See the very simplified schematic down. As what you can see: Two inputs are in the form of a DC signal. We operate each input individually. RL represents a motor. The two inputs are initialized by a micro controller. The transistor used is an NPN type (BJT) (or General Purpose one).

The problem is this: When we input one signal, the two motor spin while there should be only one of them spins.They affect each other. We do not want that.

Suggestions made by ourselves:

  1. Note that the signal is amplified by an op amp for each input's path. We used diodes for each input's path but it did not work.
  2. We tried to buffer the the power source rail but it did not work.
  3. We tried to create two separated power sources for each input (but with a one common ground and one micro controller) and it did not work.
  4. We put also diodes with each motor and it did not.

So, how can we isolate these two signals? Note that we are beginners with this.


simulate this circuit – Schematic created using CircuitLab

The physical circuit is this (I am pretty sure it looks messy) enter image description here

  • 2
    \$\begingroup\$ You have a serious problem with your schematic: in the center, you have +5V directly connected to ground! Obviously, this does not reflect your actual circuit as nothing would work. Please check and correct your schematic. Better yet, edit your question, press control-M, and use the schematic editor to create a better readable schematic. \$\endgroup\$ – DoxyLover Jul 12 '15 at 4:18
  • \$\begingroup\$ Can you add a photo of your circuit? Your schematic doesn't show any errors that would explain the problems you're having. Knowing exactly what is your source and the current requirements of your loads might also help. \$\endgroup\$ – The Photon Jul 12 '15 at 6:30
  • \$\begingroup\$ Also, whatever's producing the input signals---is it inherently current limited? Is there any connection between the circuits producing the two inputs? \$\endgroup\$ – The Photon Jul 12 '15 at 6:32
  • \$\begingroup\$ @The Photon Yes, both signals come from one single micro controller (I mentioned that in my question). But each one has its own pin.For the current: We are supposed to be supplied with 40 mA when producing a signal from the pins. Lastly, the motor is 5 ohms total. \$\endgroup\$ – Gold_Sky Jul 12 '15 at 7:00
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    \$\begingroup\$ Please post a photo of your physical circuit. Also, if you don't limit the current from the uC pin through the BJT bases somehow, you'll likely damage the uC eventually. \$\endgroup\$ – The Photon Jul 12 '15 at 15:33

You need to add flyback diodes and base resistors to your circuit, and you need something more substantial than a 2N2222 as a driver.

Also, if the opamps driving the bases can't source enough current to drive the transistors into saturation, you might want to consider using N MOSFETS instead of bipolars for the motor drivers.

Finally, if you're getting interaction between the two sides, it wouldn't hurt to stiffen up your 5V supply and make sure all your grounds are solid, or use a separate supply for the motors, still making sure all the grounds are adequately sized and solidly connected.

enter image description here

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  • \$\begingroup\$ would the NPN TIP 120 BJT be good relatively in case if we can't afford for the MOSFETS ones? \$\endgroup\$ – Gold_Sky Jul 12 '15 at 23:26
  • \$\begingroup\$ @Gold_Sky: Dunno... When it's loaded, how much current does a motor draw if it's connected directly to the 5 volt supply? And how about posting the rest of the circuit? \$\endgroup\$ – EM Fields Jul 13 '15 at 0:23
  • \$\begingroup\$ @ EM Fields I just used TIP 120. It seems more better. The results are more consistent now. More than that, I created two different circuits for each signal. The only thing that combines them is the source of the signal AND IT WORKS. IT WORKS. But, it is still inefficient in terms of size and resources used (like two power sources!!! and two breadboards). \$\endgroup\$ – Gold_Sky Jul 13 '15 at 7:46
  • \$\begingroup\$ @Gold_Sky: YAY! How's chances for an as-built schematic and some motor part numbers or links, please? \$\endgroup\$ – EM Fields Jul 13 '15 at 11:36
  • \$\begingroup\$ I will. Now: you suggested that using N MOSFETS instead of the op amps. But, can they be switched with 40 mV? And if they can't, how can we make the UNO arduino to provide a signal like 5 volts instead of just that small 40mV? \$\endgroup\$ – Gold_Sky Jul 17 '15 at 1:57

The problem isn't in the schematic provided. Something is causing voltage from one input to affect the other. If you're using op amps to buffer the signal, and are powering those from the same +5v supply, there may be a voltage drop when the motor turns on, as each will draw nearly an amp. If this is powered by usb, that can not supply enough current, it will drop noticably under 1a load. Depending on your op amp setup, this may cause the signal to go high, this activatin the other motor. Include more of the schematic for a more definite answer. otherwise, try going to a higher current supply.

Also, you absolutely have to add overshoot diodes if you're switching motors! Without them, you could send enough voltage back to fry your microcontroller! Just add a diode across the motor leads, with the cathode on the 5v rail (yes, it will look backward). It may run for a while without the diodes, but it is very likely to break something, particularly with repeated switching.

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  • \$\begingroup\$ I noticed many times that the uC disconnects itself and reconnects again while testing the circuit out. How can I know that the uC is still OKAY? We still get the same amount of voltage for each signal "as if" we test it for the first time. \$\endgroup\$ – Gold_Sky Jul 13 '15 at 6:04

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