biasing a transistor

[aruna1]

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 )

 

[MikeMl]

Think of it this way: the 470K resistor (if the photo transistor is completely turned off) can supply a maximum of E/R=(3.6-Vbe)/470000 = 6uA. If the Beta is ~ 100, that means the collector current is ~0.6mA, hardly enough to turn the motor, which probably requires ~100mA

 

[crutschow]

You will need more gain, either an op amp, or another transistor stage (an emitter follower at the IR receiver output may be sufficient).

 

[aruna1]

i can reduce 470k resistor value but that will greatly reduce senstivity

 

[audioguru]

Your schematic shows an IR diode wrongly labelled an an IR receiver. What is it?
If it is an IR diode then the transistor is slightly turned on by the 470k resistor when there is no IR and the transistor is turned off when there is IR.

The LED will probably instantly burn out since it has no current-limiting resistor in series if the transistor conducts enough current but the 470k resistor value is so high that the transistor has a very low base current so its output current is very low.

 

[aruna1]

sorry for the wrong labbeling.its a IR photodiode.and yes 470k is too large and transistor isnt biasing to saturation region.but problem is if i reduce this resistor value it greatly reduces the sensitivity of IR diode. and about current limiting resistor-I just forgot to draw it sorry

 

[audioguru]

Please do not post circuits that are very wrong "sorry for the wrong labelling, its an IR photodiode".

The IR diode needs an amplifier, probably an opamp. Then the transistor might be able to have enough base current (1/10th its collector current regardless of its hFE) for it to saturate and drive the motor or LED properly.

Please do not post circuits that are missing important details "I just forgot to draw it sorry"

 

[0y1k]

wah you really do not care in designing the schematic.
oh iyah, approximately how much the estimated cost to create a robot with this series? Would have very much different than using a MCU?
then how about the ability of robots to cross the line? Is it good?

 

[JCLrd]

Hi,
I never worked with a IR diode as a detector, but see if some thing like this would work for you.

 

[audioguru]

Hi,
I never worked with a IR diode as a detector, but see if some thing like this would work for you.

1) Your photo-diode is upside down. It should be reverse-biased do it can "leak" a small current when it receives IR radiation.
If want to use it as a tiny very low power solar cell then it should have zero bias voltage.

2) The gain of the two transistors is very low but a photo-diode needs to have lots of gain, usually from an opamp.
The circuit will just apply 0.8V to the motor continuously.

 

[JCLrd]

Hi,
I took the time to actually build this to see if it would work, for a small motor about the size of a quarter in dia. runs quite fast at around 2.5 volts, across it.
This was designed to work as low as 3v. so 3.6v. will work fine.

I used a IR transistor, for the detector.

Here's how it works,

The motor has a DC resistance of around 12ohms.

The first stage (motor driver), is biased to have at the very least around 100mA. of collector current.

With this biasing and testing the motor runs very fast with 2.6v. dropped across it.
The second stage (detector), is biased to put its collector voltage at a value that would make the output transistor go into cutoff. At this point the motor shows 0v. dropped across it.

The I-R diode used in this experiment, was the kind you can get at radioshack, this was hooked up with a current limiting resistor of 470 ohms.

This diode was held at a distance of around 1" and turned on, the detector (I-R transistor) picked up this signal and brings the voltage at its collector low, thereby bringing the transistor invertor to a low base voltage cutting it off to the point where the output transistor motor driver can go into conduction to drive the motor to full voltage.

The voltage drop across the motor reached 2.6v. continuous running as long as the IR transmision was happening, this was very directional, with the tip of a pencil point I was able to break the IR beam very acurately and by moving the pencil point slowly different directions was able to vary the speed of the motor depending on the amount of IR light was recieved by the detector.

You can give it a try and see if you can get it working for yourself.
Hope this helps in some way...