Update - 22.01.2020

Here is my schematic. I tried to use all of the comments guys below:

New Schematic

  • Should I use a fly-back diode as commented after I cleared the schematic? If yes, where?

  • Is it required to protect the Arduino or any part of the circuit in this diagram?

  • If the solenoid used 24V DC in this diagram, will 24V DC reach my Nano and damage it? If yes how to protect it?

  • Is the circuit protected from using both way to open the solenoid accidentally by finger print and the push button of the headset? If no how to do that?

Here is the post before update to the schematic:

I am a newbie to electronics, but I did read for a long time and search but this is my first circuit it may look easy for you but I need help.

Here are two schematics and I need them to be joined together with op protection and reverse voltage protection as well.

Here is the Schematic

The project is a solenoid door lock I will add a finger print module operated with Arduino Nano to open it.

In fact, the solenoid is operated by normal doorbell intercom, so the project here is to make both ways to open the solenoid the fingerprint module and the intercom.

Please note:

  • The solenoid needs 1.8amp to open.
  • Reverse voltage protection due to the power will come to Arduino when solenoid opened from the intercom.
  • I have a TIP122 transistor if needed.
  • Arduino Nano will be used in the project.
  • Pin 9 or any will be used to send a signal to the transistor to pass the power required to open the solenoid.
  • Over 12V protection for Arduino Nano if dual input power source will be used.

The scenario:

  • Normal intercom button should open the solenoid.

    1. Fingerprint module read the fingerprint
    2. Arduino Nano received the data approved then send a signal from pin 9 to transistor TIP122 to open
    3. Transistor TIP122 opens and passes the current to open the solenoid.
    4. The finger print module goes into sleep mode if no activity for specific time.

Please try to consider the size of the circuit if possible need to be a small or good size to be placed behind the door of the house.

Edited 19.01.2020:"

I need to know how to connect these both isolated circuit to do one work which is open the solenoid without conflict with each other if both power used intermediately?

  • \$\begingroup\$ What exactly is your question? Please, edit the post to actually have a question. \$\endgroup\$
    – jDAQ
    Jan 10, 2020 at 8:14
  • \$\begingroup\$ Also, is that "handset" some momentary button? \$\endgroup\$
    – jDAQ
    Jan 10, 2020 at 8:21
  • \$\begingroup\$ iIs this an assignment or something you wish to build for yourself. \$\endgroup\$
    – Russell McMahon
    Jan 10, 2020 at 8:42
  • \$\begingroup\$ See if there is any help from these image links to design ideas. google.com/search?q=arduino+solenoid+driver&&tbm=isch \$\endgroup\$
    – KalleMP
    Jan 10, 2020 at 10:08

1 Answer 1


Considering you want the solenoid to be activated by the transistor OR the button (which you call the "handset"), this is what you need to do.


simulate this circuit – Schematic created using CircuitLab


  • I just pointed the circuit topology and you should pick the proper diode and transistor to fit your needs.
  • 1
    \$\begingroup\$ With a ~2A solenoid the 1N4148 also might be a little light (perhaps literally). And the transistor. Better use a MOSFET or Darlington. \$\endgroup\$ Jan 22, 2020 at 14:47

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