Replacement Front Panel for HP3468A multimeter - Page 1
Forum section: electronics / projects

author: intabits
author_rank: Frequent Contributor
timestamp: January 27, 2023, 06:48:32 am
content: I have three HP3468A's, but almost never use them because of the poor visibility of the display.So I've been working on a project to to replace the front panel, incorporating a better display. I call it the HP3468A-E (for "Enhanced").This is the first prototype, which has enabled me to identify many issues that will be corrected in the next version. I've been working on this for over a year off-and-on, and only a few weeks ago realized that there is significant problem with the current design, for which I have a solution, but have not yet implemented. I can't believe it took me so long to see it. Anyway, here are some pictures:-The original front panel and LCD, which is not backlit, making it hard to read in poor light or when not facing directly forward. It's looks good here, but that is not always the case.The new front panel with a 320x480 TFT graphics display with touchscreen.Very rough around the edges (literally!), but fixable. This is a PROTOTYPE. These boards have some size issues, and so do not fit the case properly. USB connection still to come.The back of the panel which is wired to IC pads on the underside of the main board. (I didn't want to do anything that makes it hard to remove)The mess at the right is a pair of analog switches to simulate keypad actions, this stuff will be integrated in the next revision.Also the LCD/controller assembly needs to be moved upward to give more clearance above the main board - it currently presses hard onto a couple of the chips.This is the front of two PCBs used to make the front panel. It has a paper decal stuck to it, with a sheet of transparency film over that. The rough edges are from trying to cut the film and paper to reduce the thickness at the edges. The proper solution is to use a thinner PCB for this.The back side of the front plate.Front side of the back plate. The notches and corners are there to avoid moldings in case, inside the slot that the panel fits into. Those moldings affect only this back plate.Back side of the back plate. The spacers support the LCD assembly, so that the touchscreen sits nicely just at the back of the transparency film.The LCD/Controller board assembly.With the LCD module removed, showing the ATMega1284 controller, voltage regulator, and connectors. The pins to the LCD are a variation of an Arduino UNO pinout. (that the LCD module was designed for)Back of the controller. The wires are just a bodge to supply power the analog switch mess.Bottom of the main board, showing wiring to the new front panel connectors. These wires are too thick, and the connectors were manually crimped. The next version will use pre-made JST connectors with thinner wires. All of the wiring should route to the bottom edge of the board instead of two edges as shown here.The power switch push rod will be replaced by a toggle switch to provide room above the ROM chip on the motherboard, so that a 27x64 replacement can be socketed, and also for the adapter board that will probably be required for the earlier 24pin version of these ROMs.The 7805 regulator runs from the meter's unregulated 7V supply, but gets fairly hot. I haven't yet checked the currents and voltages involved, though the transformer does not get even slightly warm. It may turn out that a switching regulator is required, but I'm not sure about having that inside a sensitive meter. I'm assuming the AVR will not be a problem, just like the existing digital electronicsThe firmware is based on the excellent work done by forum user "Xi" who decoded and published details of the signalling protocol used by HP in talking to these types of displays. The thread for that is:-www.eevblog.com/forum/projects/led-display-for-hp-3457a-multimeter-i-did-it-)/Unlike Xi's implementation where he added a few gates and flip-flops to enable use of the AVR's SPI subsystem to receive HP display control sequences, I chose to do this in code via bit-banging. Maybe it's worth adding the logic to take advantage of hardware, but I've had no problems with my method.I hope to use the bottom part of the display area for extended functionality such as high/low/average tracking, or graphical trace of measurements, etc - You know, all the good stuff that modern meters have.No doubt there will be those who consider this project a waste of time and effort, but for others, here is what I've been working on. Your helpful comments and suggestions would be most appreciated...Video:-https://youtu.be/IY4tkyuX1Eg