Convert 12V to 3V Without affecting Total Resistance

Thread Starter

Kalindu Aragorn

Joined May 22, 2018
9
Hello Guys,
This is my first thread here..

I have a device that contains a circuit like this.
View attachment 41188


The 1K Potentiometer RV1 is actually a Foot Pedal where the resistance changes when the pedal is pressed and outputs the Voltage to another Device through OUTPUT.
however, The OUTPUT Voltage is set to a fixed Default Value of 1.2V when the pedal is Not pressed and It'll go upto 12V when the pedal is fully pressed.

I need to tap into that output Voltage and connect it to a ESP8266 Analog Input. Since the ESP only works with 3V, I need to convert that output Voltage to 3V.

I tried a voltage divider connected to the output like this.
View attachment 41189

The Converted Out can be taken down to 3V. BUT the Voltage of the main OUTPUT that goes to the device changes when the Voltage Divider is connected. The Maximum 12V that is supposed to be achieved when the pedal is fully pressed is never reached because the Voltage Divider affects the Total Resistance.
Note that the RV1 Resistor cannot be changed. It is fixed inside a device which is very hard to dismount AND there are 1000s of devices that needs this conversion.

So what are my options? How can I get the Output Voltage of 0 - 12V range of the Device and Convert it into a 0 - 3 V Range ?

Thank you for your support!



 

crutschow

Joined Mar 14, 2008
34,285
If the first attachment I see no RV1.
And the second attachment gives me an error.

But if you want to tap off a voltage without affecting the resistance, you could use a non-inverting (follower) op amp circuit (below) to drive the voltage divider.
Such a circuit has a very high input impedance.
upload_2018-5-22_21-9-12.png
 

danadak

Joined Mar 10, 2018
4,057
Where are you connecting divider to, the output of the opamp ?

If so what is the value of the divider in ohms ?

Regards, Dana.
 

Thread Starter

Kalindu Aragorn

Joined May 22, 2018
9
Hello Guys!
Thank you for all the replies!

looks like alot of confusion going on. :D

Anyway Let me Rephrase the question.
Lets say I have a simple Circuit like the attached image.
There is a DEVICE that outputs a Voltage between 0V and 12V.
I want to get the SAME Voltage but converted to a 0-3V without affecting the above 0-12V.

So the OPERATION X would convert that 0-12V into 0-3V without changing the original Voltage.


cct3.png

Thank you for all your contributions!


Regards,
Aragorn.
 

ebeowulf17

Joined Aug 12, 2014
3,307
Hello Guys!
Thank you for all the replies!

looks like alot of confusion going on. :D

Anyway Let me Rephrase the question.
Lets say I have a simple Circuit like the attached image.
There is a DEVICE that outputs a Voltage between 0V and 12V.
I want to get the SAME Voltage but converted to a 0-3V without affecting the above 0-12V.

So the OPERATION X would convert that 0-12V into 0-3V without changing the original Voltage.


View attachment 153254

Thank you for all your contributions!


Regards,
Aragorn.
I wouldn't say there was a lot of confusion.

@crutschow has answered your question.
@danadak asked for a few relevant details.
@ArakelTheDragon raised a fair point about ADC voltage ranges. The ESP8266 ADC is set up to accept 0-1V, not 0-3V. If your ESP8266 is part of a development board, that board may or may not have some built in scaling.

Regardless of the other details, an op amp voltage follower is a great way to accomplish your signal tapping. With it in between the "device" and "Operation x," the device output voltage will be unaffected by whatever operation x does.
 

ArakelTheDragon

Joined Nov 18, 2016
1,362
We understand the question, but we need to know the details, because otherwise we will tell you how to lower it to 1v, while you need 3.3V.

You need a device with low input resistance and high output resistance (check the standard transistor circuits or opamp circuits).
 

Externet

Joined Nov 29, 2005
2,202
The existing output 0 to 12 V can be branched with a divide-by-4 as "operation x" to yield 0 to 3 Volts. Which is two resistors in series outputting the 0 to 3 V at the node between the resistors.
The values of the resistors depend on the current delivery capability of the "DEVICE" and the current requierement of whatever works with 0 to 3 V.

0-12V------------------------------------------R1----------A----------R2-----------------------------------------------------common

In general, R1 is to be 3 times value of R2. Point 'A' is the 0-3V output desired, and will be proportional to 0-12V.
 
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