DEC H-500 Computer Lab Reproduction

Reproduce the Digital Computer Lab H-500, a training tool from the late 60's aimed at teaching people the basics of logic circuits.

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Most people reading this will be familiar with the Digital Equipment Corporation (DEC) lines of PDP machines. I would guess though that far fewer have encountered the H-500 Computer Lab. Launched in the late 60's the H-500 was part of a COMPUTER LAB curriculum to introduce students and engineers to digital electronics. It's not surprising that DEC would undertake this since more than half of it's PDP machines at the time were installed in educational institutions.

The machine itself shipped with a wonderful workbook that contained a complete course in digital electronics. Together the COMPUTER LAB package was intended to accompany courses in binary arithmetic, Boolean algebra, digital logic or computer technology. While not a true computer, the H-500 could be "wired" to perform many of the underlying operations of a true computer using a point-to-point patch cord mechanism.

I'm very much looking forward to this build.


So why would I want to undertake an H-500 Computer Lab reproduction? Because most of the projects that I have been working on lately follow the same pattern:

  • They are reproductions of cool computer toys and devices from the 60's.
  • Have tremendous educational value (when first released and even now IMHO). To that end they usually shipped with well written and instructive manuals.
  • Feature unique and noteworthy designs.
  • Due to their age they have become rare and thus expensive and hard to get.
  • Perhaps most importantly I think that the devices themselves and their designers deserve to be remembered and honored.
  • As a bonus for this particular project, I'm Canadian and the H-500s were manufactured in Canada. 

So the H-500 is a very good fit for my interests and skill set. Plus I really want one ;-)

Design Goals

With this reproduction I hope to provide the best H-500 Computer Lab experience possible. What do I mean by that? Well...

  • Visually my reproduction will be clearly recognizable as an H-500. I will strive to provide an outwardly accurate appearance of the original to the best of my ability and within the constraints of the implementing technologies.  Bear in mind that I can't afford one of these so I will be working from images and the kindness of actual owners to provide me with details.
  • Operationally my H-500 will perform like the original. A user will be  able to work through the experiments in the Lab Workbook without change or issues.
  • I would like for this project to be easy to well, reproduce. To this end I will only be using 3D printed parts and readily available components and materials. 
  • I will try to keep the total project cost to a minimum.
  • A complete set of instructions, parts list, and STL files will be posted to Instructables at the completion of this project so that others could make one should they choose to.

So hopefully my H-500 reproduction will look and work like an original, but I can guarantee (at least for version 1) that the very cool PCB with "surface mounted" components used in the original will not be part of the final design in order to meet cost and reproducibility goals.

 (Maybe version 2?)

Rocker Man

Well rocker switches is actually the reason that I started considering this project.  At the end of my Mostly 3D Printed Rocker Switch project I posted the following:

With a workable and customizable rocker switch in my tool kit now, I'm looking at you PDP-8/I ;-)
The remark was made somewhat tongue in cheek, but it did get me thinking about it since I love the look of the switches that they used. I actually started researching the possibility of making an 8/I replica. I soon discovered however that Oscarv had already created an awesome 2:3 replica the PiDP-8/I which I purchased (I'm just waiting to finish this project before assembling it). So not much incentive to duplicate his great work.

A short while later I was looking at the System Source Computer Museum Toy Computers page and I saw the DEC Computer Lab. WOW. This was actually a much better fit for both my interests and skill set. I have made replicas of six of the other computer toys on this page five of which I created myself.  And the DEC Computer Lab uses the same rocker switches as the PDP 8/I. Perfect.

Making the Switches

My starting point was an STL version of the PDP 8/I rocker switch that I found at Vince Slyngstad's PDP-8 Stuff page.

I modified it slightly to have a hole at the pivot point instead of a shaft. Then I modeled a "counter weight" to hold the switch magnets and a corresponding base for the "reciprocating" magnets and the reed switches. You will notice that there are two kinds of counter weights. That's because there are two kinds of rocker switches required, a momentary "spring" return for the three H-500 pulse switches, and an ON-ON variant for the eight input switches.

The base has slots...

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Adobe Portable Document Format - 7.89 MB - 07/01/2020 at 02:56


DEC H-500 Computer Lab STL

All of the STL files required to make a DEC H-500 Computer Lab reproduction.

x-zip-compressed - 2.53 MB - 06/25/2020 at 17:42


7400 Series Breakouts.pcb

The DesignSpark PCB design file for the Plug Panel breakout boards.

pcb - 51.00 kB - 06/24/2020 at 20:43


7400 Series

The Gerber files used to produce the PCB breakout boards for the Plug Panel.

x-zip-compressed - 9.06 kB - 06/24/2020 at 20:40



Workbook used in conjunction with the DEC H-500 Computer Lab.

Adobe Portable Document Format - 6.52 MB - 06/24/2020 at 20:37


View all 6 files

  • 1 × Arduino Mega 2560 Used to implement the clock and switch funtionality.
  • 11 × NO Magnetic Reed Switches Digi-Key part number 2010-1087-ND
  • 24 × 2-pin Male Headers Amazon - Gikfun 40 Pin Strip Breakable Pin Header Tin PCB Panel IC Male 2.54 mm for Arduino
  • 48 × 1-pin Female Headers Amazon - PTSolns 40 Pin 2.54 mm Single Row Female Header Gold Plated
  • 350 × Rolled Flange Head Eyelets Spaenaur part number E-412

View all 16 components

  • A Better Incandescent Look

    Michael Gardi07/01/2020 at 04:34 0 comments

    BrightBlueJim pointed out in a comment to Dan Maloney's blog entry ( that the lamps looked blue, which as he pointed out is not very incandescent like (the original used incandescents).  The LEDs are actually white shining through translucent PLA diffusers, but they do look a little blue in the videos. 

    So I tried printing the diffusers in both yellow and in the same brown that I used for the rocker switches. 

    Hard to see in the picture but the yellow (left) doesn’t look too bad (better than the transparent IMHO), but the brown looked great after I changed the limiting resistor to about 330R to brighten up the LED.  If I were doing it again I would definitely go with the brown diffuser.

  • Wiring the Lower Half

    Michael Gardi06/19/2020 at 23:02 0 comments

    The first thing that I noticed when I started the wiring was that the switch assembly wires (purple) and the light panel LEDs (grey hexes) were overlapping, making my life harder than it had to be.

    So I removed the switch assembly and rebuilt it, moving the eight brown rocker switch wires to the bottom of the assembly (the pulse switches were OK). There is a nice channel on that side to run the wires between the frame and the assembly itself.  Following this wiring diagram (which has a small change to the potentiometer power wire from the one posted previously)... was pretty clear sailing and here is the result:

    For reference the wire colors are:

    • purple - switch inputs to Arduino
    • grey - switch outputs from Arduino
    • white - clock out and clock delay control from and to Arduino
    • yellow - potentiometer input to Arduino
    • red - power +, 9V to Arduino, 5V from Arduino
    • black - ground

    The LEDs are wired directly to the lamp inputs with a 4K limiting resistor (large enough to keep the brightness down to incandescent levels).  While wiring the LEDs I noticed a pretty big goof on my part. The label for the lamp terminals should have read LAMP INPUTS.  Doh!

    Oh well nothing to do about it now. I have updated the patch panel STL file but will live with my error on this build.

    All that's left to do now is run power up to the PCBs and the HIGH and GND terminals. Very excited to be this close.

  • Framing

    Michael Gardi06/11/2020 at 14:47 0 comments

    Well my lumber and stain finally "arrived". It took my local Lowes 17 days to put together my "curbside pickup" order. Sigh. If I had access to my local maker-space (kwartzlab) I probably would have ripped something down to the required 1/2" x 3 1/4" board size that I needed, but I was "lucky" enough to find some "craft board" at Lowes in the required size.

    The other maker-space thing that I missed was access to their fine selection of corner clamps to put the frame together, so I had to improvise. I ended up printing some corner guides that worked quite well.

    I mitered the corners of the frame and used the guides to align the boards while I glued and popped in a few brads.

    Once dry I stained the boards then applied a few applications of clear coat. I was a little disappointed in the stain as I was expecting it to be a little lighter and redder like the original's "simulated teak". But given how long it took to get the stain I went ahead and used it.

    I printed some supports to hold the Plug, Light, and Switch Panels in place at their appropriate depths (1/2" for the Switch Panel and 1 3/4" for the Plug Panel) within the frame and attached them to the inside faces with 1/2" wood screws.

    I attached the panels onto the frame supports with some E6000 Adhesive. 

    Finally I printed a mount for the Arduino Mega...

    ...and installed it in the corner of the frame underneath the potentiometer.

    This feels like a major milestone to me. I basically just have to do the "integration" wiring and make a back panel for the box. For sure I can now see the light at the end of the tunnel for this project.

  • Patch Cables

    Michael Gardi05/28/2020 at 16:18 0 comments

    I started making the patch cables. 

    I found some relatively inexpensive 2 mm banana plugs at about 56 cents CDN each. The wire is silicon based and very flexible. From Amazon:

    • 22 Gauge Electric Wire,Tinned Copper Wire Kit 22 AWG Flexible Silicone Wire(6 Different Colored 26 Feet spools) 600V Electronic Hook up Wire
    • 20PCS Gold Plated Banana Plug for Speaker Wire, Home Theater, Wall Plates and More(Five Colors Choices)

    The cables are colored coded by length and are based to the original H-500 except black was substituted for brown and there are no orange cables.  Here is what the original cables looked like (from

    And here are the plugs that were used. If anyone knows what this type of plug is called and where one might find a supply I would be eternally grateful.

    As I stated before the 2 mm banana plugs I'm using work great with the rivets so I'm fine with them for now.

    Starting with 50 cables.

  • Credit Where Credit Is Due, Eh

    Michael Gardi05/28/2020 at 01:58 0 comments

    I'm waiting for a lumber and stain order to be ready for "curbside pickup" so I can get started on the H-500 frame. In the mean time I thought I would jump ahead and make a very important part for the build.

    On the back of each Computer Lab sold was a plaque with the model and serial number as seen above. What's important to me as a proud Canadian is that so far as I can determine all H-500s were Made In Canada. So I had to make sure that my reproduction had such a plaque.

    Ready to go.

  • Plug Panel Wiring

    Michael Gardi05/21/2020 at 21:16 0 comments

    Well I've started to wire up the Plug Panel. The circuits represented by logic symbols on the top of the board are implemented with a dozen 7400 series ICs underneath. 

    By looking at the logic symbols and using pictures of the wiring for the original H-500 I was able to determine the chips used:

    • SN7400  - 2 Quadruple 2-Input Positive-NAND Gates 
    • SN7410   - 2 Triple 3-Input Positive-NAND Gates
    • SN7420  - 2 Dual 4-Input Positive-NAND Gates
    • SN7450  - 2 Dual AND-OR-Invert Gates
    • SN7473   - 4 Dual J-K Flip-Flops with Clear 

    So this is what I have so far:

    There's a few things to note here. First of all I decided to make my life a little easier by creating some breakout boards for the 74XX chips. My initial though was to just "dead-bug" the chips and solder the leads directly to the pins.  It would have worked but the appeal of re-arranging the pin-outs to be more consistent with the layout of the panel, plus having a label on each pin won out. Here's the PCB:

    I created a small base for the PCBs so that I could stand them upright since there was no room to lay them out.

    My plan was to solder the wires coming from the break-out boards directly to the brass rivets. It turned out that this was a lot harder than I thought it would be. I really should have tested this much earlier in the process. I don't know if the brass was coated with something but the soldering iron had to be applied to the rivets way too long to get a good join. As a result the panel holes melted and got too large to hold the rivet securely in place.  I needed a plan B.

    They say the necessity is the mother of invention, and I think the solution I came up with is actually better than if the soldering had worked. I made some mechanical "fittings" to securely attach the wires to the rivets:

    Just insert the wire into the horizontal hole and slide the fitting over a pair of rivets representing a circuit lead. These work great, are quick and easy to install, and actually reinforce the rivet's attachment to the panel. An additional benefit is that a whole PCB "circuit" can be removed from the panel should the need arise to troubleshoot. Here's a closer look.

    So with the bugs ironed out, it's time to finish the Plug Panel wiring.

    And here you go. All the logic elements are now wired except for power which I'll run when I put all of the pieces together. Next I'll knock together a frame to mount everything in.

  • Plug Panel(s)

    Michael Gardi05/15/2020 at 15:17 0 comments

    After a day and a bit of Fusion 360 modelling and twenty-six plus hours of printing, the Plug Panel "panels" are done.

    Printed in multiple parts to accommodate my print bed, and the text is slightly larger than original to work at 3D printer resolutions, but a pretty good result I think.

    My big "discovery" for these panels was the "Hilbert Curve" option for the top layer. 

    I was unaware of this PrusaSlicer feature until now, and would certainly have used it in some of my other projects had I known about it (Minivac 601 and Digi-Comp II for sure).  It produces a beautiful even matte finish unlike the somewhat "streaky" result I see with the default Rectilinear option. The downside is that Hilbert is quite a bit slower. It added an extra fourty minutes or so to each of the four panels I printed. Totally worth it in my books.

    I glued the four panel pieces together. Since the Plug Panel itself needs to be pretty rigid to support the insertion and removal of the patch cables, I designed and printed some support beams and glued them to the underside.

    Then it's just a simple matter of installing the 335 rivets onto the panel.

    I had done a number of test to try to get the perfect hole size so that the rivets could be just pushed in with a nice friction fit. Unfortunately halfway through printing the panels I made a small adjustment to my first layer height. So in the end the two right panels worked as expected, but the ones on the left required a small dab of glue applied to the side of each rivet. 

    Time to start wiring these up.

  • Tic-Toc, Click-Clack, and Blinkenlights Too

    Michael Gardi05/08/2020 at 16:44 0 comments

    All of the active controls for the H-500 are located in the lower part of the device. 

    These included (taken directly from the Computer Lab Workbook):

    • Rocker Switches - Rocker switches can be used to provide either a HI or a LO logic level. If the upper side of the rocker switch is depressed, the two corresponding switch output terminals (directly in front of the switch) are taken to a HI level. If the lower side of the switch is depressed, the two corresponding output terminals are taken to a LO level.
    • Pulser Switches - The outputs of the pulser switches are normally LO. When a pulser is depressed, the corresponding two terminal terminals go to a HI level. When the pulser is released its output terminals return to the LO condition. Internal circuits connected to the pulsers make sure that when a pulser is depressed or released, electrical noise generated in the switch is not transmitted to the pulser output. This special circuitry makes the pulsers useful in applications requiring noise-free transitions from one level to another. Rocker switches do not have this feature.
    • Clock - The clock provides a continuous train of HI pulses. Clock pulses are 50 nanoseconds wide. The frequency of clock pulses can be continuously varied from less than one pulse per second to over 10 million pulses per second. The slowest range of the clock is obtained by connecting the common clock coarse terminal to the left-most speed-selecting terminal. The repetition rate of the clock increases with each terminal to the right. The fastest repetition rate is obtained by leaving the clock range selector disconnected. Repetition rates within each coarse range can be varied using the clock fine control. (Fully counterclockwise gives the slowest repetition rate; fully clockwise provides the fastest rate.) The clock range coarse terminals are to be used only for selection of clock repetition rate. The clock output is obtained from the two terminals labeled CLOCK OUTPUT.
    • Lamp Indicators - The operation of experiments constructed on the COMPUTER LAB patchpanel is monitored by the lamp indicators.  A lamp will be ON if its corresponding input is at a HI logic level. A lamp will be OFF if its corresponding input is at a LO logic level. If no connection is provided to a lamp, the lamp will be OFF.  Lamps will respond to sustained logic levels and pulses of sufficient duration to activate the lamp filament.

    So how were these functions implemented back in the late 60's. Like this:

    I apologize for the image quality. I'm trying to find a better photo. Looks like this schematic was taped to the back of the frame's rear cover. 

    Now I'm not going to pretend that I completely understand all of what is going on in this schematic. Electronics is not my strong suit, I'm more of a digital guy. (if anyone wants to help me out here with a short description of what the above circuit does I would be happy to include it in the write-up.) Suffice it to say that the complexity of reproducing this circuit goes counter to two of my primary design goals with this project: make it easy to reproduce and not too expensive.  

    Fortunately I do know what the circuit is supposed to do. So what would be an easy and cheap replacement, well understood by today's community of makers, well an Arduino of course. This is what I came up with:

    Rocker and Pulser Switches - The magnetic reeds of the rocker and pulse switches are monitored by eleven inputs that are set with their pull-ups enabled. When the normally open reed switch is "activated" by a magnet its input line will be pulled down. The Arduino sketch monitors changes to the reed switches and will only change the state of it's corresponding output line once the switch has been suitably debounced (three consecutive reads 20 milliseconds apart with the same state).  In this implementation all switches are debounced, not just the pulsers. We are already at 22 I/O lines so it's a good thing...

    Read more »

  • Light Panel

    Michael Gardi04/28/2020 at 14:45 0 comments

    This piece separates the Switch Panel from the Plug Panel, holds the eight output lamps and the fine tuning knob for the clock speed, and also transitions from the higher switch level to the lower plug level within the wooden frame. 

    It's impossible to know from just pictures what kind of bulbs they used in the original, but if I had to guess based on the estimated size (about 4 mm) I would say grain of wheat bulbs.  I could have ordered some but given the current lead times for delivery of non-essential items I decided to go with what I had on hand, LEDs.  I modified my Panel Mounted LED Socket by adding a 4 mm diffuser printed with transparent filament.

    When put together it looks like this.

    These bulb assemblies were slotted into holes on the back of the Light Panel. The panel itself was printed in two pieces and glued together with the aid of a reinforcing strip. You can see the clock fine adjustment potentiometer mounted on the back as well.

    From the front here is the fine tuning knob that I modeled...

    ... based on the original.

    The "bulbs" ended up looking pretty good. I found that I will have to add a 4K limiting resister to each LED to get the brightness down to incandescent levels.

    The light panel is ready to go.

    Updated May 7, 2020: Last week Bob Roswell, curator of the System Source Computer Museum, was kind enough to spend some time on the phone with me talking about the H-500 that they have on display. He also provided me with some invaluable first hand measurements and some close-up pictures. While my light panel was pretty close to the original measurement wise, I had learned that the potentiometer on the original was also the power switch so I redid the panel to fine tune the measurements and accommodate the new pot with switch. The photo of the tuning knob Bob provided also allowed me to produce a more accurate replica. Thanks Bob. I have update some of the pictures above with the new parts.

  • Switch Panel

    Michael Gardi04/24/2020 at 22:01 2 comments

    With all preparations complete I forged ahead and built the bottom part of the H-500. 

    Here is what I did.  I started by preparing the eleven rocker switches required, three momentary "spring" return types for the H-500 pulse switches, and eight ON-ON variants for the input switches. I  took care and made sure that the magnets were all oriented the same way and used my 3D pen to secure them in place. See Rocker Man for details.

    Next I prepared the eleven rocker switch bases. They are all the same as the one below. You will notice that there is only a single reed switch.  I had already decided that I would use an Arduino to manage the clock and pulse signals (reading the clock rate, generating the clock signals, debouncing the pulse switches, etc., more on this later in the project) so I thought I would take advantage and generate the input switch signals based on a single reed per switch knocking about $20 off the BOM. Also I'm not sure what the final assembled layout will be so I attached extra long leads.

    I designed two "cradles" to hold the switch bases.

    The bases slide in snugly.

    I made some simple clips to hold the base pieces securely in place.

    Same for the eight switch cradle. I'm running a bit low on filament so mixing colors a bit.

    Next I printed some mounting plates. They have slots for M3 nuts to ensure a strong bond.

    Connected the cradles to the mounting plates with M3 x 10 bolts.

    Cut a 1/8 inch steel rod to span the length of the combined switch assembly and printed some small spacers to help align the switches.

    Starting from one end I began slowly sliding the rod through the pivot holes attaching the rocker switches with their counter weights as I went. Between switches I inserted one of the gaskets.

    When done the switch assembly should look like this.

    I printed the face plate for the panel (in two pieces, need a larger volume printer). I paused and changed the filament color multiple times to print the text and the logo. All that's left is to attach the switch assembly to the face plate. (Or is it the other way around, the switch assembly is quite massive.)

    That's it. The switch panel is done. It is tweaked a bit from the original. I had to make the characters bigger by about 20% to get them "print" correctly when extruded. The switches when assembled ended up being a little wider than the originals. Since I had already printed the switches, I made the panel holes a little larger to accommodate them. When I get the chance I'll get some black steel bolts so the heads are less noticeable. All-in-all though I'm extremely happy with the result. 

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Enjoy this project?



Kaleberg wrote 2 days ago point

Totally awesome. It's funny how much work you had to do to make those rocker switches. Ken Olsen gave a talk at my college and explained that they chose the switches for the first DEC computers by going down to Lechmere's, the discount appliance store, and looking at washing machines and driers. They wanted cheap, available stock switches to keep costs down.

  Are you sure? yes | no

wtnh wrote 6 days ago point

Your Minivac replica is great too - the rotary selector always reminded me of a toy I had in the 1950's called the Z-Man (see:

The "brain" under the helmet was a circular pc board with tiny switches which could be in either of two positions: turn right or turn left, which actuated motors on the front wheels. So you could "program" a zig-zag course for the robot. He would also fire spring-loaded rockets at a sort of preset interval. Anyway - that was my introduction as a 6-year-old to "programming" and I guess it always stuck with me.

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wtnh wrote 6 days ago point

Great project and well done! This sure brings back memories - I was in high school in the mid - 1960s and a classmate got a Minivac 601, which was interesting but relay logic based. I got a follow-on product from the same company which was a logic trainer with resistor-transistor gates, a patch panel, and switches and lights. It was a kit. I donated it to my school when I graduated. Then around 1970 I saw the manual for the DEC trainer and wondered if there was a market for a look-alike and considered trying to make them. Gave up on that idea, but in 1976 I actually went to work for DEC and saw one of the trainers first hand.

  Are you sure? yes | no

Michael Gardi wrote 6 days ago point

Man I love hearing stories like this.  Thanks for sharing.  I was more of a late 60's high schooler.  Unfortunately the DEC Trainer and I never crossed paths because I know that teenage me would have loved it. Better late than never I guess. BTW I've done a Minivac 601 replica as well:

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Ken Yap wrote 05/21/2020 at 22:17 point

I think TTL of that vintage were not LS but the standard series. LS has about ¼ of the drive capacity of standard TTL. If all the chips are LS then it works out. Where the output drives something you need to look at the current sinking capability. Then again, LEDs don't need as much current as lamps.

  Are you sure? yes | no

Michael Gardi wrote 05/21/2020 at 23:26 point

Oops. Hadn't thought about that. They are all LS so I should be OK.  Thanks for the tip.

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Randy Wright wrote 04/29/2020 at 07:24 point

Great reproduction! But can this be applied using pcb?

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Michael Gardi wrote 04/29/2020 at 13:43 point

A PCB would be too expensive for a full size reproduction.

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Karl wrote 04/12/2020 at 15:44 point

Have you thought about using a PCB for the panel ,and real receptacles that are meant for wires?  I use a PCB house that builds 2 sided boards up to 60 sq in for $30 in sets of 3 or more, and Mill-Max receptacles # ED90580 from Digikey are about $.10 each if you order 250 & up.

You could then use the cheap jumpers used on solder less breadboards [the ones with stranded wires and solid pins with black rubber over molded on them].

The PCB could be scaled to about 6.6 by 9 inches, and the silk screen could have the symbols, etc on the 'front' side..

  Are you sure? yes | no

Michael Gardi wrote 04/12/2020 at 21:16 point

Thanks for the suggestions Karl. I toyed with the idea of using a PCB for a while, but at the end of the day decided it was too expensive and would make the project less accessible if anyone wanted to make their own reproduction based on my plans. If I were doing a scale model as you suggested it would make a lot more sense but I have my heart set on a full size reproduction.

 I really like the Mill-Max receptacles. I have never encountered them before and will certainly keep them in mind for future projects, but for this one I feel that the rivets will look more authentic. 

You know Karl the world is big enough for a full size H-500 reproduction and a scale model as well. Just saying...

  Are you sure? yes | no

Peabody1929 wrote 03/06/2020 at 20:43 point

There is an object in the Computer History Museum catalog that appears to be a real DEC H-500 Computer Lab.  It is catalog number 102630125.  It is possible to submit a research request to the museum to examine/study on object in the collection.



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Michael Gardi wrote 03/07/2020 at 02:36 point

Wow. Great suggestion. Thanks.

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Michael Gardi wrote 04/12/2020 at 21:20 point

Since you posted this I have been in touch with Bob Roswell curator at the System Source Computer Museum. Bob has graciously offered to provide me with photos and other useful information from the H-500 in their collection when the museum opens again. 

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Dave's Dev Lab wrote 05/22/2020 at 00:21 point

that is awesome!

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