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My last question for my project!!! — Parallax Forums

My last question for my project!!!

Angela1Angela1 Posts: 25
edited 2009-03-22 14:39 in Robotics
Can someone please help me!!! This is like the last part of my project and I can't figure it out I tried posting this question on the "Stamp" category but things didn't work out...

My problem: I need to increase the voltage of a I/O pin which is currently at·5V to about·8V

I am trying to·put one of the I/O·pins' voltage into a·741 noninverting amplifier that multiples the voltage. I understand the configuration·BUT THE PROBLEM IS THE V-. I don't·know·how to get the negative voltage from a I/O pin or from anywhere else for that matter...·If I could figure out the V+ and V- aspect of the 741 Op Amp then I can finish my project. I don't know how to get the V- using the stamp configuration or what to do to get it... I don't know!!!

Any ideas how to·use the voltage·comming from·an I/O pin and putting it through some type of circuitry that makes the voltage about twice or three times or·maybe adding 5 volts to it?? This is the last part of my project so it is really getting to me!!!·Please HELP me·or I will cry... seriously...and i'll be out of your hair for good (probably) shakehead.gif

Please Help me!!!!!!!

Comments

  • kwinnkwinn Posts: 8,697
    edited 2009-03-20 23:06
    Angela1, referring to the attached data sheet, the 741 op amp needs a V+ (typically + 12V) on pin 7 and a V- (typically -12V) on pin 4 of the dip and metal can packages. To amplify an input signal to 8 volts those supply voltages must be higher than the voltage you want out. To have the signal go from 0 to +8 volts when the stamp pin goes from 0 volts to +5 volts the gain has to be set to 1.6 and the signal must go to the non inverting input (pin 3). If you want the signal to go in the negative direction when the stamp pin goes positive then the signal must go to the inverting input (pin 2).
  • Angela1Angela1 Posts: 25
    edited 2009-03-21 02:00
    kwinn said...
    Angela1, referring to the attached data sheet, the 741 op amp needs a V+ (typically + 12V) on pin 7 and a V- (typically -12V) on pin 4 of the dip and metal can packages. To amplify an input signal to 8 volts those supply voltages must be higher than the voltage you want out. To have the signal go from 0 to +8 volts when the stamp pin goes from 0 volts to +5 volts the gain has to be set to 1.6 and the signal must go to the non inverting input (pin 3). If you want the signal to go in the negative direction when the stamp pin goes positive then the signal must go to the inverting input (pin 2).
    Kwinn, your post gave me the greatest idea idea.gif·seriously!!
    I·have two 741 op amps so why don't I use one as an inverter and feed the output to the second op amp that is non-inverting and get the result I wanted!!!... and i've been trying that for almost 3 hours now... but it hasn't worked either... lol...*sigh* confused.gif
  • roadrunner3groadrunner3g Posts: 36
    edited 2009-03-21 05:37
    instead of the op-amp , use a NPN transistor. use 100ohm resistor from pin to base and 9v on the transistor and load. there are examples in the nuts and volts columns.
  • Luiz mauricio mionLuiz mauricio mion Posts: 77
    edited 2009-03-21 12:10
    Try to using switched transistor with coletor resistor of 220 pluged in a 8 volts batery

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  • Angela1Angela1 Posts: 25
    edited 2009-03-21 16:06
    roadrunner3g said...
    instead of the op-amp , use a NPN transistor. use 100ohm resistor from pin to base and 9v on the transistor and load. there are examples in the nuts and volts columns.
    Ok I'll try this! You said "there are examples in the nuts and volts columns"? Is this on the parallax website? Little confused about where this is.
  • kwinnkwinn Posts: 8,697
    edited 2009-03-21 16:19
    Angela, the transistor is a good idea. If you have problems with that post a schematic of what you are trying to do with the op amp and the transistor. Include the stamp and the circuit you are trying to drive so we know how much current it needs.

    Post Edited (kwinn) : 3/21/2009 4:24:59 PM GMT
  • Luiz mauricio mionLuiz mauricio mion Posts: 77
    edited 2009-03-21 21:15
    use only switched transistor for digital outputs like a pwm , ,, the 741 needs simetric source voltage or voltage divisor , but a minimum +-6 or +-5 volts , and offset null adjustment much components on board with a trimpot to adjust these offsets

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  • Luiz mauricio mionLuiz mauricio mion Posts: 77
    edited 2009-03-21 21:17
    try using opamp with single voltage suply , without offset null

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  • GWJaxGWJax Posts: 267
    edited 2009-03-22 14:39
    Angela1, Here is a great tutorial for the 741 OpAmp that may help you and others out here that was written by Tony van Roon, from his papers on Electronic Circuits For the Hobbiest. This should help you understand where the 741 came from and how they work plus he has circuits that will help you out in getting your 5v to 8v as you need for your project.


    Jax

    ps.. Make sure you zoom up on it at least 1200% to read it, sorry for that.

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    If a robot has a screw then it must be romoved and hacked into..
    pdf
    pdf
    OpAmps.pdf
    129K
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