I am trying to build a little circuit to help measure voltages at various points of projects I build. Any links to similar circuits will help.

The project circuit (PrC) will connect circuit under test (CuT) to a DMM + lead

So...
Inputs to the PrC are from CuT
Output is to a DMM's + lead

PrC will have up to 20 momentary push buttons, normally open.
When one of those 20 buttons, say button A, is pressed-
current to flow between corresponding input (A) and DMM+ (so latching is required)
an LED (3mm type will do) next to it will light up stay lit

This state will only change when a different button, say button D, is pressed, then circuit A will open and circuit D will close. Pressing button for currently active circuit will make it open but this feature is not necessary if it leads to more complexity/components.

Each line will carry under 100mA, under 12VDC. Resistance to be as close to zero as possible.

I looked into relays as they have the lowest resistance, but are there better ways?
Are there boards already built with some/all of these features sold on ebay etc.?

 

how simple do you want it..
You could always just use a switch for each circuit before the meter. flip the switch and its reading.. then close it and flip another. no "circuits" required.
or a mechanically linked multiswitch would do exactly what you want again without any circuitry..
or just use banana jacks on each uut and plug them in one at a time.

relays will work fine too.. contact resistance is low.

 

If you want minimum connection resistance (< 1Ω) then I would use a relay. Here are some ideas on how to design a radio push-button circuit (which is what you want) which can be used to control your 20 relays.

Note that I believe there's an error in the circuit using the 4017 (you could use two 4017's to control 20 relays) so if you want to use that approach, I can suggest a correction.

 

What is the point of avoiding a few ohms of resistance when you are measuring voltage with a DMM that uses 10s of meg ohms input resistance.

What voltage range are your signals and what are the driving? Will a set of dumb switches work? If you have a bunch of low voltage 24vdc or less circuits, I would go to the hardware store and buy a bunch of mains switches - 10 for $4.90

http://t.homedepot.com/p/Leviton-15...10-Pack-Light-Almond-M26-01451-2TM/100684041/

Cheers.

 

As GopherT noted, if you are measuring the voltage with a high input resistance voltmeter, then the small resistance of typical analog switches will not significantly affect your measurement accuracy. That way you can build an all solid-state circuit (except for the switches of course) which would be smaller and more reliable than one using relays. For example a CD4066 has 4 analog switches in one package.

What is the highest voltage and polarity you want to measure?

 

You might want to look at ADG406 which is a 16-channel analog multiplexer.

http://www.analog.com/en/switchesmultiplexers/multiplexers-muxes/adg406/products/product.html

 

To simplify, instead of using push-buttons, you could use a rotary switch such as this, which is a single-pole multiple-throw switch, to select the channel. That requires no additional circuitry -- all you need is the switch.

 

The below was what I did that it have 2 sets and 10 Pins for oscilloscope and multimeter multi selector that allows two tracks simultaneously detecting 20 points, but it only can see two points at one times, and it just using the positive for power, so only for testing the DC and logical signal.

That is just using google to translate, so you may see the strange sentences.