# Taking away 1/6th or so from a variable voltage

Discussion in 'General Electronics Chat' started by Hipervids, Sep 28, 2008.

1. ### Hipervids Thread Starter Member

Sep 28, 2008
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Hi guys, I'm new at posting here as you can see but have been a reader for a long time.

I have 1 question that I am hoping someone might be able to answer.
I'll try explain as best as I can I hope I dont confuse anyone while trying to be as clear as possible.

Is there such a way to take a voltage that rises and falls say from 2 volts to 4 volts depending on a sensors input and either add say 30% more or 30% less as the final output voltage.

For eg
Say a temperature sensor is giving 2 volts at 20 degrees and 4 volts at 40 degrees but I want 3 volts at 20 degrees and 6 volts at 40 degrees from the original 2 and 4 volt signals while still maintaining the original output curve from the sensor.
Can this be done?

The way of adjustment I was trying to experiment with was not with the sensor but with the output voltage that it gives out.
Say changing 2 volts coming from the sensors final output to 4 or 2 volts to 1 volt etc.

As the sensor voltage rises and falls the output voltage is changed at the same ratio but except for adding 1 volt or subtracting 1 volt.

I dont know if there is a circuit type or a product out there for doing this as I am only a hobiest and not an expert.

Thanks for taking the time to read.

Apr 5, 2008
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3. ### SgtWookie Expert

Jul 17, 2007
22,182
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See the attached for one way you might approach it.

R1 and R2 form a voltage divider. Depending on the setting of potentiometer R1, the voltage at point B (yellow trace) will be 100% to 50% of the input voltage at point A (cyan trace).

R3 and R4 set a fixed amount of gain for the opamp of (R4+R3)/R4, or (20k+10k)/20k, or 30k/20k=1.5

So, the total gain of the circuit is 50%x1.5 = .75 to 100%x1.5 = 1.5.

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4. ### Hipervids Thread Starter Member

Sep 28, 2008
14
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Thanks alot bertus and SgtWookie.
The schematic looks very simple but as they say the simple things in life are often the best but I am only working with small currents and voltages so I think it will work fine for my experiment.
Thanks alot for helping me I have the day off tomorrow so Ill test it with my application.
Thanks again

5. ### Hipervids Thread Starter Member

Sep 28, 2008
14
0
Hi guys well after a week of fiddling around I havent been able to get it to work.
Ive been trying it with TL071CP & also tried the LM358N will these chips do the same job as I couldnt find the LT1007 at my local electronics shop.
When I use a 1.5v battery as the input and connect the amp with 10v I get nothing at the output of the amp
This is using SGT WOOKIES circuit above but with a TL071 or LM358.
Do I need the exact LT1007 or is there a similar item that I could look for.
Thanks guys I know im stupid but its just something I am experimenting with.
Garry

Last edited: Oct 6, 2008
6. ### SgtWookie Expert

Jul 17, 2007
22,182
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That is distressing
The TL071 should do it. It's far better than the LM358N.
Ahh, I think we have a problem here.
Garry,
You are NOT stupid! I would be indignant if anyone besides yourself called you that.

However, I can suggest that perhaps you did not wire up the circuit properly. First, please check all of your connections.

Secondly, please ensure that you have the proper supply voltages applied. Your TL071 will need both + and - supplies.

Electronics is fun - really. I'm not kidding.

If you're still having problems, please post back a schematic diagram of your circuit - because it must be different from what I posted.

7. ### Hipervids Thread Starter Member

Sep 28, 2008
14
0
Thanks for helping me through this.
I went and bought a few more chips and bingo, the first new chip I put in the socket worked, there is one problem that I still have though and have tried to fix this myself but cant.

The voltages run fine using the exact circuit that SGT WOOKIE drew but when I go under 0.80 volts on the input, the output from the amp runs at 8.64 volts.

The supply to the OP AMP is 9.55 volts from a brand new 9v battery.
I am using a variable power supply for the input from 0.10 volts to 2 volts ideally but have tested this all the way up to 6 volts with no problems at all from the voltages past 0.80 volts and up.

The thing I am affraid of is that the sensor I am using has a range of 0.1 volt to 2 volts, mainly using 1 - 2 volts range but sometimes it does go to 0.5 volts and it will be a problem if that happens and in turn the op amp outputs the 8.64 volts

Is the range of the amp no good for this application? Any suggestions to protect from the voltage spike if the sensor actually does go under the 0.80 volt range?

Heres some data.
0.10 volt in = 8.64v out
0.50 volt in = 8.64v out
0.70 volt in = 8.55v out
0.75 volt in = 8.44v out
0.80 volt in = 1.48v out
1.00 volt in = 1.45v out
2.00 volts in = 1.79v out
4.00 volts in = 6.17v out

Thanks alot for reading I dont know what I can do now to get past this.
Garry

Last edited: Oct 13, 2008
8. ### SgtWookie Expert

Jul 17, 2007
22,182
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It sounds like you are trying to use just a single battery to power the opamp.
You must use two batteries, or you will experience phase reversal, or "lockup".

This happens in many opamps when the input gets too close to the negative rail.

See the attached for how to connect the batteries.

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9. ### Hipervids Thread Starter Member

Sep 28, 2008
14
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Ahh cool, thanks for that, ill try it tonight. I was using 1 battery.

10. ### Hipervids Thread Starter Member

Sep 28, 2008
14
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No go
Done exactly as the circuit is drawn but it still runs high voltage with an input under 0.8 volts.
Should I be grounding the power inputs to the op amp with the rest of it all? which i am.

11. ### SgtWookie Expert

Jul 17, 2007
22,182
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OK, I re-drew the circuit using all wires instead of the ground symbol.
See the attached.

In general, all of the ground wires should be connected together.

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12. ### Hipervids Thread Starter Member

Sep 28, 2008
14
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Sorry for the late reply I had given up on this but came back to check and found your post.
It works exactly right now.
Thanks SGT WOOKIE I have fiddled around with virtual ground aswell and all is good, works great.
Thanks again for all your help.