What is "Logic Voltage"?

Hello.
This is probably a very beginner question to ask, I am very sorry.
I was just reading through tutorials when I saw "logic voltage". I have no idea what this means, could someone explain?

Thank you very much,
Omar

"logic voltage" is the input or output voltage(s) to/from a digital circuit.

On a motor controller, there is usually a pin (which you connect to the microcontroller's output pin), there's +, ground and the obvious output pints for the motors. The logic voltage would be connected to what exactly?

Hope this information will help you

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A & B is the inputs from microcontroller

Note that the transisters are for a small motor.adequate transisters need to drive a big motor.

Omar.M said:

Hello.
This is probably a very beginner question to ask, I am very sorry.
I was just reading through tutorials when I saw "logic voltage". I have no idea what this means, could someone explain?

Thank you very much,
Omar

Click to expand...

Logic voltage refers to the two possible voltage levels of a binary state. For example the popular TTL logic family has states of 5vdc for a high state and 0 volts for a low state. Other logic families can have similar or vastly different voltage levels for their states.

One can learn, study and even design logic circuits without concern of the actual voltage levels. That can be decided after a specific logic family has been selected based on cost/benefit/power consumption, etc.

That make sense?

Lefty

Typically logic level voltages are 0 and +5, they can be defined as 1 or 0 (high or low) in either voltage state completly at the discretion of the requirements of the circuit or the whim of the designer. "Common sense" says low or 0 should be 0volts and high or 1 should be +5volts but there are many different reasons that might not be true.
The problem with saying all of that is modern micro controllers no longer use just 5 volt logic. They also use 3.3volt logic and some micro controllers go down to as low or lower than 2.7 volts before the circuit ceases to function. Modern micro processers use 'core logic' voltage in the 1.5 volt range (the less the voltage swing the faster it swings so the faster it can run to a certain degree) Most I/O logic levels are 3 to 5 volts. Just to confuse things even further you can tri-state an I/O line which causes it to go 'high z' basically act like it's been disconnected from the circuitm there are ways of detecting this high-z state.
The more you learn about digital logic, the more you'll find there are ways to blur the difference between analog and digital

So, if I understand this correctly the logic voltage is also coming from the microcontroller.

So if used a fancy h-bridge module, the logic voltage would be coming from my motor controller, as well as the other pin would ALSO be connected to my controller?

Kind of yes, you seem to still misunderstand a bit though, logic level doesn't really mean anything, it's a sometimes used phrase to refer to low voltage low load control lines. In your case the h-bridge would be controlled by the logic level signals which are low voltage (usually 5 volts) and low current (usually only a few miliamps) which it would do what it needs to do to provide high voltage high current pulses to a motor to control it.

Logic voltage is whatever voltage your logic circuit runs from, it depends on the components you are using for you logic circuit. Logic voltage is normally taken a around 5V, but can be from 2V to 6V for HC CMOS, 3V-15V for ordinary CMOS or even 24VAC if you're using good old fashioned relays.

Ohhhh. I am sort of beginning to understand. So when using a motor driver that one buys, you cannot just hook it up. You need some kind of chip that will convert the voltage to logic voltage? If so, what chips do this?

But if I recall correctly, logic voltage refers to low voltage. Under 5V like Sceadwian said. Does that mean I can hook it up to my + voltage like (that is being regulated with a voltage regulator which outputs 5V)?

Sorry about asking so many questions.

Omar

A motor driver actually "converts" (if you really want to use that word) logic voltage from PWM into high currents to drive the motor.

If it is 5V logic (and there are different kinds like TTL and CMOS which define different thresholds between a logic 0 and 1/HI and LO), you can pull up the line to 5V supply through a resistor to define a one.

So: in a motor driver does the logic voltage define if the motor is spinning? That is usually done from the microcontroller to the microcontroller input on the driver right? So if it is high, the motor will spin and if low it wont?

If that is the case, then I can simply connect logic voltage pin to +5 volts through any resistor?

Yes. THat is correct. The logic voltage drives the transistors (or the gate drivers that drive the transitor gates). PWM is used on the logic voltage to turn the transistors on and off in such a way so that the motor does the same (but it happens so fast it look like the motor is travellign slower).

What are you trying to connect? If you connect something to +5V through a resistor, then a low signal connecting to the line can pull the line low (this means by default the line is high if no other logic signal is present). If you just connect it to t +5V through a resistor (or without a resistor), then the line is just always high and your motor would never turn off.

Well, I just want to make a line following robot. I've learned making your own motor driver isn't the best. So I was looking to buy one. Most of them have a logic input line, and I had no clue on what I would do with that pin.

http://www.robotshop.ca/home/produc...ntrollers/pololu-micro-dual-serial-motor.html
That's the one I was looking to get. Or one like it.
The one pin is an logic voltage.

So if I connect that to the 5V line, I can still turn the motors on and off from the PIC pin?

Okay, your question was a bit misleading. Logic voltage levels are what everyone has been explaining to you (2.5V, 3.3V, or 5V being HI and ~0V being LO).

In this case, that pin that says logic voltage is a power supply pin. It says "logic" to differentiate this power pin from the motor power pins (labelled "motor power supply") and to tell you that the power going into that pin is meant for the PIC and other digital logic on the PCB.

It's a pin to supply power to the logic on the PCB- so do not use a resistor. The resistor thing was just talking about pull-up resistors (connected to +3.3/5V) and pull-down reistors (connected to ground) to make digital signal lines have a default value in the absence of any other signals.

Logic voltage is just the power supply used for control, it doesn't actually provide any current to the load, all it does is drive the transistors connected to the load.

I am guessing based on the manual provided that the logic supply pin powers the motor controller circuitry. I am not entirely sure but it seems that the chips do not power themselves off of the motor battery. They need a separate power source from 4.5V to 5.5V. So I would say that your Vcc supply for your microcontroller needs to connect to that pin (pin3). The motor controller probably isolates the motor battery from Vcc (except Ground, or course). Having the same Vcc for the microcontroller and motor controller allows them to share the same "logical high" voltage level.