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Topic: Simple transistor switch (Read 3369 times) previous topic - next topic


Hello everyone,

I have been trying to implement a simple transistor switch to shut off a motor in a remote control car and I can't seem to figure out where my problem is. I tapped in to the motor's wire with my transistor circuit. I am trying to make all of the current coming from the black box go to through the transistor and then to ground when 5 Volts is applied to the base of my transistor from the Arduino. This should make the motor stop. Originally I used one transistor, but there was a lot of current still flowing to the motor. So I decided to use a second transistor to increase beta, which allowed more current to flow through Ic  based on the equation Ic= Beta*Ib. This slowed the motor down drastically, but it is still spinning. The currents shown in the diagram attached are the currents I measured using the multimeter. I don't understand why current is still flowing to the motor and why there is so much more current in Ic than what is being input from the blackbox circuit. Any help would be greatly appreciated.

Thank you all so much and sorry if this is unclear.


Your measurements of current can't be right. The sum of the currents flowing into any junction must be equal to the sum of the currents flowing out.
The "T" junction at the top has 1.44 amps flowing out but only .36A flowing in.

Don't send me technical questions via Private Message.


There must be a second wire between the "black box" and the motor.

If this second wire is GND, then you are doing a short cut, (which is a bad thing, usually).
If this is an independent line, switching the one you draw to GND won't change very much ( which about is what you describe, if I understand half of your post ).

The current feeding your motor has to run through the transistor  ( C - E ), in order to be switched by  Ib.
Check your transistor's data sheet, if it can handle that amount of current.
( If it survived the shortcut already, all seems fine, though )


Thank you all for your response.

Thanks for the advice El_Supremo. I know that the currents don't make sense. I was hoping someone would help me by explain that.

Thank you Michael_x. I forgot to include the second wire to the motor from the blackbox. You are correct, there is a second wire. I just measured the voltage through the second wire and it is 0.06 Volts. I believe the second wire is not ground and is used to reverse the motor for the RC wheel as an independent wire.

I also looked at the transistor specifications:

Vceo = 40V
Vcbo = 60V
Ic = 600mA
Pd = 350mW
fr = 250Mhz
Vce = 0.4 V
Ic(max) = 600 mA

I also know that the current must flow through C-E in order to be grounded, but for some reason all of the current is not flowing directly to ground as it should.

I am not entirely sure what you mean by the current has to be switched by Ib. Can you please elaborate.

Thank you guys again. I really appreciate your help.


im just curious what are you trying to do? why not instead of wasting the power as it seems you are trying to do diverting the amps from the motor to your switch, why not switch the motor itself? use a npn for a low side switch or a pnp for a high side switch
maybe im reading it wrong but if your just attempting to ground the output of that black box instead of cutting itto the motor you canburn it out, the motor is still spinning because that black box is trying really hard to power your short and the motor


Thanks for your response winner10920.

I am trying to build a collision avoidance system RC car. Basically, When my ultrasonic ping sensor senses an object, it switches on 5 volts to my transistor base. The current from the black box is then rerouted and gets grounded, which would stop the wheels from spinning.

The reason I do not want to switch the motor itself is because I do not want to tamper with the manufacturer's circuit and mess something up since I do not have access to the schematic. This is why I put the transistor switch in parallel with the manufacturer's black box and motor. It allows me to switch the motor on and off with minimal amount of tampering with the original circuitry.

Thanks for your suggestion, it is really appreciated.


Unfortunetly it wont get rerouted unless you really bog down the supply,which will just kill the battery
your best bet (without modify wiring) would be to tap into the battery compartment and shut down the entire car with a transistor or mosfet(by interupting the supply not shorting it)
That could be as simple as wiring a transistor collector to the compartment connection, emittor the the battery and just a piece of cardboard or plastic to make sure the battery isn't bypassing the transistor,
then you supply your 5v and it allows the car to run, take it away and it kills the car


Thanks for your response winner.

I already thought of your suggestion. The only reason I am not tapping in to the battery compartment is because after I stop the motor from allowing the car to move forward, I still give the user the ability to reverse backwards. So I need to keep the battery powering the RC car.

My current circuit worked before with just one transistor, but for some reason it stopped working and I couldn't get it working again.

Thanks again and I appreciate any more input.


I am not entirely sure what you mean by the current has to be switched by Ib. Can you please elaborate.

I meant the same as winner10920 : You should not shortcut your "black box", but run the motor by current coming from the transistor.
If you just want to inhibit the motor running forward, but not the reverse direction, things are more complicated anyway.
Sure, just adding one wire seems less intrusive than cutting a connection. But I fear, to achieve your goal, you have to modify even more.

Good luck, and have fun ...

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but for some reason it stopped working and I couldn't get it working again If a short cut does not kill your transistor, perhaps a reverse voltage does  :(  ?


I read this tread, and ...shorting the V supply to the Motor is ... NO NO.. Kiss the battery good bye... Man !

If you want to control the motor, you have no choise to remove the motor wires... well just one. If you see the color - RED and BLACK, I will bet a beer bottle that the red is positive and the black is negative. So just disconected the black wire, or cut it , remove the insulator, connected the black wire FROM the motor to your transistor circuit - Collector, the red still connected from board to motor <-- that is your Vcc, the black wire is your connection to the collector / transistor circuit, and you are ready to control the motor.

Now, That I call hacking , Man ! Sorry but you have to "hack" the RC car.


Thanks for the advice everyone.

I will try Techone's suggestion once I get home tonight and let you guys know how it goes. Thanks a lot!


Don't forget to ground your transistor circuit.

I was thinking about it, I don't know about the RC car, but it may have a H bridge circuit to control the motor. In that case, remove both wires ( red & black ) Connect the red to the battery + terminal, the black wire to the collector of your transistor circuit and the gnd to the - terminal. 

If you want to re-connect the wires back, just cut the wires ( red & black ) and removed the insulator. To re-connected, just connect the wires with the insulator remove.

I hope you understand. Sorry for that, but you have to take risks.


Maybe a triac will work to interupt one leg of the motor?  Or a 5v pc relay?

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