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$1 DVM as a Digital Input Display — Parallax Forums

$1 DVM as a Digital Input Display

My 5V circuit has a potentiometer to adjust a timing delay and I wanted to add a digital display for precise calibration. I realized that 0-5 seconds could be displayed as 0-5 volts on a dollar DVM, so I just used the pot as a 5V voltage divider and connected the DVM to the wiper. (This requires a 3-wire DVM, since a 2-wire unit won't work below ~3V.) Micro reads the voltage via ADC and the code converts into seconds.

If desired, the input range could be limited or shifted by adding one or more fixed resistor(s) in series with the potentiometer. This would also reduce the sensitivity of the pot adjustment and help stabilize that last flickering digit on the DVM. I used a $1 Powerbank for my 5V supply. One advantage to leaving the upper limit at 5V is that I can use the DVM & pot to check for full 5.0 V output. Less than 5V indicates a low 18650 battery in the Powerbank and time to recharge.


  • WhitWhit Posts: 4,191

    Nice work erco!

  • @Whit said:
    Nice work erco!

    What he said. Nice work. Let's see. the powerbank was from your local dollar store? But who supplied the display?

  • ercoerco Posts: 20,232
    edited 2021-10-12 03:05

    All pre-Covid Ebay $1 purchases. Most of erco's dollar deals are gone now. Even if you can find something for a buck today, Chinese Ebay sellers want $3-4 for shipping. :( Ken may be happy but erco is not! I am buying more on AliExpress now that they take Paypal. The best deals are buying multiple units to find a good deal on bulk shipping. Now I'm even MORE of a hoarder!

    For instance... I got 4 of these green DVMs for $4.96 (incl tax & shipping). But ordering a 5th unit, the shipping jumps $2. It's a game...

    Another: Free ship on ONE FM radio kit, but two will cost $2.33 to ship. So order individually!

  • sam_sam_samsam_sam_sam Posts: 2,285
    edited 2021-11-25 10:47

    PayPal users of there fairly new feature should be aware of an issue with the “ 4 payment plan “website page link has the first problem is if use an iPad to use this service you need to aware that each time you use this service you need clear your internet history and the cache and cookies because this site does not function correctly if you do not do this first

    Here is what happens if you do not do this first what I said above when you get to the screen that tells you what the payment will and the date will be on when you click continue it freezes and unresponsive if you refresh the page or go back to the previous page it might take you to the next page or it might take the full payment from your bank account or to your credit card payment that you have in your wallet and not show this screen at all

    I have made the technical support aware of this issue but they keep telling me that it a user error and it is not on there end and I have suggested that they try to order something and use this option using an iPad newest version so they can see what I am talking about and not clear the history or cache or cookies you do not have to do this when you use the normal way to pay using PayPal website so why is the new payment option have so many issues

    Be careful with AliExpress if get something from them do not tell them that you received the item until you have tested completely and you happy with your purchase because once you have told them you have received it you do not have any recourse with them
    This also holds true for Banggood website as well has the same exact thing that if you tell them that you receive the items that you are happy with your purchase and they will not help you either

    Unless you file a complaint with PayPal you might get the issue resolved but do not count on for every issue you might have

    This has not happened each time that I have had an issue with something that I have bought from them but it has happened to me a few times so I do not rely on them to always do the right thing

  • sam_sam_samsam_sam_sam Posts: 2,285
    edited 2021-11-22 01:37

    @erco said:
    For instance... I got 4 of these green DVMs for $4.96 (incl tax & shipping). But ordering a 5th unit, the shipping jumps $2. It's a game...

    Are these meter any good and would recommend buying them

  • ercoerco Posts: 20,232

    Still waiting on these to arrive. But I have used similar ones off ebay and they work fine for rough voltage indications. It's rare for two to give exact same reading, so not for mission critical stuff.

    Will advise when I receive & test.

  • ercoerco Posts: 20,232

    BTW I have had good success with AE & Paypal. A couple times items arrived damaged or not at all. Opened a dispute in each case and AE always decided in my favor, no need to involve Paypal.

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