I have this shematic. With one electrical Pump and two solenoid valves I want to control via the arduino.

Here are the specs for the parts:

  • Pump: 12V max. 250mA
  • Valve 1: 12V max. 160mA
  • Valve 2: 12V max 220mA
  • Source: 12V max 2A


simulate this circuit – Schematic created using CircuitLab

My questions is:

Are the 4.7k resistors enough? Do I need different resistors for the different consumers? And I am using S8050 as transistors. Are they to small for the load? And where do I have to connect the ground of the arduino to?

  • I think you mean S8050 as transistors and in the circuit I see 2N3904's – Michel Keijzers Oct 7 at 10:56
  • 1
    Thanks. I corrected it. – Trojan Oct 7 at 10:57
  • Connect the Arduino Gnd to the 12V supply Gnd. From what I see here, digikey.com/products/en/discrete-semiconductor-products/… most S8050 parts need 80mA or more to turn into saturation - which is more than an Arduino can supply. You'd be better off with N-Channel MOSFET, which takes little drive current, and with low Rds will have little voltage across the Drain-Source. Example: digikey.com/products/en/discrete-semiconductor-products/… – CrossRoads Oct 7 at 12:29
  • (5v/4700r) * 100hfe = 0.106A; not enough current for any of them. Try a 330R: good for 1.5A (or still 600ma at the worst-case 40hfe), while only pulling 15ma from the MCU. You can go up to 680r... – dandavis Oct 7 at 19:29

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