# Thoughts about wiring up a voltmeter on my motorhome

#### zirconx

Joined Mar 10, 2010
154
Hi, I'd like to add a voltmeter to the dash on my motorhome. It's on a Ford E-450 chassis. I'd like the voltmeter to come on with the key (accessory power), and I'd also like to install a pushbutton switch on the dash I can use to see the voltage without the key. I probably should have bought a 3 wire one, but I like the size & style I picked out and it happens to be a two wire one.

I can see two fairly straightforward options. I'm looking for feedback on them or other ideas.

The first one is to use a relay. I would need to use diodes so I don't backfeed power into the accessory circuit. This option would also draw more current from the battery due to the relay. Which isn't an issue when the vehicle is running and honestly probably isn't really an issue when it's not running either. I would use switch when, for example, I want to check the voltage to see if the 5W solar charger is keeping the battery charged. Or if the battery was discharged and I'm charging it from the motorhome electrical system and want to keep an eye on it.

One downside to this idea is if you toggle the switch while driving, the voltage reading may change (due to where it's being sampled from). Which is fine, just kind of weird.

The 2nd idea is to use a normally open / normally closed switch to toggle where the voltage is read from. In "normal" mode the voltmeter is connected to accessory power, so it just comes on with the key. But if I toggle the switch it samples from the battery or some other always-hot source in the fuse panel. This kind of switch is a little harder to find, most of the simple round push button switches on ebay only have two contacts.

Thoughts? Thanks.

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#### djsfantasi

Joined Apr 11, 2010
8,263
Search eBay for “spdt round toggle ebay”

#### zirconx

Joined Mar 10, 2010
154
Search eBay for “spdt round toggle ebay”
I want this style of push button switch that blends in with the dash. I can find spdt in this style, but they are 4x the cost. I'll go that route if I have to.

#### Jon Chandler

Joined Jun 12, 2008
160
Here's the route I went. I used a dual digital meter to monitor chassis and house batteries, and added a dual high-rate USB charger. These are powered from the cigar lighter socket via a relay controlled from ACC (i.e., key on) power. A push-buttom switch bypasses the relay to read voltages with the key off.

The dual voltmeter is powered only by the primary input, so the house battery connected to the second input doesn't need to be switched.

A laser-cut panel fits in the radio opening in the dash. The rectangular 'switch' is the control for a headless radio/Bluetooth system located elsewhere.

#### Jon Chandler

Joined Jun 12, 2008
160
The USB power supply also has a circular battery voltage bargraph as shown in the first picture. Fortunately it's in good agreement the the dual voltmeter (upper display).

#### MisterBill2

Joined Jan 23, 2018
11,646
Interesting question. And the button with the N.C./N.O. switch will be the best approach. But there are fewplaces WORSE than ebay to buy electrical parts. Most honest electronics distributors will have the buttons you seek available in quite a few styles and sizes, and various price ranges. Digikey , Newark, Avnet, are those that come to mind first. Using diodes for isolation can work but there will always be a diode voltage drop to confuse the readings.
So the solution is locating the right supplier for the button. And you do want to have the switch be a momentary action one, to avoid unwanted battery drain when the vehicle sits for a long time between uses.

#### zirconx

Joined Mar 10, 2010
154
That looks good @Jon Chandler .

There is very little space on my dash to mount anything, so I went with a simple round voltmeter. I had been set on a voltmeter / ammeter combo, to show how much power the house battery was drawing from the alternator. But gave up on that because I can't figure out how to mount it. Also cutting circular holes in my dash (step bit) is much easier than a rectangular one.

I'm attaching a picture of the dash and circling the spot I plan to mount the voltmeter and switch (two separate holes in this area).

@MisterBill2 the diodes would only be used on the trigger inputs to the relay, so they would not affect the voltage reading. I see your point about supply houses, but I like to do things very very cheap. Sometimes that comes back to bite me, but usually it's ok.

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#### Tonyr1084

Joined Sep 24, 2015
6,864
You don't need a relay. Depending on what diode you use you WILL drop a small voltage. Common diodes drop about 0.7V, Shottkey drops less. I think 0.4V maybe?

#### MisterBill2

Joined Jan 23, 2018
11,646
Just using the correct push button from a reliable honest source will work. No need for a diode or a relay. And a lot of suppliers sell the correct type of button, but not on amazon or ebay.

#### ErnieM

Joined Apr 24, 2011
8,275
No need for anything but a momentary single pole double throw (SPDT) switch, though more poles will also work.

Accessory power (switch with the key) connects to the normally closed (NC) terminal, and the always on battery power to the normally open (NO) terminal. The common goes to the meter +, and the meter - to the chassis.

I found some nice looking ones on Ebay searching "spdt momentary push button switch"

#### MisterBill2

Joined Jan 23, 2018
11,646
No need for anything but a momentary single pole double throw (SPDT) switch, though more poles will also work.

Accessory power (switch with the key) connects to the normally closed (NC) terminal, and the always on battery power to the normally open (NO) terminal. The common goes to the meter +, and the meter - to the chassis.
View attachment 251831

I found some nice looking ones on Ebay searching "spdt momentary push button switch"
The original complaint from the TS was the lack of a suitable switch at a reasonable price. Amazon has limited choices and ebay even more limited choices. And product information at Amazon is generally limited to price and weight.
Certainly the circuit shown is the preferred choice, it was finding the correct switch that has been the issue.

#### zirconx

Joined Mar 10, 2010
154
Good points, thank you. Right now I'm leaning toward using a "ugly" spdt toggle switch, I have a bunch of those, and mounting it just under the dash, out of sight. I think this will be a good compromise.

#### Tonyr1084

Joined Sep 24, 2015
6,864
Just using the correct push button from a reliable honest source will work. No need for a diode or a relay.
I agree with "No need for a • • • relay" but I incorporated the diode to prevent the PB from back feeding the auto electrics when pushed. Admittedly there is a drop in voltage through a diode, but it's not significant. The PB is only for checking battery voltage without having to start the engine or to switch everything on, which will significantly drop the voltage.
No need for anything but a momentary single pole double throw (SPDT) switch
I'm going to give this some consideration too. A momentary SPDT should be able to be configured without the use of the diode. Didn't originally think of that.
The original complaint from the TS was the lack of a suitable switch at a reasonable price.
I got that too. But the TS DID say he'd go that route if necessary. I don't think the goal was to keep it as cheap as possible, rather, to be able to read the battery voltage when the engine was running and to be able to check the battery status without the engine running but rather, charging from solar panels.

I'll bang out a new drawing. I just wish I would have kept the first. No problem though.

#### Tonyr1084

Joined Sep 24, 2015
6,864
OK, no diode - no forward voltage drop.

#### Tonyr1084

Joined Sep 24, 2015
6,864
Took a little searching but here's one on Amazon. It can be lighted or you can ignore the LED. Costs less than $10.00. If you drive an RV I wouldn't think$10 is going to hurt you.

The LED can be independently connected regardless of the switch position and I'd consider wiring it to the dash lights. When you turn on the lights the button will illuminate as well.

 In the reviews some people have complained they fall apart, others have said they don't work. But the majority of reviews seem to like the switch. And some people run the LED on 5V even though it's rated for 12V, and on 5V it's bright enough. So I would assume that on 12 volts it may be rather bright. An additional external resistor may drop the current to a level where you prefer for brightness.
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Joined Jan 15, 2015
6,552
All you want/need is a simple normally open push button switch. A Grayhill 4001 N/O would do fine and they are inexpensive. If you want to get cool get the button guard. If you want smaller you can run with a Alcoswitch Alco mspm101c-1. Both are inexpensive simple switches. As to how to wire it:

I have seen the Grayhill online through Amazon but they are carried by a wide range of distributors. If you have Amazon Prime you are spared the shipping.

Ron

#### Tonyr1084

Joined Sep 24, 2015
6,864
@Reloadron The problem with that is when you push the button you're simulating the key. Which means you're powering up the entire vehicular electronics. A small PB would probably either burn out or weld. That's the reason why I put the diode in the first drawing. In the second drawing there's a SPDT momentary switch to select (for the panel meter) which DC source it's operating on. In the NC position the meter is powered when the vehicle is on. In the NO position, the meter only reads when the PB is pressed.

And SPDT momentary switches are more expensive and harder to find. But I linked to one.

It was @ErnieM who first proposed the SPDT momentary switch. And I'd stick with the momentary function because otherwise you might switch the meter on and inadvertently leave the meter constantly reading battery voltage. If it should go unattended for months it will drain the battery. Hence, the momentary switch. The TS said he may just go with a toggle switch; but that might end up draining the battery. I'd advise against the toggle.

Joined Jan 15, 2015
6,552
@Reloadron The problem with that is when you push the button you're simulating the key. Which means you're powering up the entire vehicular electronics. A small PB would probably either burn out or weld. That's the reason why I put the diode in the first drawing. In the second drawing there's a SPDT momentary switch to select (for the panel meter) which DC source it's operating on. In the NC position the meter is powered when the vehicle is on. In the NO position, the meter only reads when the PB is pressed.

And SPDT momentary switches are more expensive and harder to find. But I linked to one.

It was @ErnieM who first proposed the SPDT momentary switch. And I'd stick with the momentary function because otherwise you might switch the meter on and inadvertently leave the meter constantly reading battery voltage. If it should go unattended for months it will drain the battery. Hence, the momentary switch. The TS said he may just go with a toggle switch; but that might end up draining the battery. I'd advise against the toggle.
Uh Oh, I never gave back feed a thought. I should have read this more closely. Yes, enter the diode on the ACC line.

Thought of something else too. Make sure the meter used is capable of measuring the voltage powering it. Not all of these handy little meters are.

Thanks again Tony for pointing that out and just maybe I should have more thoroughly read the thread and placed some thought into it.

Ron

#### Tonyr1084

Joined Sep 24, 2015
6,864
@Reloadron - I do that same thing too. That's why we all need to run things by others, so they can spot potential mistakes.

#### Tonyr1084

Joined Sep 24, 2015
6,864
Removed comment. Put it on the wrong thread. Sorry.