# DIY multimeter

#### SeanV123

Joined Nov 12, 2020
109
Hello, I'm building a DIY multimeter that can measure between +- 5V. However, my microcontroller (SDP-K1) analog pins can only read up to 3.3V. Is there anyway I can read +- 5 volts with it? I need to use to probes (like a regular multimeter) so I can measure minus voltages aswell. Thank you

#### Papabravo

Joined Feb 24, 2006
21,322
You can use an opamp attenuator to map ±5V into 0 to 3.3V. So -5V will map to 0.0V, 0V will map to 1.65V (= 3.3/2), and +5V will map to 3.3V. Do you know how to design such a thing, or do you need some help?

#### SeanV123

Joined Nov 12, 2020
109
You can use an opamp attenuator to map ±5V into 0 to 3.3V. So -5V will map to 0.0V, 0V will map to 1.65V (= 3.3/2), and +5V will map to 3.3V. Do you know how to design such a thing, or do you need some help?
Yes, schematic would be helpful Thank you!

#### Papabravo

Joined Feb 24, 2006
21,322
Yes, schematic would be helpful Thank you!
Well I don't have one right handy and I don't know what you have available to allow this to happen. For example is 3.3 Volts the only supply voltage you have or is that just the power required by the processor. Do you have ±5 Volts that can be used for analog stuff like this? Anything else I should know?

#### SeanV123

Joined Nov 12, 2020
109
Well I don't have one right handy and I don't know what you have available to allow this to happen. For example is 3.3 Volts the only supply voltage you have or is that just the power required by the processor. Do you have ±5 Volts that can be used for analog stuff like this? Anything else I should know?
My microcontroller can output 3.3 and 5v. However the analog in ports can only read up to 3.3v. I want it to be able to read voltages up to 5V. I have seen a few examples online where they use a voltage divider to be able to determine the input voltage

#### SamR

Joined Mar 19, 2019
5,100

#### SeanV123

Joined Nov 12, 2020
109
Which is?
Wow! Hi Sam. The microcontroller is an SDP-K1 by Analog devices. I use mbed to program it! Peace out!

#### upand_at_them

Joined May 15, 2010
940
This should help:

#### MrSalts

Joined Apr 2, 2020
2,767
If you need to measure to -5v and your power supply is 0-5v, you’ll need a precision rectifier plus an 8ndicator (comparitor) to let you know if the input is positive or negative.

#### Papabravo

Joined Feb 24, 2006
21,322
As I understand the requirement the TS wants to map ±5 VDC onto the interval [0,...3.3] or close enough to not be worth mentioning. Once you use the precision rectifier you lose the sign information. (Edit: Never mind I see where you provided for the sign), I have a solution that allows the A/D converter to measure the whole 10 volt range. that simulates, but it can be improved. I originally thought I might have to use two inverting stages because I needed a gain of less than one. Turns out that is not the case. I think I can do it with a single non-inverting amplifier.

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