrigation valves are AC operated, typically using 24VAC at about 250mA (Double that to 500mA for the inrush current).  The best way to drive these is via a triac.  If you want isolation between your controller and the valve voltage an opto coupler with a triac output is the best way to go.  A good choice given the typical irrigation valve is the Vishay VO3023:  5mA LED input current and 1A output current and only about $0.50 per unit.  Triacs are pretty tough devices and have no moving parts like relays so they are quite reliable.</p>
 31105: <p>An Arduino can control (at least) 8 devices. Instead of a transistor, it might be much easier to use an 8 way relay controller, something like this:</p>

<p><a href="https://rads.stackoverflow.com/amzn/click/B0057OC5WK" rel="noreferrer">8 relay module</a></p>

<p>Just connect all 8 valves to the relays, you power the relay with a separate source for the valves, and connect the pins to the Arduino.</p>

<p>These ones have opto couplers, which give an extra security between the valves and the Arduino. </p>

<p><a href="https://www.elegoo.com/product/elegoo-8-channel-dc-5v-relay-module-with-optocoupler/" rel="noreferrer">8 relay module with opto couplers</a>
(note I just put the first entry I found, you might find better/cheaper similar items, e.g. at AliExpress for $4).</p>

<p>As Andre said in a comment below, you can use an Arduino Mega if you need more outputs. Or if you want a challenge you can use use a multiplexer IC like 74HC595 (plenty of examples to be found with Arduino).</p>

<p>For controlling one relay with an Arduino enough examples can be found, for 8 it's similar (just use 8 GPIOs), or a multiplexer IC.</p>
