External scanner for DMMs

[klausES]

I am planning to build an small external scanner for DMMs.

Since I often measure 3, 4, sometimes 5 digits, for which you can of course also use 3, 4, 5 different DMMs (so far ...) but in the long run,
so the thought came to me that my bench DMM would also log that might prefer.
(Of course I could have bought my last Bench DMM with a scanner card, but unfortunately I missed that ...). 

First of all, everything is thought of except current measurement, i.e. voltage, resistance, frequenzy in low range,
(maybe or if current, then actually only for the max. a few hundred mA range).

I already have the hardware of the measurement / signal path in mind (decent solution with high-quality SDS relay contacts).
A small box with an external DC plug power supply. Input sockets for the DMM 4mm standard, the individual channels 2mm sockets
(I mostly use 2mm cutlery with clamps for the circuits).

You still need a timer with which you can set the following:
Cycle and pause times of the channels (relays), the possibility of reducing the number of channels
(unused channel relays should not appear unnecessarily in the order).

As an optical display on the box which channel is currently active, either a led 7 segment, or even more clearly 5 individual led in 5 different colors.

Now there is also no "detection" in the log data which e.g. Voltage of which channel is.
As a current solution, I only think of each channel for a second shortly before it becomes active (before the relay to the measuring point closes),
staggered e.g. 1V for channel 1, 2V for channel 2 ... 5V for channel 5. 

Marking assignments for the log data would certainly be much more elegant with software,
especially if the relays were also switched with it.

Can you think of something good ?
Did I forget something important ?

[beanflying]

Have a good look in the Metrology section or use the Google Site search option up top Right as SMF's native search sucks.

This thread for a start but there is several others too https://www.eevblog.com/forum/projects/diy-low-thermal-emf-switchscanner-for-comparisons-of-voltage-and-resistor-stand/

[klausES]

Thanks for the hint and the link. 
I'm already reading ...

[klausES]

I have now designed a circuit for an self-sufficient (non µP) external DMM scanner for my requirements.

It (the control of the relays) is still running on the breadboard.
Housing with socket buttons, displays and the PCB is in the works.
(Material costs (except power supply) about 30 USD).

The following conditions (important to me) are met:
6 channels (10 would be possible with the circuit, but I will never need more than 6).

Features:

Separately adjustable cycle times and pause times.
All functions can be selected with one button each:
Automatic (the adjustable cycle and pause times take effect),
manual (switch to the next channel per button cycle),
Preselection of the number of channels used (can be selected between terminated after channel no. 2 and 5 to jump back to the beginning, applies to both, automatic and manual).
Display of which modes are active (automatic, manual), which preselection (after channels 2 to 5) from which to start again,
Clock and pause time (also adjustable in the background in mode manualy).

Defined state after switching on:
Mode manual, preselection of channels jump to the beginning inactive, display for setting the cycle and pause times also visible in mode manually.

Socket input (to DMM) 2x 4mm plus 2x 2mm standard (gold-plated all 19mm apart)
Socket Cannel Inputs 6x pairs of 2mm standard (gold-plated all 19mm apart).

Power supply: external plug switching power supply DC 12V