A 67 Cent Postal Scale




Introduction: A 67 Cent Postal Scale

About: Lazy Old Geek

PROBLEM: Once or twice a year, this Lazy Old Geek could use a postal scale. Coming up are the elections. Colorado is set up so that I can always vote by mail. I am lazy so I do so. I think you may have to request to vote absentee. Now sometimes the absentee ballot tells you how much postage but often they’ll say to use appropriate postage. So is it over an ounce?

The miser in me figures this way: I live about five miles from the nearest town. That’s ten miles round trip. My car gets around 30 mpg, so a trip to the post office (or to a polling place) is going to take one-third of a gallon or a little less than a dollar. To my friend who is taking college Algebra, here’s an example of applied Algebra. So it’s actually cheaper for me to mail in a ballot, than it is to drive in and vote. (I would do it anyway. I hate waiting in line). But I also don't want to underpay or overpay.

Okay, so I thought, I bought an Arduino clone, how can I use it for weighing a letter? I found some sensors that could do it but they were quiet expensive, so I decided to go low tech. Another Instructable has a good postal scale:

Pretty Good Postal Scale from Old CDs
by Phil B

And it’s probably cheaper than mine. But I wanted something that looked more like a scale. Since I’m OLD and will probably only use it a couple of times a year, it has to be cheap and easy to build so I can amortize it over several years. (HA! HA!)

NOTE: This scale is designed for use in the U.S. Adjust accordingly for other countries.

Step 1: Parts and Tools

So after my annual visit with my shrink, I went across the street to the Dollar store and found a pretty-close-to-what-I-wanted ruler.

Plastic ruler 3 for $1
2 Clips 10 for a $1
Dental Floss Free (my dental hygienist always gives me a sample)
Paper clip
5 quarters

Total cost per scale: 67 cents ( the quarters are refundable) since there are parts enough for three scales.


Step 2: Ruler Modification

STEP: Ruler Modification

Many of you probably took some physics in school and many probably developed Teeter Totter skills.

A balance scale (see picture) is like a Teeter Totter (see picture) and both work on the same physics principles (see Garfield picture). The basic idea is equal lengths and equal weights balance (see Garfield picture).

Non-essential Information: Algebra: I like to think of the balance scale (see first picture) as a visual example of the equal (=) sign. Everything on the left has to have the same weight as everything on the right. Which means whatever you do to one side, you have to do the same or equivalent to the other.

Unfortunately, the ruler wasn’t quite what I was hoping for. I was hoping for a hole in the middle and a hole equally spaced near the ends. So I needed to drill one hole to match up with the other end.
Scientific: The scientific me say, how am I going to get the hole the exact same distance from the center and what if it’s still not balanced? Will I have to shave it off?
Practical: The practical me says it’s not that critical, I have a calibration scheme to compensate and all I really care about is ‘Is this letter less than an ounce or more than an ounce.’

So I had to drill a not so critical hole in the ruler. What I did was: take my calipers to the side with the hole and adjusted them to the edge of the hole and the edge of the ruler, then I went to the other end and marked a line with my soldering aid tip. This made a square but you can’t quiet see it in the picture. Then I scratched a line from corner to corner to get the center of the square. This matched up to the center of the hole on the other end. This may sound pretty complicated. Depending on the ruler you are using, just try to make holes to match the other side.

CAUTION: Drill out the hole to the same size as the other one slowly and carefully. The plastic on a 33 cent ruler is thin and cheap. I know; I broke my first one.

Step 3: Assembly and Calibration

In attaching the clips, I did use a little physics. My first thought was to bolts or just glue the clips to the ruler. But depending on where you clip the letter, the center of gravity will change and that would affect the balance, so I suspended the clips with some dental floss so the distance from the balance point would always be the same.

Since most of us don’t have calibrated weights in our junk pile (I used to), we need something common of a known weight and what is probably the best candidate are U.S. coins. Phil M already figured out that five quarters, 1967 or later = 1oz.

Here’s a reliable supporting website, the US Mint:

The weight of a quarter is 5.67grams. 1 gram = 0.0352739619 ounces.
What? Another Algebra problem? Well I already did this: one quarter = 0.20003oz. So five quarters are 1.000017oz. Close enough for government use.

Non-essential Information: You may notice that all of the coins are a mixture of metals and the silver dollar is no longer a silver dollar and even the copper penny is no longer just copper. Over the years, prices of metal kept rising until the metal in the coins were worth more than the value printed on the coin. That’s how the ‘sandwich’ coin was created. I just read an article that the metals in a 1932-1964 quarter is worth about $3.75.

Calibration: I suspended the scale from a screw through the middle hole and clipped five quarters on each side. (See picture) It is a good idea to pull the ruler away from the door so the clips aren’t dragging along the door. If it isn’t balanced then take a paper clip starting near the center on the high side and slide it out until the ruler is level. (See second picture). I taped a piece of paper behind the ruler so that I could mark the balanced point.

Step 4: Scale in Use

Here’s the scale in use. I put five quarters (1oz.) on the left side and the letter on the other. If the ruler is slanted up as in the picture, than it is under 1oz. and doesn’t require extra postage. If the ruler is right over the calibration line or below it than I would add postage (currently 17cents per extra ounce).

Now this could also be used for 2oz, by putting 10 quarter on the left side. Like Phil M, you could put them in a baggie as it would be hard to clip 10 quarters.

For Algebra students, this picture is an example of an inequality.
The left side is greater than (>) the right side.
Which means the right side is less than (<) the left side.
Which means the letter < one ounce (five quarters).
Believe it or not, there’s some use for Algebra.

Lazy Old Geek

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    6 Discussions


    9 years ago on Step 4

    Though I do not have a drill to use to make an extra hole in the ruler, I think if you just tied the clips at the same point on either side and taped them down so they stay in the same place, and counter balance with paper clips/binder clips until level--this would still would. Good to know that 5 quarters are about an ounce, I usually just go down to the P.O. because it's only 1 or 2 miles away, but this is just such a cool really useful real life/real cheap use of math. Love it!!


    Reply 9 years ago on Step 4

    I agree, this is not a very critical application and if it's balanced at some point, then all you care about is if one side is heavier than the other.
    I haven't used my scale more than once but it is nice to know it's available if needed.



    9 years ago on Introduction

    Very nice. I like simple and effective solutions to problems. One thing though. I think you have a typo in the last paragraph of the last step. "The left side is greater than (>) the right side. Which means the left side is less than (<) the right side."


    Reply 9 years ago on Introduction

    Thank you. You are right. I usually try to sleep on these before I publish so I can look it over with a refreshed mind.

    I am getting OLD.

    I will correct it.



    9 years ago on Introduction

    Great project, and great physics (anyone can probably get a Feature flag by using Physics :-)! But, I gotta ask, where did you hook up the Arduino? And how much does it weigh? Could you have used it in place of the quarters?


    Reply 9 years ago on Introduction

    Thanks for the tip about Physics. I guess the same doesn't go for Algebra! Unfortunately, I couldn't use the Arduino in my 67 cent budget. It 's coming up in some future Instructables I hope to get going next week.