A variety of questions from a newbie.

I've spent the last few months teaching myself electronics and I've come to an impasse; a few things I can't seem to figure out. If anyone can answer any of my questions, I'd be very appreciative.

• 1. I'm trying to make a motor run from the output on a microcontroller. The output from a microcontroller will be linked to either a transistor or an opamp in order to drive the motor. what would be the simplest way to allow the DC motor to reverse polarity (thus changing the direction of the rotation)
  
  Currently I put the motor in series with the opamp, which switches between 0V and 5V but a short circuit occurs when I try to connect the terminal to both the + and - rails. I put resistors on each but the motor doesn't seem to recieve any voltage. I'd like to use as few parts as possible because this may become a solar powered project. Also, the motor never stops, it only goes forward and reverse.
  
  Summary: I'm trying to make a small DC motor move and change directions based on the output of a microcontroller.
  
• 2. Since I've been working with microcontrollers and logic chips. Let's assume I want to be able to drive a pin high (say ~+4 to +5V) with a switch. what would be the approprate way to divide the votage? does the switched (high) portion require a resistor as well? can you show me a sample of the voltage division with the switch?
  
• 3. How on earth does a crystal oscillator work? sometimes it's placed in series, sometimes in parallel with a capacitor. how does the oscillation get started? wouldn't the crystal stay stationary if presented with a voltage?
  
• 4. If a transistor controls current based on the base current, does the current remain constant over varying voltages?

I have more questions but I'll do some reading before I hassle you guys with these simple questions.

Any help is appreciated

Hi there m8y

1} use an hbridge ic or make an active circuit using mosfets you need 4 i/os look on the net for relevent circuits there are many check out the links pages on vsmlibs for some nice hobby stuff like this to look thru

**broken link removed**

2} To drive an i/o high from a switch on another i/o you just load the state of the switch into the button i/o pin
use a 220R resistor in series with an i/o will limit the current and protect the gate
and is advised
again on the links page is some usefull stuff to look thru esp the ftps

3} a crystal oscillator is like a little very flappy disk in electrical terms much like a bass drum
you only need to supply voltage to a capacitor{water tank} with a certain decay a tap {base current of the active device} this diminishes then the reflex capacitance{another tank } of the crystal causes an emf to form on the other pole
{it fills and empties remember the more it fills in some tanks the quicker it empties the capacitors voltage apature}
all the time the transistor is going around about its own switching thresholds
of .6 v base emiter
much like audio you bias the transistor to stop the device clipping
and youll get a nice sine wave

but the capacitor is a kind of bellows that fills the third tank
that feeds the other two
so as you see this machine is easier to grasp

ever wondered why you get feedback??? from an amp and a mic

4}
a transistor base is a valve in a pipe the collector its inlet
the emiter the outlet for current {water}
and the valve has an auto return
you pull it on it shuts its self off at a given rate
this is the easy way to think
and the way to calibrate to thinking
about how to work out values and there effect

this way a nice slow system in the mind but efficient

so this will also build in a curves to the setup you use


there are many paradyms to use

Let me answer your last question:

The primary factor in determining the collector current is the applied base current in a BJT. This is the beta relationship (beta is also referred to as hfe):

Ic = beta * Ib

While this is a good approximation, there are other factors that affect the transfer of current from base to collector.

In real life, beta is NOT constant. It changes based on things like temperature, frequency, and collector current. To explain it in a way for real life BJT behavior, the collector current depends on beta and the base current, but beta depends on the collector current. It is a differential equation of sorts because Ic ==> beta, and beta ==> Ic.

Also, the effective transistor beta changes with the applied collector-emitter voltage, Vce. This is called the Early effect.

If I remember correctly, the equation that describes the Early effect and its impact on beta is this:

beta(actual) = beta(ideal) * (1 + Vce/Va)
Where Vce is the collector-emitter voltage, and Va is the Early voltage.

Generally, as Vce increases, the effective beta of the transistor increases as well. This is another kind of differential equation of sorts because

collector current depends on beta,

Vce depends on collector and emitter current (depending upon what resistances you have in the collector and emitter),

beta depends on Vce.

There are a lot of factors that go in to determining collector current from the base current, but often times, these factors are small, and we can approximate collector currents if the conditions are right.

hi, dear abt your last question,

roughly speaking does not matter abt voltage, because, not going in semiconductor physics details, BJTs are current operated devices not voltage, yes FETs are voltage operated devices.

Tarkus said:

I've spent the last few months teaching myself electronics and I've come to an impasse; a few things I can't seem to figure out. If anyone can answer any of my questions, I'd be very appreciative.

• 4. If a transistor controls current based on the base current, does the current remain constant over varying voltages?

Click to expand...

i dont think it is feasible to keep the constant current at a port by varying the volatge at the same port..
replies are expected to further explore this useful topic

Thanks for the help you guys... I've been doing alot of reading on transistors.

A question though, do you guys think using a set of optoisolators would be good for changing motor direction based on the high and low output from the op-amp? An H-bridge would be ok if I had alot of juice, but I'd like to keep the power requirements as low as possible. Also I'd like to keep the cost low.