How to build a cheap LED voltmeter

[BT Humble]

Over the past week I've been experimenting with various means of getting a cheap, robust and reliable voltmeter for small solar lighting systems.  I need it to be cheap, because I'll be building them by the dozen (it's for remote village lighting systems in Fiji, a project which a fellow Otherpower contributor introduced me to).

Anyway, I considered the options available to me:

1. Analogue meter - too expensive, subject to corrosion (sea air), you need to be nearby to read it and know what "12.2V" means in terms of battery capacity;
   
2. Digital panel meter - again, too expensive & the number needs interpretation;
   
3. LED meter kit - A bit expensive at $14.92;
   
4. Pre-built LED meter cheaper at $9.97, but I think I can do better. ;-)
   
5. Design and build my own.
   
   

Here's the schematic I came up with.  The idea was based on my hazy recollection of a circuit from "Dick Smith's Fun Way into Electronics Volume 2", which was being used as a level meter for stereo systems.

Here's a picture of the finished product, and the prototype on its breadboard:

My initial PCB designs, made using a piece of cardboard and a pin:

Cutting the blanks out to size - be sure to use a 32tpi blade in your hacksaw!

Sicne I'll be making lots of these, I made a drill jig.  It's basically a little aluminium box with the pinout holes in the lid, and a block of wood the same size as the blank.

Blank goes on the block:

Jig goes on the blank:

Let's drill some holes!

A finished board on the left, and the drilled blank on the right.

The lands are drawn in place using a special "resist pen" - this is why I decided to go against tradition and drill the blanks first (it makes everything easier to align).  

Tie a piece of nylon fishing line through some convenient holes, and dunk the blanks into the warm Ferric Chloride solution for 15 minutes.

This eats away the copper that wasn't covered with resist ink.  I've cleaned up the one on the right with steel wool.

Coming up next, the power box that this was designed for!

BTH

[domwild]

Keep up the good work! I like the idea to assist third world countries by using low-cost solutions.

dominic

[Nando]

BT :

I have circuits that could be built for that purpose, please connect with me directly.

Using the LM3914 and some ancillary parts

nando37-at-comcast-dot-net

Regards

Nando

[BT Humble]

BT :

I have circuits that could be built for that purpose, please connect with me directly.

Using the LM3914 and some ancillary parts

Thanks Nando, I've already looked at those (in fact, the $14.95 kit I mentioned is based on that IC).  Unfortunately the best price I can get for LM3914 chips is $4.45 each (in quantities of 25 or more), whereas I can build this entire circuit (including the PCB) for $4.60

Additionally, this cicuit only draws 30mA, while the LM3914 circuits that I've seen (at least 3 variations available on the web) are around double that.

BTH

[nothing to lose]

Great Board and great instructions.

I may try that someday myself. Book marking the post

So, what's the deal on Fiji?? WHere could I get some info on that type of stuff?

Might be just what I need, Nice long peacefull vaction working hard to help some others.

[jimjjnn]

Nice work. You may wish to spray a conformal coating on both sides after assembly and testing to guarantee against corrosion in humid environment. Maybe varnish or lacquer or some other coating(that is cheap) might do the same.

[jimjjnn]

The 3914 circuit is a great one. Radio Shack also has a circuits book showing various configurations using the 3914. It may even simplify the circuit for you and will use fewer parts( I think)

[Old F]

BT

You my want to try replacing the 1K resistor on the led that indicates 13.5 or so volts with a reed relay with 1k coil resistants.

Then you would have a shunt type regulator you could turn on a dump load

Or disconnect the panels to keep the battery from over charging.

It wouldn't add that much to the cost and would protect a more costly battery. Just a thought.

Old F    

[wpowokal]

BTH are you talking about Richard Cross here (a fellow Otherpower contributor) if so has he changed his 'e' mail as I have lost contact with him.

regards Allan

[BT Humble]

You my want to try replacing the 1K resistor on the led that indicates 13.5 or so volts with a reed relay with 1k coil resistants.

Then you would have a shunt type regulator you could turn on a dump load

Or disconnect the panels to keep the battery from over charging.

It wouldn't add that much to the cost and would protect a more costly battery. Just a thought.

Good idea, thanks.

Also, I think I've messed up a bit there and built a circuit that will fry trimpots if they're incorrectly adjusted - I think I should be using a lower-voltage zener with a 1k resistor between it and the trimpot. (ie. zener->resistor->connection to the transistor bases->trimpot).

I'll post an updated diagram once I've sorted that problem.

BTH

[BT Humble]

Yes, but since I hadn't seen him mention it here I thought I'd let him keep his privacy.

E-mail me directly if you like, and I'll let you know his new address.

BTH

[johnlm]

if you ar going to use the meter to control things ths looks ok.  if you want to measure voltage, you can get a digital (cheap one but seems to work fine) from harbor frieght for $8.00.  I got one on sale at $3.99.  Compared it to my two good HP meters and found it was off about 0.5% on the dc volts setting.  Opene it up and adjusted a trimpot to read the same as my HP's.  Not a bad meter for 4 bucks.

Johnlm

[Ungrounded Lightning Rod]

You my want to try replacing the 1K resistor on the led that indicates 13.5 or so volts with a reed relay with 1k coil resistants.

Then you would have a shunt type regulator you could turn on a dump load

Or disconnect the panels to keep the battery from over charging.

While reed relays have considerable hysteresis, you may still find that turning on the dump load will drop the voltage enough to cause it to drop out, creating an oscilator.  B-(

I though of a way to give you an adjustable cutin/out with just a few more components but it's a tad too elaborate to go into here.

[Ungrounded Lightning Rod]

Also, I think I've messed up a bit there and built a circuit that will fry trimpots if they're incorrectly adjusted - I think I should be using a lower-voltage zener with a 1k resistor between it and the trimpot. (ie. zener->resistor->connection to the transistor bases->trimpot).

How about, insteading of grounding the tap of the trimpot and hooking the diodes to the top, grounding the bottom of the trimpot and hooking the diodes to the tap?

[troy]

I have half a dozen of the HF cheepie meters and a nice Fluke for precision work.  I did make a discovery about the Harbor Freight meters that could affect your use of that meter.  They apparently don't have any means of compensating for a low 9V battery.  And I have gotten into trouble on my big battery bank by using readings that were 3V high because of a low 9V battery in the meter.  The low battery indicator was not on at the time the error occured.

Also, some of the recent HF meters have a lighted display available with a switch.  I discovered that turning the light on caused a big change in the indicated voltage, so if you have one, don't use the light while reading anything that matters.

Not saying they aren't useful, just saying to throw away the cheap battery that came with it immediately, and make sure to have a fresh battery in there regularly.

Good luck and have fun!

troy

[BT Humble]

How about, insteading of grounding the tap of the trimpot and hooking the diodes to the top, grounding the bottom of the trimpot and hooking the diodes to the tap?

That would probably work too, but I gave this a try yesterday and now the whole thing works pretty much the way I intended it to:

If you want to increase the voltage spacing between the LEDs you can add an extra diode in series (as I've done with Q1).

I've revised the PCB layout a bit to accomodate the changes, and it now measures 100x25mm (4x1").  I'll post the artwork here if anyone is interested.

BTH

[BT Humble]

if you ar going to use the meter to control things ths looks ok.  if you want to measure voltage, you can get a digital (cheap one but seems to work fine) from harbor frieght for $8.00.  I got one on sale at $3.99.  Compared it to my two good HP meters and found it was off about 0.5% on the dc volts setting.  Opene it up and adjusted a trimpot to read the same as my HP's.  Not a bad meter for 4 bucks.

Basically my reasons for using this rather than a cheap digital meter are the same as listed here:

http://www.homepower.com/files/bat-o-meter.pdf

Essentially, you don't have to keep replacing 9V batteries in this one, and you can tell from the other side of the room if your battery is getting low (without having to refer to a chart of voltage levels).  Less is more! ;-)

BTH

[jacquesm]

I like it !

One note: if you really make this thing in 'volume' don't discount your work (if you can produce 100 instead of 50 in the same time that's a $ value too...), and the cost of your copper, if you add those two IC's may start to look attractive again.

Other than that it's a great little circuit !

thank you.