# Modifying fluctuating DC voltage

Discussion in 'General Electronics Discussion' started by XmodAlloy, Mar 21, 2012.

1. ### XmodAlloy

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Mar 21, 2012
Hello! I am currently working on a Hall Effect driving scheme which, in the end, needs to put out -0.1 to 5.5 volts. The signal I am starting with is a 2.35 to 4.7 volt linear positive output from the Hall Effect sensor.
So, what is the easiest circuit to change 2.35 to 4.7 volts into -0.1 to +5.5 volts?
I'm certain there's some IC that exists which I am simply overlooking or some other shortcut.

2. ### Sid723

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Jan 28, 2010
You could try an Op amp set up as a comparator with hysteresis.

When the input to the comparator reaches at or below 2.35 (that will be the low trip point) that is when the output goes to -0.1 or the low rail of the comparator.

When the input to the comparator reaches at or above 4.7 (that will be the high trip point) that is when the output goes to the high rail voltage which is set to 5.5 volts.

You will have to set the high voltage rail of the comparator to 5.5 volts and the low voltage rail to -0.1 volts.

Last edited: Mar 21, 2012
3. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,481
2,830
Jan 21, 2010
Or does he want a linear range of "2.35 to 4.7 volts into -0.1 to +5.5"

Personally I think there may be some misunderstanding there, the output range he's specified sound more like the absolute maximum ratings for the input of something, but I'll accept it for now.

This is a relatively common question (surprisingly) and uses a standard technique with an op-amp. Look here.

Note that you'll require a power supply that is double-ended and exceeds the 0.1 to 5.5V range by at least a couple of volts. +/-9V (essentially 18 volts centre tapped) would be fine.

4. ### XmodAlloy

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Mar 21, 2012
I want a linear output of the circuit, I apologize for any miss-communication. I could run a positive/negative setup without much trouble, now to figure out the schematic... <.<
So far my best results have been with a single NPN transistor and an Op-Amp, giving me 0.5 to 5.25 volt modulation.
I COULD just get by if the minimum voltage coming form the circuit was a smidgen less than 0.3 volts as long as it reliably never exceeded that voltage in testing.
-Don

5. ### BobK

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Jan 5, 2010
What device are you putting the output to? -0.1 to 5V seems like a very strange range.

Bob

6. ### XmodAlloy

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Mar 21, 2012
I have the system running on a computer model right now. It's rather unorthodox, but it is working if I use the 0.7 volt voltage drop on diodes to lower the incoming signal near the zero line.
Here is a schematic, I know it looks ridiculous with all the diodes hooked in series (and probably won't work quite as well in reality). Nevertheless, it illustrates the principal of what I want.
http://img689.imageshack.us/img689/3281/hallcircuitwithpwm.jpg

The unmarked IC is a 555 timer used to create a PWM signal which I plan to feed into a MOSFET driver.
Any input is welcome!
-Don

7. ### twister

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Feb 12, 2012
You have that 555 set up as an astable oscillator running a a very high freq. Unless you have a hefty op amp it will not be able to sink the voltage on the control. If I remember right, the control puts out over 100ma. The only thing you are going to do is vary the freq.
If you would explain what you are doing someone might help.

8. ### XmodAlloy

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Mar 21, 2012
Okay. What I am trying to do is make a ~16khz pulse width modulation system which uses a Hall Effect sensor as the input. This output will then be used to drive a MOSFET system and eventually a motor.
That's the short of it.
I will be using only one magnetic pole with varying distance on this HE sensor: http://search.digikey.com/us/en/products/A1302KUA-T/620-1022-ND/1006597

If you find a better linear HE sensor on Digiky or Mouser, let me know.

I do not want to do any programming to the HE sensor (Some have microprocessors which need to programmed) nor do I want to use an embedded micrcontroller such as a PIC or ATMEGA.

-Don

9. ### BobK

7,682
1,688
Jan 5, 2010
Too bad, the circuit would be far simpler and more flexible if you did use a microcontroller.

Bob

10. ### twister

172
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Feb 12, 2012
Ok, a 16KHZ freq. has a period of 0.624m seconds. You need to have a monostable oscillator set to 0.625msec. A 625ohm resistor should do that. Hook it to pin 7 and 6 and your .1 cap. to ground. Hook the output of your op amp oscillator to the trigger. Hook a 100k pot to V+ and to ground, with the wiper to control pin 5. The pot will let you modulate the pulse width. I might be wrong about the pot hooked to +V, it might work better just hooked to ground.
Try that in your model program. It might work, no guarantees. My math may be all wrong. Check it first.

I now notice that you want a 16kHz freq. but your schematic says 2HZ. Where does the 2HZ come from? You will need another 555 oscillator running at 16khz and use that to trigger the monostable. R1+R2=1k with a 0.1 cap should get you somewhere close.
Then hook the op amp to pin 5 of the monostable, to vary the pulse length.
Hook a amp meter to ground on pin 5 to see how many milliamps you will need to sink. You might have to use a transistor on that pin and control it with your op amp.

Last edited: Mar 23, 2012
11. ### XmodAlloy

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Mar 21, 2012
Okay, just when I think I have everything explained I find myself neglecting to tell more parts of the story.
This system is a throttle for an automotive application, the entire reason I wanted to use a HE sensor was to avoid potentiometers, I have had bad experiences with them failing on me in the past and will not trust them in this particular application. If all else fails, I will use a potentiometer with a circuit like this: http://www.robotroom.com/PWM.html
I have used this system in the past and it works well, using a potentiometer... (I know, I'm seriously paranoid about them now...) If that is my only option, then I will find a massively overpriced 30 degree loop of carbon just so I feel it won't fail...
The frequency is by no means going to hold at two hertz, it was just what I was using as a sample input to test the circuit, low enough to not count as AC with most capacitor sizes, and large enough to watch the circuit work on the simulator (It doesn't allow playback in real-time, so even the 2Hz signal played at an eternally slow speed.

Thank you all for the great ideas so far though, I do appreciate it and will examine each!
-Don