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Circuit Diagram

In the above circuit, the transistor is an NPN 2N2222 model. The RC resistor is actually a very small water pump (which I believe is a DC motor) that uses about 60mA across a 5V potential difference, hence the 85 Ohm resistor to represent it.

My issue is that when I set this up in real life with a Raspberry Pi supplying the 5V and 3.3V from its pins, the motor does not turn on.

Sometimes if I power the base then disconnect and reconnect the power, it briefly turns on.

Wierdly, when I measure the current going through the pump (with the base pin high), it increases to about 240mA then goes to 0. If I disconnect and reconnect the power this happens again.

I'm quite new to transistors so would be grateful for any help possible. If changes need to be made to my circuit I would be grateful to end up with as simple circuit as possible with as few transistors as possible as well.

Many thanks in advance!

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    \$\begingroup\$ We generally draw schematics with ground at the bottom and supplies at the top. There doesn't seem to be anything wrong in principle with your circuit, though I would use a logic-level FET and not a bipolar. You will also need a flyback diode across the motor coil, could be you are damaging your transistor with the inductive kick on turn-off. Also make sure you aren't reversing the emitter and collector. \$\endgroup\$
    – John D
    Dec 6, 2016 at 22:27
  • \$\begingroup\$ Thanks, I'm not very experienced with drawing schematics. I did originally mix up the base and emitter but have definitely sorted that out now. I considered putting a diode in but surely if it works without a diode without the transistor it should work the same way with the transistor too? \$\endgroup\$
    – Zak
    Dec 6, 2016 at 22:30
  • \$\begingroup\$ Did you connect the grounds? hackingmajenkoblog.wordpress.com/2016/12/06/… \$\endgroup\$
    – Majenko
    Dec 6, 2016 at 22:32
  • \$\begingroup\$ @Majenko Yes, I did \$\endgroup\$
    – Zak
    Dec 6, 2016 at 22:33
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    \$\begingroup\$ You can a) reduce the value of your base resistor to allow more current through. b) replace the BJT with a logic level FET. Either way you want to add the diode connected backwards across the motor to absorb any back-EMF generated by the coils. \$\endgroup\$
    – Majenko
    Dec 6, 2016 at 23:48

1 Answer 1

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I think your switch is not strong enough for the starting current required by the motor. try reducing the base resistor to 330, or boost the base current with an emitter follower.

schematic

simulate this circuit – Schematic created using CircuitLab

THe mosfet suggested by others is also an option.

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