Using large / high current 7 segment LED displays with the MAX7219

I have some largish 7 segment displays which are 0.8″ in height but I found that they were dim when driven from a MAX7219 even at full brightness. I found that this was because the LED displays I were using need around 15 mA for a decent amount of brightness, 25mA ideal. However the MAX7219 only has a maximum peak current of 40mA per segment; now that is peak current so that means with a multiplexed display that will be divided by the number of digits to give an average current of 5mA, or if you prefer the equivalent DC current. Now as my displays need 15mA that simply isn’t enough hence the dim display.

So to get around that problem I used a UDN2982 as a source driver between the segment outputs and the LED segment anodes. 47 ohm current limiting resistors were used (more on that later) and a ULN2803A was used as the sink driver on the digit outputs. Now as the digit outputs will be at a logic low level I had to use an 74HCT04 inverter between the MAX7219 and the ULN2803A as the ‘2803A needs a logic 1 to ‘turn on’ it’s outputs.

I used an LED supply voltage of 7V as 5V wasn’t enough to get a decent amount of current through the displays as the UDN2982 drops about 1.6V across it and the ULN2803A about 1 volt. Together with the voltage drop across the LED display I was only left with 0.4V to play with at a supply voltage of 5V. Both the ‘2803A and the ‘2982 have a absolute maximum current of 500mA for the entire chip so I aimed for 60mA peak current per segment which gives an maximum current through the UDN2982 of 480mA when the worst case scenario of the displays showing 8 and the decimal point is lit. 60mA divided by 8 digits still only gives 7.5mA average segment current – still not enough but as I was using only 4 digits I set the MAX7219 scan limit to only 4 displays which doubled it to 15mA equivalent DC current. This resulted in an acceptable display without overloading the driver IC’s.

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P&G aircraft cockpit voice recorder (black box) teardown

This is probably going to be the last aviation related teardown video but to finish this teardown series I’ve got something pretty cool; a black box from an RAF Nimrod MRA4 which was a failed project and numerous aircraft were built but were never delivered to the RAF and were later scrapped. This one does show some signs of wear and tear although it may well be damage due to storage and this unit has never actually been used. The locator beacon on the front which is a circular tube containing a battery and the radio beacon itself is missing however everything else is present.

Penny & Giles Nimrod MRA4 cockpit voice recorder photo.

It is a solid state recorder utilizing 128Mb of flash memory to store the audio recordings and it is buffered and compressed by the electronics to reduce file size and therefore increase recording time. I found the recorder to be functional as I managed to power it up but unfortunately I was unable to access the recordings or the built in serial menu via the RS232 port on the front panel.

The recorder is powered by two microcontrollers; a Philips 80C51 clone which serves as the boot processor and a 32 bit AMD AM29200-20 as the main application processor. The boot ROM is stored in the 8051 and there is a separate ROM for the 29200 CPU which looks like it contains the operating system (probably some kind of RTOS) as well as some flash for containing the application that actually runs the device. This looks like it is flashed / updated via the serial port on the front panel. I managed to dump some of these ROMS so I could get some idea how it works.

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Clone Wemos D1 mini boards intermittent resetting and other problems

I’ve recently been experimenting with the ESP8266 modules and made a YouTube statistics counter as the first and experimental project. I used a NodeMCU for that which gave me no problems at all. However there is a smaller board, the Wemos D1 mini which on first impressions looks like a more ideal board for final projects and can easily be soldered to veroboard and prototyping boards.

The problem I had was my boards were clones as the originals no longer seem to be made and sellers advertising genuine boards (even showing a photo of the genuine board) are sending clone boards instead. Now that may not seem like a bad thing but I had constant intermittent reboot problems and the board would stick on the bootloader in flash mode if the 5V supply rail was ever so slightly above 5V. That meant it would boot connected to some devices that provide USB power and not boot on others.

This was driving me nuts and I wasted a lot of time trying to find the problem thinking my code was at fault believing the intermittent reboot was due to a stack overflow. I came across this reddit post which shows that mostly these clones come with voltage regulators that cannot handle the load the ESP8266 puts on them. Uh oh.

This post is dated 2018 and a quick look at my boards, from different sellers had voltage regulators marked 4A20 which indicates it can supply a maximum current of 150mA. However the ESP8266 draws a peak current of 430mA especially at boot.

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Aircraft instrument teardown TCAS / TCRD LCD display unit

I obtained for teardown via the wonders of eBay another aircraft instrument. This time it’s a Honeywell LCD display unit that can be used as a TCAS (Traffic Collision and Avoidance System) display or TCRD display. It is for older aircraft that do not have glass cockpits that need to have a TCAS system installed as otherwise such TCAS data would be shown on the main MFD’s. This unit is actually from a helicopter and has modified firmware to turn it into a TCRD display which I’m not entirely sure what that is. I think it stands for Torque Cruise RPM Display but I stand corrected if anyone knows otherwise.

It’s a well engineered unit consisting of a couple of power supply boards, an analogue board and a couple of processor boards containing a custom ASIC and FPGAs with the main CPU being a TMS34010 which is actually a combined CPU and GPU. It is capable of generating 2D graphics for the various symbols and graphics needed to display the data. This chip was used in several arcade cabinets of the late 80’s and early 90’s including the Terminator 2 game cabinet. Which is kind of cool. Speaking of which data is supplied over the ARINC-429 bus and this device generates all of the graphics required; it is a fully self contained computer system in a tiny box. The only disappointment with this is that it runs off 115V 400Hz rather than 28V DC so I’m unable to power it up. It’s also pretty much unsuitable to be used for parts so it’s going to end up either as a man cave item or be re-sold on eBay.

Here’s some photos and a video of the teardown

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A YouTube statistics counter for subscribers and views (2020)

This month’s project; a YouTube statistics counter I made from spare parts I had laying around and also a good opportunity to play around with the ESP8266 module which is a WiFi chip that contains an 80Mhz 32bit CPU with some flash space free to run some additional code on. It has a USB bootloader in ROM to enable you to install custom programs which can be written in lua, python or C++ to increase it’s functionality. It’s basically a high speed Arduino as it can be programmed with the Arduino IDE amongst others. The Arduino IDE is what I used as I was already familiar with that and C++. I have a crap ton of spare 7 segment LED displays so I thought I’d put some of them to use and designed a YouTube stats counter that shows your current subscriber count and views to your YouTube page.

Now YouTube made some changes to how the subscriber counts are displayed last year and now instead of it showing you the exact number of subscribers you have they are now approximated and rounded down on a sliding scale. For example subscriber counts below 999 will show exact numbers and above 1000 they are rounded to the lowest 10 e.g 1234 subscribers will show 1230 or 1.23K depending on where it is displayed. This counter gets it’s statistics via the YouTube API which only returns numbers so in this case it would return 1230. So I decided that I would settle for a 4 digit display for subscribers and 8 digits for views. It is ideal for smaller channels with less than 10,000 subscribers who want to keep an eye on their channel growing. When you are just starting every new subscriber counts and this will show it in relative real time; the counts are updated every 5 mins.

As you can see I decided to make my own PCB as I didn’t want to assemble it on prototype board and connect loads of wires as that would just take too much time and it’s a tedious process. You can get boards made now for very cheap; I used JLCPCB and it only cost $17 including shipping which even with the current restrictions was very quick. The boards were manufactured and sent to me within 2 weeks. Not bad.

The circuit consists of an NodeMCU ESP8266-12E module, 2 MAX7219 chips and a 74ACHT125 buffer which serves to convert the 3.3V logic signals of the 8266 into the 5V the MAX7219 requires. Some vintage Fairchild MAN74A 7 segment LED displays finish the design for a retro look. I plan to fit it into a photo frame or perspex display case to finish the counter. Power is supplied via USB although I have provided PCB pads for a 5 volt standalone power supply.

I do have some spare blank unpopulated PCB’s for this project. Please let me know in the comments below if you would like one or even if you want one of these building. They would be sold via eBay UK. Link to buy a PCB here.

Code and all other files can be downloaded here Please click on read more for setup instructions.

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Oddball military hardware – GRF Control Panel

I bought some more crap; this time another eBay find which appears to be a control panel for a video processing unit that takes the recorded video from a Tornado jet’s onboard VHS video recorder (yes, really!) and exports it to another media format along with some video processing. It has several analog switches and consists of a keyboard with 7 segment displays showing the mission time etc. It was made by GEC-Marconi Avionics (now BAE Systems) in the early 1990’s and is called a “GRF Control Panel” which I think GRF is an acronym for GR Force – as in Tornado GR1 Force. But I’m not really sure. I asked BAE Systems about this unit but it appears to be something they forgot about or contracted it out and stuck their name on it. The boards say Tactus International on them as a clue. That company now seems to be defunct.

Unfortunately it does nothing on it’s own as it is simply a dumb keyboard and display assembly. Internally it is filled with CPLD chips and EPROMS which are used as combinational logic rather than for storing firmware. There’s some ICM7228 LED display driver chips, a few 74xx series chips and that’s about it. The CPLD chips could be possibly resold as they are expensive and obsolete and easily reprogrammed. The rest can go in my spares bin as I managed to control the LED display with an Arduino using the ICM7218 library which works with the ICM7228. The whole thing runs off 5 volts so it was easily powered up and it communicates with the host system via a RS232 port which spits out a continuous stream of garbage. Hmm. It’s not quite as fuckaboutable as I’d thought. I was thinking of connecting it to an Arduino or even a PC to display something useful like the time / date, Youtube stats etc but that’s not gonna work.

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Merlin Helicopter CDU update – serial port probing

I have been trying to see if the Merlin Helicopter CDU I wrote about in my previous article could be repurposed. I found that it has a number of serial ports which are a mix of RS485 and RS232 and was hoping that key presses would present something on the serial port(s) or the display could be controlled via the ports. The pinout of the large 26 pin connector can be found here.

I found that one serial port is connected to the processor board and outputs diagnostic information once booted. This is a strange one as all ports are 57600 baud, no parity and 2 stop bits. The ports all output 0123456789 briefly a second or two after power on and during loopback test but the port connected to the processor board switches to 19200 baud, no parity and 2 stop bits then dumps some diagnostic information shown in the image below.

I wasn’t able to get anything else by pressing the keys nor able to get it into maintenance mode either by pressing “S” (53 hex) nor via the serial menu. I tried sending various commands but it would not move off the CCS Failure message on the screen. The error message entry point not declared seems to imply that the RTOS cannot find a particular task (application) to execute. Have it been purposely removed or is it just because it isn’t connected to the rest of the aircraft’s systems? I tried the other RS232 port but that outputs nothing at all nor responds to commands. That leaves just the RS485 port and the TTL level serial output which also just sends 0123456789 at boot and loopback test. Nothing is output when keys are pressed and the unit does not respond. The garbage shown is 0123456789 if switched to 57600 baud as in this image:-

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Merlin helicopter CDU teardown and power up demo. Three microprocessors inside!

With lockdown continuing I bought some more military surplus aircraft avionics to mess around with to pass the time and try and figure out how they work of if they could be modified for some other purpose. This time I have a RAF Merlin helicopter CDU (Control Display Unit) to strip down and see if I could power it up. This is the most modern piece of hardware I have got so far dating to 1998 with a sticker suggesting it was repaired in 2002 and it consists of modern surface mount components. It is manufactured by Racal Avionics (now Thales Avionics) in the UK! which is not something you see often nowadays. Actual British electronics and not made in China!

Front view of the CDU from the Merlin helicopter

The unit was marked as unserviceable due to broken fixing bolts so it would not stay secured however it is fully functional otherwise. Furthermore the RAF transferred all of their Merlin helicopters to the Navy where they were upgraded and hence these units are now obsolete and were picked up from a military surplus seller. There wasn’t any information on this unit and searching Google brings up no information either other than photos of the Merlin cockpit with the pilot interacting with one of these units; there are two per aircraft. Prince William may have even flown one of these models of helicopter and could have even used this particular unit however I couldn’t find anything suggesting that.

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Found a collection of old ceramic CPU’s. Exposing the dies

I have a ton of crap I have collected over the years and I come across a bag of old CPU chips – early pentiums, some DEC alphas and some SUN ultrasparc chips that I had kept for some reason. There’s even the main VLSI chip from a Wang 2200 2nd generation system my dad’s work were still using until the late 1990’s. Seem as they are so old now I thought I’d crack them open and expose the dies to see what’s inside of them. Here’s some photos of them. Click on the photo for a description of what they are and where they came from.

More RAF Tornado parts – a look inside the bearing indicator display

There are a lot of parts from the RAF Tornado on eBay due to them being retired from service back in 2019. I did a teardown on one of the avionics boxes on one of my previous posts which revealed lots of 1970’s silicon tech. Not very impressive by today’s standards but pretty cool to pull apart and see what an aircraft’s avionics consists of. The box itself would make a great chassis to build a bench power supply into once I pull out all the old electronics. Boxes full of 1970’s era electronics were still in use in 2019 although some of the aircraft’s systems were upgraded at some point such as replacing the cockpit displays with colour LCD’s.

RAF Tornado Bearing indicator display

Which brings me on to this item. What this appears to be is a bearing indicator going by the description on the back. This is the display for the Radar Warning Receiver system which shows which direction incoming missiles are coming from so the pilot can release countermeasures and avoid being hit. Many thanks to the curator of BAE Systems’ Rochester Avionics Archive for help in identifying this item. BAE didn’t make the display as the radar division of Marconi who made this wasn’t absorbed into BAE systems. It is a green screen CRT with no video processing circuitry inside the box at all so no composite video input. All the circuit boards consist of is an X deflection board, a Y deflection board both of which appear to be identical and a beam control PCB. The EHT and other high voltage is produced by a module in the top part of the chassis.

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Panavia Tornado Avionics LRU teardown

Whilst browsing through Ebay for weird and obscure things that people sell I came across several sellers selling surplus military and old aircraft parts. Two particular aircraft seem to be the most popular; the RAF Tornado and the Nimrod with the former retired from service in 2019, the latter much earlier back in the early 2000’s. Rather than the parts coming directly from the aircraft scrapyard I think these have been in storage for a long time and have come from avionics repair centres or spares stores as these parts are no longer required due to aircraft retirement. Some of the parts were ridiculously expensive and some were even brand new especially the LCD screens which were fitted during an avionics upgrade back in 2000-2001 I believe?

Anyway, some parts were very cheap and several LRU’s (the black boxes that sit in the avionics bay) caught my eye. I have often wondered how aircraft electronics works and exactly what is inside those boxes and how they communicate with each other. I took a punt on one of the cheapest LRU’s that was complete as a lot of the ex military stuff has had the electronics stripped out and they are literally empty black boxes. The thing I bought was an “Interface Unit 1” which after some searching on google it appears to sit between the main computer and several other systems within the aircraft and external sensors. There was also an “interface unit 2” which looked similar amongst other things which had their electronics stripped out. I presume this was done as the circuit boards were classified; they could have contained ROM’s containing code that was confidential for example. It was also probably to stop reverse engineering, not that anyone would probably want to for something so obsolete and ancient.

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1940’s National watch cleaning machine repair

I was given this machine to repair & restore – a National Electric watch cleaning machine Model 1 which dates from the 1940’s. It was in a very poor condition, rusty and all electrical parts visibly corroded and the original fabric insulation on the wiring had rotten.

National watch cleaning machine
National Watch Cleaning machine model 1

This was a very dangerous machine even when new; they didn’t care much for electrical safety in them days! If you turned it over all the live wiring was accessible as was the rheostat which controlled the motor speed. Simply picking it up whilst plugged in could have resulted in an electric shock. Also the heating element, directly connected to the mains had a jar sat on top of it full of water which is splashed about with an impeller.

I decided that in order to make this thing somewhat safer it would need to be re-wired and some sort of plate fastened over the base to cover the exposed live parts. However upon further inspection I deemed it too unsafe to even plug in so I didn’t bother testing it with power applied. I tested all the parts individually and found that the motor had burnt out with it visibly heavily water damaged and rusted, the power switch was open circuit even in both positions, the rheostat was visibly corroded and the heating element was busted. In short every component on it was faulty with nothing salvageable or repairable at all. It was questionable what I would be able to do with it given the lack of parts, tools and a decent work area as this had to be done on the kitchen worktop.

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