Reverse-engineering avionics: Bonzer Digital DH Converter for radar altimeter - Page 1
Forum section: electronics / projects

author: D Straney
author_rank: Frequent Contributor
timestamp: September 09, 2023, 06:34:17 pm
content: Here's a piece that (kind of) goes along with the radar altimeter I looked at here:https://www.eevblog.com/forum/rf-microwave/avionics-reverse-engineering-bonzer-mini-mark-radar-altimeter/It's a Digital DH Converter, which was being sold essentially as scrap, came in a nice bright red enclosure, and seemed worth a look.From the little I was able to find, "DH" stands for "Decision Height": the height threshold during an aircraft landing approach at which if you're not in position yet, you can't safely attempt the landing and need to pull up and go around again for another try. So I'm guessing there's some kind of setpoint/comparison/warning-or-alarm functionality in here.Inside there's a single board attached to standoffs on the metal backing plate, with a voltage regulator heatsinked to the plate:Can see a lot of CMOS logic, plus the MC3302, which is a quad comparator (same series as LM339 apparently). There's also a good number of jumper wires on both the front and back, but not quite like the radar altimeter which seemed coated in wiring on the backs of the boards.At first I thought the empty DIP socket was for some kind of debug or production-test interface, but then realized after seeing an installation manual preview and a photo of another similar unit, that there's a ribbon cable that's supposed to plug into the socket, exit through the now-obvious small gap in the enclosure (although it doesn't come through in the photos), and connect to the display unit mentioned by the label on top. You can see a photo of one of these athttps://www.ebay.com/itm/303856623842, and the display looks too small to have any actual electronics in it, so all the actual functionality should be in this unit here.After tracing out the schematic... (PDF copy attached at the end)The main feature is a counter that creates a descending analog ramp by selecting different resistors in a voltage divider. Some of the connections and the order of the resistors I was unsure about (as the resistor values are hard to read when they're on the bottom), but I was able to power up the unit and give the counter some clock pulses to watch the descending output voltage. Due the short pulse-generating circuit with a NOR gate (IC2D), the counter cycles back to its initial value as soon as it "falls of the bottom" of its range. This circuit also acts as a power-on reset, taking full advantage of the high input impedance of CMOS logic gates by using some large resistances.The current counter value is also sent to a BCD-to-7-segment decoder, whose output goes over the ribbon cable. It looks like the display unit has a single 7-segment display, and so I'm guessing that the "Set" button is what feeds the debouncing circuit that clocks the counter, to cycle through the choice of 8 different setpoints and display the corresponding number.There's a comparator that compares a scaled analog input (which likely represents the current altitude) against this setpoint created by this primitive DAC, which seems to be the "decision" part of "DH". This comparator feeds a very....quirky...idiosyncratic...circuit with a few other inter-connected comparators and some various RC delays. Normally I'm pretty good at doing some mental simulation and figuring out how a circuit is supposed to work, but this one I had to simulate in LTspice to figure out. It turns out that what it does is actually pretty simple - just creates a 3-second pulse on a rising edge. I feel like there's definitely some much simpler & more straightforward ways to do that than with two comparators and 3 capacitors: was the Bonzer EE responsible for this on an acid trip that day?ANYWAYS, the upshot is that when the external input is below the setpoint (meaning that the altitude is below the DH?) it turns on 5V to something on the display for 3 seconds - maybe a warning light? (not sure what that rectangular area is above the display) It also blanks the 7-segment display for the same time, probably as a warning, and drives an external output high, which could be the alert buzzer mentioned briefly in the snippet of the installation manual.The "self test" button on the display is probably what activates the debounce/timer circuit in the bottom-right corner of the schematic: this creates a 32-second 5V pulse on an output to the main connector. I'm not sure exactly what this activates - maybe some kind of self-test functionality in the altimeter? (the Mini-Mark altimeter I took apart doesn't have anything like this, but maybe one of the more complicated Bonzer altimeters does)I'm completely puzzled about the circuit in the bottom-left corner of the schematic, which does something with connections from the display, and some components seemingly connected the wrong way around. It seems to expect a negative voltage on the pad of the mounting hole (which connects to the metal enclosure through a standoff)...even though the ground tab of the 7805 regulator is also connected to the enclosure through a metal screw and a metal washer, so the enclosure should be at circuit ground anyways. I double- and triple-checked the connections here, inc. with my meter for continuity, but it came up the same every time. I also have no idea what the 3 connections from the display would be doing here, unless this is current-source control for a warning light in the display, or something like that. Maybe Q3 is activated by the 5V possibly-self-test pulse, and it brightens the display's warning light through a low-side connection on pins 7 or 13? The 2 diodes across Q4 could be there to feed a lower voltage to the warning light in steady-state, to create 2 different levels (dim and bright, for "power on" vs. "warning"). I still don't know what the mounting hole connection and "backwards capacitor" are about though.