Up Close And Personal With Some Busted Avionics

When he found this broken Narco DME 890 that was headed for the trash, [Yeo Kheng Meng] did what any self-respecting hardware hacker would do: he took it back to his workbench so he could crack it open. After all, it’s not often you get to look at a piece of tech built to the exacting standards required by even outdated avionics.

DME stands for “Distance Measuring Equipment”, and as you might expect from the name, it indicates how far the aircraft is from a given target. [Yeo Kheng Meng] actually goes pretty deep into the theory behind how it works in his write-up if you’re interested in the nuts and bolts of it all, but the short version is that the pilot selects the frequency of a known station on the ground, and the distance to the target is displayed on the screen.

Inside the device, [Yeo Kheng Meng] found several densely packed boards, each isolated to minimize interference. The main PCB plays host to the Mostek MK3870 microcontroller, an 8-bit chip that screams along at 4 MHz and offers a spacious 128 bytes of RAM. It doesn’t sound like much to the modern AVR wrangler, but for 1977, it was cutting edge stuff.

Digging further, [Yeo Kheng Meng] opens up the metal cans that hold the transmitter and receiver. Thanks to the excellent documentation available for the device, which contains extensive schematics and block diagrams, he was able to ascertain the function of many of the components. Even if you’re unlikely to ever go hands on with this type of technology, it’s fascinating to see the thought and attention to detail that goes into even seemingly mundane aspects of the hardware.

Hungry for more airworthy engineering? We’ve taken a close look at some hardware pulled from a civilian airliner, as well as some battle-hardened electronics that once graced the cockpit of an AH-64 Apache attack helicopter.

Tricking A Vintage Clock Chip Into Working On 50-Hz Power

Thanks to microcontrollers, RTC modules, and a plethora of cheap and interesting display options, digital clock projects have become pretty easy. Choose to base a clock build around a chip sporting a date code from the late 70s, though, and your build is bound to be more than run-of-the-mill.

This is the boat that [Fran Blanche] finds herself in with one of her ongoing projects. The chip in question is a Mostek MK50250 digital alarm clock chip, and her first hurdle was find a way to run the clock on 50 Hertz with North American 60-Hertz power. The reason for this is a lesson in the compromises engineers sometimes have to make during the design process, and how that sometimes leads to false assumptions. It seems that the Mostek designers assumed that a 24-hour display would only ever be needed in locales where the line frequency is 50 Hz. [Fran], however, wants military time at 60 Hz, so she came up with a circuit to fool the chip. It uses a 4017 decade counter to divide the 60-Hz signal by 10, and uses the 6-Hz output to turn on a transistor that pulls the 60-Hz output low for one pulse. The result is one dropped pulse out of every six, which gives the Mostek the 50-Hz signal it needs. Sure, the pulse chain is asymmetric, but the chip won’t care, and [Fran] gets the clock she wants. Pretty clever.

[Fran] has been teasing this clock build for a while, and we’re keen to see what it looks like. We hope she’ll be using these outsized not-quite-a-light-pipe LED displays or something similar.

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