Society of Robots - Robot Forum

Electronics => Electronics => Topic started by: aruna1 on May 08, 2010, 05:44:21 AM

Title: biasing a transistor
Post by: aruna1 on May 08, 2010, 05:44:21 AM

Hi
guys I'm making a robot using two pager motors. its a line follower without any mcu brain.it uses two transistors to turn on and off motors as it follows the line.

problem is

transistor is not biasing properly.it doesn't go to saturation region. with this circuit i can control a LED but not the motor.motor never turns on (due to low current )

Title: Re: biasing a transistor
Post by: Jak24 on May 08, 2010, 06:27:50 AM

Hi

for high current devices could you just use a relay like this :
http://interactive.usc.edu/members/phoberman/relayCircuit.gif (http://interactive.usc.edu/members/phoberman/relayCircuit.gif)
just replace the bulb with a motor ...it should work ...

regards

Jak24

Title: Re: biasing a transistor
Post by: billhowl on May 08, 2010, 06:31:28 AM

Resistors R1 470K Ohms is too high, you need 470 ohms will do, change R2 to 470 ohms too.

Title: Re: biasing a transistor
Post by: aruna1 on May 08, 2010, 07:40:58 AM

Quote from: Jak24 on May 08, 2010, 06:27:50 AM

Hi

for high current devices could you just use a relay like this :
http://interactive.usc.edu/members/phoberman/relayCircuit.gif (http://interactive.usc.edu/members/phoberman/relayCircuit.gif)
just replace the bulb with a motor ...it should work ...

regards

Jak24

hi relay is not an option as I'm making a tiny robot

Quote from: billhowl on May 08, 2010, 06:31:28 AM

Resistors R1 470K Ohms is too high, you need 470 ohms will do, change R2 to 470 ohms too.

well i can turn on the motor with using 470ohm but that will make IR photodiode useless.it becomes non responsive

Title: Re: biasing a transistor
Post by: TrickyNekro on May 08, 2010, 09:14:04 AM

Try replacing your transistors with some Darlington and Sziklai pair transistors....
This should do the trick...

But pay attention to the voltage at the base of the transistor...

Title: Re: biasing a transistor
Post by: aruna1 on May 08, 2010, 10:01:15 AM

hi
can you suggest me a suitable transistor for this?

Title: Re: biasing a transistor
Post by: Soeren on May 08, 2010, 11:22:44 AM

Hi,

Darlingtons and Compound transistors have a large voltage drop, so might be a problem.
The original circuit probably just need the right resistor value, which should be less than the 470k, (but not 1000 times smaller). Use a DMM to find out how it works with the 470k and deduct the answer.

If you need more than one transistor, don't use Darlington or Compound coupling, instead, use a PNP to control the existing transistor (and swap the position of photodiode and the 470k). That way you'll experience the lowest voltage drop and hence get more voltage to the motor.

Title: Re: biasing a transistor
Post by: aruna1 on May 08, 2010, 06:57:55 PM

hi soren, i tried different resistor values but didnt do any good.

and i didnt get your advice about using PNP.can you explain more
thanks

Title: Re: biasing a transistor
Post by: Soeren on May 09, 2010, 06:54:47 AM

Hi,

Quote from: aruna1 on May 08, 2010, 06:57:55 PM

and i didnt get your advice about using PNP.can you explain more

A picture paints a thousand words:
(http://That.Homepage.dk/Img/Motor_Ctrl.png)
If the motor current allow it, lowering I_c/I_b will lower the saturation drop.
I assume the purpose of the photo diode is to make it a photovore?

Title: Re: biasing a transistor
Post by: aruna1 on May 09, 2010, 11:13:04 PM

hi soren  my objective is to make a line follow robot