Continue to Site

Welcome to

Welcome to our site! is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Different type of DC Motor with PIC + Bluetooth

Not open for further replies.


Newbie level 3
Feb 11, 2011
Reaction score
Trophy points
Activity points
Hi All,

I have an issue with DC motor and Bluetooth.
I'm using PIC, the motor driver is TIP 31 NPN transistor and 4 x AA rechargeable battery. Each battery is about 1500mAh.
The board has 2 modes, 1.simple run and 2.Bluetooh mode.


I have 3 motors and all are in good condition:
1. Motor A: 2.1 mAh
2. Motor B: 20 mAh
3. Motor C: 70 mAh

In "Simple Run" mode, the 3 motors run without an issue.
But when in "Bluetooth" mode with PWM, Motor B had difficulty in running and Motor C had a slight issue. Motor A is perfectly fine.
I have an android app to control the motor using bluetooh.

It looks like the current issue. But I already use 1500mAh battery.Somehow Motor C is run better than Motor B.

What may be the issue?

It looks like the Motor draw away the current from Bluetooth module and the microprocessor. This cause the signal send from the Android App break.

Or could it be the current from the motor fall back to the board?

What's the different about Mosfet and TIP 31 NPN Transitor?



  • MotorDriver.jpg
    5.9 KB · Views: 78

In a switch case the MOSFET and bipolar TIP 31 transistor will both work in the same fashion, as switches. Probably the most notable difference is the MOSFET will only require gate current when switching where as with the bipolar there is base current required to keep the transistor turned on. This current more of a problem with low power battery designs though the choice is not so clear when running PWM.

As for your other issue I'm going to guess it's a noise problem from specific motors. The fact that the higher current motor works better than the lower one says it's not a power problem per se. I would add decoupling from the power supply 3.3V to ground. This cap should be connect close to where the + of the motor and the TIP31 emitter are connected at the board. Also twist the pair of wires feeding the motor to prevent radiation.

Thanks rhaynes. This helpful, although I don't know how to apply it to my board. I'm a newbie (hobbyist). My MCU is PIC18F4520-E/P. But I don't use Motor Driver chip. The board created by someone else.

Instead of placing the decoupling cap on the board you could try adding a 0.01uF or 0.1uF @10V or greater ceramic cap right on the back of the motor across the power leads. This is better as will reduce the RF level even coming down the wires.

I don't know the configuration of the motors. If there are terminals on the the back the cap goes right across the terminals. If there are wires directly out of the motor then twist them together and somewhere further down the wires, even right at the board place the cap across there.

I really think this will help you situation and if this is not clear enough, I'll need more details of the board to motor connection configuration.



Not open for further replies.

Part and Inventory Search

Welcome to