New project: Fluke 8840A/8842A continuity buzzer - Page 1
Forum section: electronics / testgear

author: bikeNomad
author_rank: Contributor
timestamp: February 01, 2024, 03:14:43 pm
content: I've designed a board for adding a continuity buzzer to these wonderful old meters.I'm having 5 made by JLCPCB, and should have them in hand by the last week of February.Kicad design files and MicroPython code (untested) are here:https://github.com/bikeNomad/fluke_884x_continuity_buzzer.The board uses an RP2040 microcontroller to watch the signals going to (and from) the display/keyboard module. It plugs in to the 16-pin DIP connectors on the display board, and the ribbon cables from the meter's main board plug into my board.The next version may add a USB isolator and the ability to fake keypresses; this would allow for USB SCPI control of the meter (the USB port on the board is only for programming presently).[ Attachment Invalid Or Does Not Exist ]

author: Kleinstein
author_rank: Super Contributor
timestamp: February 01, 2024, 10:10:12 pm
content: Given the effort for the ciruit, why not a completely separate contuity buzzer with just the buzzer, no ADC or display.

author: bikeNomad
author_rank: Contributor
timestamp: February 02, 2024, 03:24:09 pm
content: I designed this because I have several of these meters, and the one complaint I have about them is the lack of a continuity buzzer. Since I have one living at my bench easily at hand, this will keep me from grabbing my portable multimeter (and waiting for it to start up). These meters have a very fast reading rate on the ohms range.

author: Chris56000
author_rank: Frequent Contributor
timestamp: February 02, 2024, 06:35:32 pm
content: . . .That's even more reason to :–a) . . .spend my strictly rationed pocket–money on an '8840/8842 – I've bought a Display Replacement Kit!) ;b) . . .add yet another chapter to my already gigantic T & M Theory and Repair Book!!Chris Williams

author: bikeNomad
author_rank: Contributor
timestamp: February 09, 2024, 07:06:38 pm
content: Well, I got the boards early, and then discovered that I'd made them too tall and they wouldn't fit.But after having pulled a meter apart, I can at least test the board and my code before ordering new (shorter) ones.I am a bit sad about having to get rid of the really nice routing of my first board. I had the autorouter re-do those traces so it is no longer as beautiful.Perhaps on the next spin I will add 3.3V->5V level converter/buffers so I can also fake keypresses.

author: bikeNomad
author_rank: Contributor
timestamp: February 12, 2024, 02:06:55 pm
content: Well, I re-spun the board and added a 74LVC245 to allow for driving the keypad. Still no USB isolator though.Now I get to wait for Chinese Lunar New Year...

author: bikeNomad
author_rank: Contributor
timestamp: February 26, 2024, 02:48:51 pm
content: Boards came in and the firmware is working great.Still haven't done anything with the keypad reading or driving, but at least the circuitry is there to do so.If you set the speed to "M" and choose the 20Ω range, it's quite fast with the buzzer.I still haven't decided what threshold to use for the continuity indication; right now it's 10Ω and it doesn't respond if you're in the "kΩ" or "MΩ" ranges.It seems like if you're looking for continuity the 20Ω or 200Ω ranges would be most appropriate.

author: unexpectedbrunchguest
author_rank: Newbie
timestamp: May 03, 2025, 12:03:09 am
content: Do you have any of these boards left? It seems like you did have a successful end result yeah? I tried sending a private message but I think I'm not allowed because it's my first post? This project is very appealing to me and I would love to avoid manufacturing multiples when I need one especially with the tariff chaos that's been going on...