Design a solenoid controlled valve. The solenoid has a coil resistance of 280 Ohms. Although the coil is rated for 12-14V, it will reliably open the valve with 11.0 V applied and will keep the valve open until the voltage applied drops to less than 1V.
Available components: a MOSFET, a diode, resistors, and push-button SPDT switch rated for 10mA at 24V.
Design requirements: The solenoid opens the valve when you press the button and closes when you release the button.
My question:
1) I found the design below online. Could someone explain how that works?
2) Could someone explain how the SPDT switch works and where I could place it in my circuit?
enter image description here

  • \$\begingroup\$ its not a question. \$\endgroup\$ – drtechno May 3 '18 at 16:17
  • \$\begingroup\$ What is your question? \$\endgroup\$ – AlmostDone May 3 '18 at 16:18
  • \$\begingroup\$ I just edited my question. Sorry this is my first time to use this forum \$\endgroup\$ – ABC May 3 '18 at 16:29
  • \$\begingroup\$ Where in the datasheet does it say that it can operate at 11v in lieu of 12v? You also forget that solenoid valves also need incoming pressure to operate as well. Also, I wouldn't call this a SPDT switch.... more like a SPST unless you're suggesting that you're somehow utilizing the emitter but why would you? \$\endgroup\$ – KingDuken May 3 '18 at 16:50
Could someone explain how that works?

No, since it doesn't.

When the switch is on, it ensures the transistor is off.

When the switch is off, the transistor base is just floating. There is nothing positively turning it on.

One way to fix this is to put the pushbutton between Vcc and the left end of Rb. There should also be a resistor to ground then at the left end of Rb. In that case, pushing the button supplies base current to the transistor, which then allows more collector current to turn on the solenoid. When the pushbutton is off, the additional resistor from the left end of Rb to ground ensures that the transistor is solidly off, despite possible ambient noise.

All that said, this solenoid only draws 43 mA at 12 V. A simpler circuit is to use a suitable pusbutton directly to switch the solenoid. Any momentary normally-open switch that can handle 50 mA would do.

  • \$\begingroup\$ Could you go into more details how the diode helps the design? \$\endgroup\$ – ABC May 3 '18 at 16:56

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.