DIY multimeter

Thread Starter

SeanV123

Joined Nov 12, 2020
68
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
16,998
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?
 

Thread Starter

SeanV123

Joined Nov 12, 2020
68
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
16,998
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?
 

Thread Starter

SeanV123

Joined Nov 12, 2020
68
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
 

MrSalts

Joined Apr 2, 2020
637
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.

9C76923F-1CBC-42C1-91D4-A790EB7BD86E.png
 

Papabravo

Joined Feb 24, 2006
16,998
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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