Field desk - Part 7 - Carry strap

I finished the field desk and took it for a walk up and down the driveway a few dozen times and decided that if I had to carry the field desk any significant distance, the handle would become uncomfortable quite quickly. I determined I'd need to fit a strap of some description...

I designed a few options based around a quick dis-connectable strap which would have been based on an inset plate, with a "key-hole" shaped hole which would have allowed a strap to be connected without load, and lock in place under load. I planned on making this from the brass strip I have, and make a mushroom post to enable this function. Easily within the scope of my skills and materials, but the more I looked at the idea, something just felt wrong about it - still haven't figured out what.

Figure 1 - Front view of desk with carry strap as if carried

Under further consideration, I figured it might be easier to make a strap which would fully support the desk from the bottom, and also the sides - the previously mentioned solution would have used the top edges of the sides to carry the weight. I designed a strap which essentially followed a path under the base, up each side, and up over my shoulder. A "waist" strap would go around the desk in the horizontal plane, and have a clasp to permit it to open on the door side.
A few lengths of salvaged seatbelt material, a clasp from an old salvaged life-jacket and about one hour on the sewing machine and this is the result...

Figure 2 - Side view of desk in carry strap as if carried

Since none of the lengths of seatbelt material were long enough, I made the bottom strap as one piece, and added a brass "adjuster" at each end to permit attachment and adjustment of the shoulder strap. These adjusters were made from (50x75x3mm) 2" x3" x 1/8" brass strip. A sliding dog was made from offcuts of the same strip brass.

Figure 3 - Front view, close up of clasp

A piece of scrap denim was sewed in behind the clasp to reduce it rubbing into the paintwork of the desk, and all sewing was done with over-sewn double runs, and polyester thread.
The strap can be left in place and the door easily lowered, or just as easily totally removed, and reinstalled when it's time to move the desk.

Figure 4 - Front view of desk with strap unloaded on top

When I collected the seatbelt material from the training cars (what was left of them) I found one car had those detachable seatbelt pads - Given the final weight of the filled and complete desk, I elected to place those seatbelt pads on the shoulder strap to even the load and reduce any chance of bruising from carrying the desk for a long time.

Figure 5 - Front view with clasp undone to permit door opening

A lesson learnt on this construction - when sewing seatbelt material, the upper thread tension on the sewing machine needs to be increased - otherwise the lower side threads all bunch up - my guess is the thickness and weave of the belting makes it harder for the thread to pull up enough under normal thread tension.

Figure 6 - Door open with DPScope on desk surface

The last photo is of the DPscope I purchased with some recognition money I recieved (a program they do at work to recognise efforts put in by staff) - The DPscope is a DSO scope which connectes to a PC via USB and has 2 channel capability. Since it only arrived yesterday, I haven't had time to play with it yet, but will do so over the next week - hopefully.

That is it for the strap - hopefully I won't need to carry it far that often, but now I can. I don't know what the next article sets will be - I've heaps of books to review, many projects in the WIP box (WIP = Work In Progress), and a lot on my plate outside the shed. This past week has been a series of quite long days, and in the next 2-3 weeks I hope it will bear fruit in many forms - the least of which will be my Cert 2 in Emergency Response... the rest - that's my secret for now.

Field desk -Part 6 - contents

The contents of the field desk reflect it's intended purpose.. a portable electronics experimental/prototyping workstation.

Figure 1 - Desk opened

The bottom shelf (LHS) contains 2 deep containers - the bottom containing tools (pliers, strippers, third-hand, soldering tools, etc) whilst the top container houses the current project, some breadboard, veroboards, etc - This container should be fairly empty since I won't want my project being damaged.

Figure 2 - bottom containers out - light in place and IEC cord inserted

The middle shelf (LHS) contains 4 shallow containers - each of the type which has movable dividers. One container is filled with Resistors, another with capacitors and some mixed semis (BC (NPN and PNP), diodes, MOSFETs, 555/556, opamps,regulators, etc) The next unit has mix of parts including variable resistors, LEDs, header strips, sockets, buttons, switches... a bit of a grab bag of useful stuff. The last container contains the PICAXE chips (08M, 18M2, 28X1) plus other minor bits and pieces.
I am going to have to modify at least one of these containers so the keypad, and LCD screens can fit in there - currently they're in the "Projects" container.

Figure 3 - component trays out on display

The top shelf/drawer (LHS) is for documentation and software. It will also contain the USB/Serial adapter, connection cable, and a USB thumb drive for software. I plan on being able to use my work laptop when I'm in accomodation, or my shed PC when I'm at home.

Figure 4 - Top drawer

Figure 5 - light stowage area, and soldering station in box

The RHS compartment (under the power box) houses the 58W soldering station I bought on Ebay. Above that is a old lunch container filled with various colours and grades of hookup cable. Some of the cable is Cat5 solid cable - useful for breadboarding and veroboard work, some is stranded cable salvaged from various sources, stripped and coiled in... there are a couple of sizes of cables so I can build for signal, and power requirements.

Figure 6 - Cable container, and spare trays on open door/desk

Stats for the finished project..
Size - 600mm W x 450mm H x 300mm D (24" x 18" x 12")
Weight (loaded) = 15Kg (~33 lb)
DC supplies - -12, -5, 3.3, 5, 12 volts with total power output of up to 450W - input is 240VAC at 0.8A normal, but up to 10A for GPO loads.
Build time = Cabinet and sheet-metal construction - 4 days, painting (not including drying time) and trimming = 2 days.

The objective of the desk is to be able to work on projects, learning or simple construction, in a self contained environment. When I leave my roster, I simply stow the desk in a secure location, and take with me a thumbdrive, and "lunch box" containing the finished project to take home. The thumbdrive will contain the copies of source code, etc, and a shopping list of parts to collect and bring back with me on my next roster. The notebook, and code copies in my laptop will remind me where I'm up to upon my return.

Next postings.... hard to say - I've a few work and community commitments which will prevent me taking on any new projects. I have already started cleaning up the shed (while the varnish and paint was drying) and packing away some gear in preparation for these commitments. Over the next couple of months I hope to get these upcoming changes and commitments settled in, and then get back into the swing of designing, and completing these projects... believe me, the "to do list" is not getting any shorter.

Field desk - part 5 - Painting and trim

The painting and trimming of the field desk.
Painting was straightforward...
Undercoat,
filling,
masking and
top coats of "Bender grey" (the darker grey used to paint Bender's arms and legs)

In keeping with the Futurama theme, and tying in the fact that this desk is designed and made for use in electronics and microprocessor experimentation, I decided to theme the trim on the concept of the desk being a "Momcorp" product, a prototyping/ repair kit for Momcorp products using microprocessors.
From that concept, it wasn't too hard to look at this as an "Experimental Brain Surgery Kit" (Always wanted a tool box with that written on it!!!) for robots and robotic devices.

Figure 1 - Momcorp logo pages drying after clear-coating and spraypainting

I created a version of the Momcorp logo (The nicer one, not the authoritarian vertical MOM one) and decoupaged it to the front and back of the desk. I printed the logo I made on the thinnest paper I could find, then spray painted the back white to increase the opacity -  I clear coated the front to stop the ink running.

Figure 2 - Decoupaged logo on door of field desk

I then got to thinking... if I was in the year 3xxx and a technician, I wouldn't leave my travelling toolbox looking so bland. It would have stickers on it ....

• from places I'd been authorised into (HAL institute for criminally insane robots), 
• stickers from products (Mom's old fashioned robot oil, bachelor chow, slurm), 
• stickers which reflected the world around me (political, social fads, social commentary, humourous), 
• and at least somewhere I'd have my name.

Based on that, I started cruising the internet finding images from the show, and editing them into a sticker style format. I found a few in that "cafepress" site which were already done, and used their look as my template. I downloaded a few fonts, and made a few (how I made the Momcorp logo) and then printed them in my inkjet printer.

Figure 3 - several pages of "stickers" drying after painting

All of the "stickers" where clear coated over (to stop the inkjet ink running when varnished), and most were spray-painted white from the back to improve the contrast and opacity - but not all. A few of the images were deliberately scuffed and aged just to add a "worn look" to them. - given the less than perfect cabinetry, a few bumps and scratches here and there will be well and truly "in character: A couple of the stickers were not painted white from the back so they'd appear to be made from newsprint paper... just for variety.

Figure 4 - Back of Field desk with stickers applied

Stickers include: The vitruvian robot drawing, robo-fresh, the feminista bumper sticker, WWZJD.....

Figure 5 - Door of Field desk (Outer side)

several Slurm stickers, Mars University, Scary Door, a binary expression....

Figure 6 - LHS of Field Desk

Nixon's reelection campaign poster, robot oil sticker, "I love Snu-Snu" bumper sticker (who doesn't?)....

Figure 7 - RHS of Field desk

A hypno-toad sticker, a trekkie bumper sticker, robot oil ad, more slurm.....

Figure 8 - Top of Field desk, including "protest sticker"

more Slurm (it really is addictive!!!), the Earthican flag, a "Morbo" bumper sticker, HAL institute sticker, a NNY sticker, and a "protest sticker".
The protest sticker was written up in the Futurama "alien" font - it's the font used throughout the show. I won't give out the meaning, but it's a very well known sentiment amongst those whom nanny-states will try and govern.

That's it for the trim - I did have a bachelor chow sticker, but it got damaged during painting (Wind got it while it was wet) - I was going to put a DOOP sticker on as well, but it was starting to look cluttered... we'll see.

All that's left now it to show the unit with the contents in it. That article is as much for the PICAXE forum as any other groups since they helped me with the shopping list of things going in there.

Field desk - part 4 - Tray and lighting (additional information)

A little more information on the lighting, and a quick article concerning the tray for the top shelf.

About 5 minutes after finishing part 3 of this series, I was filing the folder for that article when I found the additional photos I took as part of the lighting design...

Figure 1 - Lamps tested for use in the desk - LED lamp on left, incandescent on right

Photos of the two lights I trialed, including the LED lamp I used in this project, and a photo of the desk with a piece of DIN rail used to measure length and placement of the lamp.

Figure 2 - Trial of lamp placement using DIN rail

Paper Tray - Top Drawer
The other photos in this article cover the construction of a "paper tray" which is designed to fill the top shelf of the field desk. I plan on keeping an A4 notebook, CDR (with manuals), a couple of pens, etc in that shelf. The easiest way to achieve this is to have a drawer.

Figure 3 - Sheet metal cut out, prior to folding

The paper tray is simply folded up from sheet metal (more of my salvaged colourbond "signwhite") to make a simple tray, with a folded top edge on three sides, and one extended edge on the front. This extended edge forms a handle which allows the drawer to be pulled out from the shelf.

Figure 4  - Drawer in place on shelf

Since I haven't had the time to build a finger brake yet (still on the ever growing "things to do" list), I did all folds using the clamp over bar, wood and hammer method. One thing I found during that exercise was that you shouldn't hammer anything whilst you have a cold - all it does is screw up your inner ear whilst your sinuses are stuffed - Not a nice sensation.

Figure 5 - drawer painted

Once folded up, a couple of pop rivets to hold it all together and then it's painted up in "Bender Grey". In fact the entire exterior of the desk will be painted "Bender Grey" and will carry a suitable theme in the trimmings.

Figure 6 - Finished drawer showing folded handle on LHS

The painting and trim of the field desk itself will be the next article, then all that's left is a brief discussion about contents and that project is complete.

Field desk - part 3 - Electrical System and lighting

Electrical services into, and within, the field desk.

Electrically speaking, the field desk will have one power lead into the desk, with a double switched GPO, and a variety of DC power sources available. A task light will also feature in the design.

Electrical enclosure

The electrical enclosure was designed to contain all components, and attach in the top of the RHS bay. The front panel will contain all interfaces - plugs, switches, etc. The original design was to use an old PSU from a 1900 series switch, but  upon checking the PSU, it was found to have some of it's pins non-commissioned (-12V and - 5V) - thank fully the size of the enclosure was dictated by the GPO, and banana sockets, this meant I had room to look at alternate options.

Figure 1 - Basic Enclosure unpainted

The front panel holds all connections, and has the receptacle for the light in the top RHS. - The receptacle is simply a short piece of  DIN rail, and the lamp holder sits inside the rail.
The front panel of the power box is made of 3-4mm thick plexiglass, drilled, cut filed to hold the GPO, IEC socket, and banana posts. To prevent scratches showing on this panel, I marked the terminal values on from the back, and then spray painted over them from the back - this means the paint cannot be scratched from outside the case. Interesting note was when the paint dried, I could suddenly see this invisible cracks around the banana posts - it looks almost surreal to see "reversed cracks" filled with paint.
The enclosure is designed to sit in the top of the field desk, therefore all ventilation is through the floor (or the front) - I simply replaced the floor with some punched mesh, and then used sheet metal shields to redirect any airflows from the back, through the PSU fan, through the PSU, and then into the front section, through the enclosure floor. Each "side" of the PSU has around 12 sq inches of floor vent available to induct, or expel air.
The enclosure (power box) is held in the top of the desk by means of some brackets, and a folded lip at the back. None of the retaining hardware obscures the ventilation grid, and removal of the power box is accomplished with the removal of one screw, since the forward brackets tilt to permit removal.

Figure 2 - Close up of front panel, with first sheet metal divider removed

The only parts of the enclosure which are painted are those parts visible in normal use - ie the front panel/s, and the bottom. The sides were deliberately not painted since the paint would simply rub off on the walls of the cabinet during insertion, or removal.

Figure 3 - Mesh base to enclosure used for ventilation of PSU

Lighting
 A 12V LED lamp was purchased from the local variety store (KMart) - I had looked at an incandescent lamp, but compared to the LED lamp, it was pale and yellow. I considered one of the halogen lamps I use when I sew, but they do throw some heat, and I considered that would not be wise in the planned location - not to mention wasted energy as heat.
The lamp was gently disassembled (no warranty voiding yet) and tested for it's ability to "hold up" from a horizontal plane - it was discovered that if held at a 45 degree angle, the lamp's "flexible arm" would support the lamp to the maximum reach. Based on that, a bracket was made (from plexiglass) to hold the base at 45 degrees. The bracket slides into a short length of DIN rail which is used as a track. This track is part of the enclosure and is accessible from the front panel.

Figure 4 - Power box in place in field desk with lamp inserted

The lamp will be removed to close the door, so the plug which supplies power to the lamp was cut through the opened switch, and additional wiring soldered on. The wiring is then terminated to the connections for 12VDC (and COMmon) so the lamp will run whenever the PSU is on. This should have worked but during final testing it was found that the light actually needed more than 12 VDC - the "wall-wart" power pack put out 14VDC unloaded - typical for a 12VDC cheap supply, so I connected it to 12VDC - the light was so dim, you'd have thought it was off. I moved the negative cable from the COM to the -5VDC (giving 17VDC) and she lit up beautifully. - Now the cables are between the -12VDC and the 3.3VDC connectors giving 15.3VDC for the light.
 
PSU
Since the planned 1900 PSU was abandoned, the next most affordable option was to use a surplus ATX PSU. There are a number of articles on the web which discuss the conversion - most centre on forcing, or redirecting the softpower "On/Off" wire, and providing a load to stabilise the regulation circuitry. I started going through my collection of surplus ATX PSUs looking for a reasonably low powered unit which worked, and could be used in this project. I tested some of my surplus ATX PSUs and found a 450W which worked OK. I originally planned on using  a "wiring harness" to connect everything up, but the space was too tight for that option.

Figure 5 - Wiring harness (Mk 1) which was too big for use

What I ended up doing was opening the PSU case, removing the IEC socket (and other mains supply switch and components) and soldering in a hardwired cable. At the same time I cut the ATX plug off, and trimmed all HDD/FDD cables at the first Molex connector. This gave me a bundle of wires about 350mm (14") long. I tied the green wire (PSU_ON) and one black wire (COM) to a toggle switch, and then grouped all other wires together based on their colours and connections within the PSU. Purple (Standby 5V) and the Grey (PSU_OK) cables were tied off inside the PSU since I didn't need them.

Figure 6 - Internals of ATX PSU being modified for use.

The rest were soldered to brass tabs made to suit the backs of the banana sockets (with 2 sets each for COM (Black) and 5V (Red)) The blue and white wires (-12V and -5V) looked so lonely on their tabs when all other tabs had 5 or more cables soldered in. A 7W wirewound 10 ohm resistor was added across the 5VDC rail for a regulating load (although most supplies theses days don't seem to need that - I'll remove the resistor for now and see if the PSU behaves)

Figure 7 - Modified PSU in enclosure with earthed divider panel.

As mentioned previously, the enclosure needed some way to direct air through the PSU fan, and back out of the enclosure. I accomplished this by bending up sheet metal dividers - the one closest to the front also being Earthed, and the one at the back being used to restrain the PSU, and block the holes in the casing where the IEC sockets were removed.

Figure 8 - Completed power box with second divider in place for ventilation redirection.

All Done!!! (although I haven't put in the 2 self tappers in the top corners yet!!)

Figure 9 - Completed power box ready for use.

As can be seen in the background of some photos, the desk cabinet is already being painted. The next article will cover the painting and trimming of the desk.

Field desk - part 2 - Hardware

Hardware fabrication



Hinges
Using the design I created, I needed to be able to remove the door to use as a work surface. I built two hinges from brass - 1/8"T x 2"W brass strip, and some brass rod salvaged from the spindle of an old household tap.

Figure 1 - Hinge materials

The hinges were patterned on a half barrel hinge style similar to those used on trailers for the tail gate - I choose that design so the door could be removed in a similar fashion.

Figure 2 - Hinges under construction - engaged

The spindle was cut and then turned in the lathe (Taig) to make the parts. Once made, the strip brass was cut and filed to suit the parts, and then all pieces soldered together.

Figure 3 - Hinges under construction - released

Once the hinges were finished, I inlaid them into the floor and door of the desk, and rebated in the backing plates on the reverse side of these surfaces. The screws securing the hardware are all 3/16" UNC (10-24 for our US cousins) into a nut plate on the reverse side.

Figure 4 - Hinges fitted - engaged

A notch has been cut in the RH side panel to permit the door to be removed similar to a trailer tailgate.

Figure 5 - hinges fitted - door released

Locks
Next thing to make was the locks. The requirements on the locks was that the door had to be perfectly flat on both sides - the outside so it would lay on a table top and not scratch the table, the inner surface needs to be flat so it can form the working surface when I'm using the desk.
I planned on making disc-locks, but after reviewing my stock of materials, came to the conclusion that cam locks would be the design. I did not have much in the way of 1/2" diameter brass rod, so I decided to use a cartridge case (.243 Win) to form the shaft. (I have a stash of around 20 of these cartridge cases which were given to me for scrap brass). The cam plate was roughly made up, and then soldered to the cartridge case at the appropriate height to permit the base of the cartridge to be used as actuating surface outside the door.

Figure 6 - Cam lock under construction

The front and back plates were made up, and the lock assembled. Only once assembled was the cam shaped to its final length, with some easing to improve its alignment when turned. The base of the cartridge was spot-drilled to match the "keys" I made - basically a small pin-wrench.

Figure 7 - Pin-wrench "key" under construction

The pin wrench started life as one of those promotional key-chain bottle openers, but after cutting and drilling a pair of nails were driven in and cut and filed to make the pin-wrenches. I made 2 of the keys so I have a spare.

Figure 8 - Finished lock - in locked position

Once the front and back plates were completed, a top plate was made and soldered to the front plate. Clearance slots for the cam operation was made, and suitably relieved for easy use. The holes in the lock were drilled to indicate the lock status - vertical holes indicate the lock is "locked", horizontal holes indicate unlocked - there is only 90 degrees of movement in the lock mechanism.

Figure 9 - Finished lock in unlocked position

Door opening mechanism - The Pusher
Since the door has to be perfectly flat on both sides, there is no handle. I considered a flip out handle, I considered simply drilling a hole to poke my finger through - both ideas had aspects which did not appeal to my sense of this project... What I really needed was something inside the desk to push the door out once the locks were released... What I came up with was "the pusher".
The pusher is nothing more than a simple spring loaded detent plunger - but instead of pressing into a detent hole, it simply pushes the door away from the locked position by about 1/2" - more than enough to get my finger on to lower the door to the table top. The pusher uses another .243" cartridge casing for the spring holder, and the plunger is made from more tap spindle stock. Who knows where the spring came from, I've boxes of salvaged "useful junk" which gets pawed through when I do jobs like this.

Figure 10 - Pusher components

The spring casing is rebated into the riser, and the cover plate relieved into the edge so there is no protrusion other than the plunger.

Figure 11 - Pusher spring casing installed

This piece of hardware is the only one secured with normal wood screws into wood - all others use the nut-plate method described in the hinges. The screws into timber should be able to hold the minor force of the spring at the end of its travel.

Figure 12 - Pusher installation nearly complete

Once the locks are unlocked, the pusher moves the door about 3/4" if the LHS doesn't grab (that bowed panel as mentioned when I built the carcass), 1/2" if it does rub... either way I can still open the door easily.

Figure 13 - Result of pusher on unlocked door

Handle
The other piece of hardware to build was the handle. Most of my toolboxes have handles which protrude on the lid - making it nearly impossible to stack things on top. Given the intended use of this desk is in a "accommodation camp" where I may need to stack a laptop, or books on top, I was insistent that the handle design had to leave the top surface perfectly flat. The ideal scenario would have been to use the folding handle from the top of a 7.62 x 51mm ammo can - but I couldn't find any. I did not trust my skills to make one, so I looked at every box and case I owned looking for a low profile handle - I stole this idea off an industrial first-aid cabinet.

Figure 14 - Handle components

The basic strap (handle) is made from 2 layers of pallet strapping - the 3/4" wide blue/black metal banding you find on pallets of bricks and other heavy things. I drilled and slotted it, then shrunk two layers of heat-shrink tuning over it to make the handle comfortable. I then made a pair of brass "sockets" which hold the handle, and allow it to slide in it slot for extending under weight, or retracting when not in use. I made the top plates for the sockets larger than required and drilled 2 large holes in each for attaching labels (addressing, or shipping labels).

Figure 15 - Assembled handle

Since I'd already glued the carcass together (my enthusiasm bit me hard there) I had to make my own "T-Nuts" to engage from the underside. Basically cylindrical spigots soldered to shim brass which were then friction fitted into holes in the timber. I wasn't feeling overly confident in the solder joints, so I backed the nuts up with some Loctite CA glue. If the nuts pull through, I'll have to look at redesigning the handle nuts, but so far OK.

Figure 16 - Completed field desk with all hardware

Since nothing was rebated in, and the finished handle is proud by 1/4", I still had protrusion to deal with, so the only quick answer was to cut an overlay board of 1/4" plywood and screw it on top to raise the surface of the top around the handle. - Not ideal, but I'd rather do that with 1/4" ply than the 1/2" ply I'd have needed for the commercial handle the local hardware store had. (trying to keep the weight down)
The additional sheet can be seen in Figures 16 and 17. It comprises some 1/4" (6mm) plywood, and an interposing sheet of cardboard (edges covered with masking tape) - this effectively recesses the handle, with minimal weight gains.

Figure 17 - Demonstrating use of tag holes in handle

All screws for the hardware had to be cut down in length so there was no protrusion to scratch the tabletop. A dab of thread-locker will be applied during final assembly just to ensure they don't come loose during travelling.

Still to come:
Electrical section
Trays and containers
finishing

- apologies for the number of photos  - next time I'll do this as two articles.