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I have set up solenoid on of based on soil moisture, The solenoid is powered by 24 volts, so I power solenoid by 24 volt and Arduino uno by stepping down to 6 volt powering through Vin and gnd.

I have used resistance of 10k ohm but I have twice burnt em. Not sure how should calculate resistance required of signal coming out from D6 pin to TP120 base !

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    Is the 24 V a DC or an AC voltage? Can you draw a circuit diagram to clarify the connections? [Click the Circuit icon on the edit-box toolbar, if you don't have a schematics program to use] Note, edit the question in reply Dec 3 '16 at 6:30
  • 24 volt dc, I have added Fritzing Dec 3 '16 at 8:47
  • Your diagram shows nothing connected to one end of the diode - is that an omission? What have you burnt twice? The resistor?
    – Mark Smith
    Dec 3 '16 at 9:31
  • sorry fixed the fritzing, yea I am talking about resistor. Dec 3 '16 at 10:59
  • 1-2.2k comes up in a web search Dec 3 '16 at 11:36
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To calculate you need to know the maximum current the solenoid uses. The TP120 has a gain of around 500-1000. So the current going into the base needs to be 1/500th of the current going through the solenoid.

So now you know the current needed. Then you can calculate the resistance value you need with R = V/I. So R = 5 / ( <solenoidcurrent> / 500 ).

So with e.g. a 2A solenoid, you get a value of 1.2kOhm.

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