I Followed The Following Circuit For The Logic NOT Gate from This Website http://www.northdownfarm.co.uk/rory/tim/basiclogic.htm


Here is My Circuit

Here is My Circuit Which is Intended to Perform The NOT Function and Drive the Motor on Low Voltage (0).


simulate this circuit – Schematic created using CircuitLab

But Its Not Working , I Don't KNow What's Wrong ??

  • \$\begingroup\$ Is R1 really 18 Ohms? Try making it something like 4.7K. In any case this isn't really the right way to perform the function because you're always burning power in R2. You should maybe use an N-Channel FET in place of R1 and put the motor where R2 is. Then just drive the gate of the FET directly from point 1. (You need a very low threshold FET.) The circuits you based this on are OK for logic gates, not so much for power control. \$\endgroup\$ – John D Dec 11 '14 at 16:21
  • \$\begingroup\$ Trying to turn a motor off by shorting it out (with Q1) is probably the least effective and efficient way of doing the job. Rather use the output of your NOT gate to drive another transistor which then switches your motor. And as John D already mentioned, 18 Ohms is way, way too low. \$\endgroup\$ – brhans Dec 11 '14 at 16:25
  • \$\begingroup\$ Yes , R2 Gets Overheated Quickly in The Circuit . So Should i Use a Resistor with Large Resistance with Another Transistor to Switch off The Motor ?? \$\endgroup\$ – Siddharth Singh Dec 11 '14 at 16:51
  • \$\begingroup\$ How do I Use another Transistor to Switch The Motor Off ?? They Don't Teach That in Xth Grade \$\endgroup\$ – Siddharth Singh Dec 11 '14 at 16:51
  • \$\begingroup\$ But The Point is Even When i used a LED To Check The Gate , It Didn't WOrk THE NOT GATE IS COMPLETELY DYSFUCNTIONAL !! ... I Wasn't Able to Construct The NOT GATE Even With the Other Circuit design in Which THere are no diodes \$\endgroup\$ – Siddharth Singh Dec 11 '14 at 17:08

My guess is that the motor will not work with an 18 Ohm series resistor. Did you check this, with just the motor and the resistor?


You need to put the transistor in series with the motor.

Here's an example (source) with your desired polarity (low to turn on), driving a relay coil. This should work for a small DC motor too. This circuit requires Vin levels to be ~ 0V or Vcc to operate properly.

enter image description here
(source: electronics-tutorials.ws)

These days, logic gates are not designed using resistors and diodes because without a transistor they have no gain and will therefore be slow and have limited fanout.

  • 1
    \$\begingroup\$ Actually, with the circuit shown the emitter will always follow the base voltage +0.7V. That's not necessarily what the OP wants either. You could use an NPN there and another to invert. \$\endgroup\$ – John D Dec 11 '14 at 17:13
  • \$\begingroup\$ @JohnD You're right, that would be best. But considering the OP's experience level I thought to keep it simple. I added a note about the required input voltage level. \$\endgroup\$ – akellyirl Dec 11 '14 at 17:29
  • \$\begingroup\$ You could also swap the transistor and the relay/diode and then you have a PNP transistor switch (with a low input to turn it on) \$\endgroup\$ – user253751 2 days ago

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