There is a gate at the posted coordinates with a sign reading No Parking - Emergency Entrance. But, if you park near the road, and off to one side, you shouldn't be in the way of any vehicles that might need entry - which is not likely to happen anyway. Durham 23 is quite busy, so use caution. The walk to the redirect, cache, and back, is < 1km.
This is the sixth part of a seven part series (6 caches plus a FINAL). Each part will build on the preceding part, so while not impossible to do out of sequence if you have the knowledge, it is suggested that you do the caches in order:
1 - GC6DDCH - Resistors
2 - GC6DKK8 - Series & Parallel
3 - GC6DR23 - Ohm's Law
4 - GC6DR2X - Magic Smoke...
5 - GC6DR3G - Breadboards
6 - GC6EG5M - PCBs (This listing)
7 - GC6DDDW - FINAL
Note: Each of parts 1 through 6 contain bonus information that's written on the top of the log. If you plan on doing the FINAL, please record this information as it will be required.
PCB:
The acronym PCB stands for Printed Circuit Board. A PCB begins as a sheet of thin fiberglass, or other material, that has a layer of copper applied to one side, for Single Sided PCBs, or both sides, for Double Sided PCBs. A typical thickness for PCBs is 0.062".
The typical copper thickness is 1oz and refers to the thickness of the copper if 1oz of copper is taken and flattened across 1 square foot of board. Various methods are used to make the traces on the PCBs that connect the components. For hobbyists, the two more popular options are Toner Transfer Paper or Photo Sensitive PCBs.
In the first, special paper is loaded into a laser printer and the desired design is printed onto the paper. The paper is then applied to a clean blank PCB, image side to the board, and an ordinary household clothes iron is applied to re-fuse the laser toner from the paper to the PCB. The board is then immersed in a copper etchant solution that dissolves any uncovered copper leaving just the desired traces. The toner is then removed to expose the copper, the holes are drilled for the components and the components are soldered onto the board.
In the second, the pattern is printed onto transparency sheets, with either laser or ink jet printers (or even free hand if you're steady enough). This method requires the use of a special PCB that has a layer of light sensitive emulsion applied to the surface of the copper. The protective plastic is removed, the transparency is applied to the board and taped in place so it can't move, and then the board is exposed to light. The copper areas that are not covered by the pattern on the transparency undergo a reaction that allows a special chemical to remove it - but leaves the covered emulsion in place. The board is then submerged into the copper etchant which removes the bare copper leaving the traces covered by the unexposed emulsion. As before, the remaining emulsion is removed, the holes drilled, components inserted and soldered in place to form the completed PCB.
Designing a PCB:
There are many software products that allow the hobbyist to design and produce the artwork (that's what the patterns are called) to make PCBs. The one I use has a schematic editor, where you draw out your circuit. It then takes that information and transfers it to it's PCB editor module, where the components are layed out and traces added to form the board. You can then print out the artwork and make your board.
So why don't we make one. We'll use this schematic, which you've seen before:
I'm going to omit quite a lot of the steps here, otherwise this might become the longest cache listing in history...
After routing the traces in the editor, and having selected bottom view, as this is a single sided PCB, the program screen looks like this:
This is a single sided PCB, which means it has copper only one one side, the bottom. The components are mounted from the top of the PCB, with their leads through holes, and soldered to the pads (round copper surrounding the holes) on the bottom. If you look carefully, you can see the components in dimmed yellow. This is because the program is showing you the bottom layer with the copper traces (red) as if you were looking through the PCB from the top. If you think about it, if you were to actually look at the bottom of the board, the copper traces would be reversed, like a mirror image, since the input connector, which is on the left of the PCB when looking at the top would actually be on the right hand side when you turn the board over. So, once you finish making the actual PCB, the bottom of the board will look like this - a mirror image of what it looked like on screen:
A PCB usually has what is called a silkscreen image applied to it to show components and other information. Silkscreen, because in the early days - and still today, the white ink is applied through a stencil using a silk screen process. The silkscreen for this PCB, which would be printed on the top of the board, looks like this:
If you imagine the green PCB above flipped over, and the silkscreen applied to the top, the holes on the silkscreen components would line up with the actual drilled holes on the PCB (shown in black on the PCB).
While this board looks pretty much exactly like the schematic layout, and would function just fine, it's not very efficient. There is a lot of wasted space. One of the goals for any hobbyist worth their salt, is to design efficiently and use the least amount of materials possible - which also saves hobby dollars for other projects!
So here's a redesign of the same schematic, with the same components, but in a smaller PCB:
Again, the left image shows the bottom copper traces (red) as if you were looking through the PCB from the top, the center image shows the bottom side of the actual PCB (remember it's a mirror image) and the right image shows the silkscreen for the PCB which would be printed on the top. It's the exact same circuit as shown in the schematic, but in a much more efficient, smaller board. In the above left diagram, you can see where the trace from the IN terminal that goes to the top of R1 and R2 has to go around the bottom lead of R1. We could improve this, and make a slightly smaller board, by using a double sided PCB:
The first and second images show the program's bottom view and bottom side copper as before. The third and fourth image show the program's top view, with the copper trace shown in green, and the top view of the PCB. The fifth image is the silkscreen. The trace from IN to the top of R1 and R2, which had to go around before, is now going from IN, through the pads for the bottom of R1 and R2 and connecting to the top of R1 and R2. The connection from the bottom of R1 to the top of the LED has now been moved to the top of the PCB. This means we can make the PCB a bit smaller than before and save even more space. It's not that much in this example, but in a large complex board, using a double sided PCB, and having circuit traces on the bottom and top layers of the PCB, allows for more complex and smaller designs.
There is one problem with this design though. In a commercially produced board, the holes are plated through after drilling so that the top and bottom pads at holes are connected electrically. But, unless the hobbyist has access to plating equipment, or special eyelets that can be installed to achieve the same result (expensive..), the pads on either side of the hole are not electrically connected. On hobbyist boards, where ever a top and bottom trace connect to the same hole's pads, the component lead is soldered at both top and bottom pads and the lead now performs the function of the plating to connect the two sides electrically. So on this board, the LED, which is mounted on the top side with its leads through the board to the bottom side, would have to have one of its leads soldered to the top copper pad. If the LED is flush mounted - that is to say mounted with no space between the LED and the PCB, you would never be able to get the soldering iron in there to make the connection. It's always best to have component copper pads connected by traces on the bottom side of the PCB for just this reason.
A way to get around this is to use what's called a via. A via is simply a hole in the PCB with pads on both sides, top and bottom, with a piece of lead that's been trimmed from an installed component, or a piece of wire, soldered through it. As mentioned above, commercial PCBs would be plated, so vias wouldn't need a piece of lead or wire soldered in it. Here are the same board layouts as above, but with a via in the trace from the bottom of R1 to the top of the LED to allow the trace to transition from the top of the board to the bottom - which results in both leads of the LED being soldered on the bottom:
Study all four PCB examples above and you'll see that they result in the components being connected as shown in the schematic. Sometimes, trying to make a design with single sided board is just really difficult - if not almost impossible. There are times where double sided is just a much easier and efficient way to go - although it's more work to produce. Here's an example of a board I made with double sided PCB that would have been much more trouble if I had stuck with single sided PCB:
The cache:
To find the cache, have a look at this PCB design:
As was the case above, the images, left to right, represent the PCB bottom copper view, PCB top copper view, and the PCB Silkscreen view. The seven resistors, R1 - R7, have values as indicated by their colour bands:
R1 : Orange, Orange, Brown, Gold
R2 : Red, Red, Brown, Silver
R3 : Orange, Blue, Black, Silver
R4 : Red, Red, Brown, Gold
R5 : Silver, Brown, Orange, Orange
R6 : Brown, Red, Brown, Silver
R7 : Gold, Brown, Grey, Brown
To determine the location of the redirect, calculate the circuit total resistance that would be measured at the IN terminals, then divide that value by 1,000. Using the following meaningless coordinates:
N44 03.349 and W079 03.715
take the value you just calculated above and subtract triple the value from the North coordinates and add double the value it to the West coordinates. At those coordinates you will find a redirect that contains the coordinates to the cache. Please make sure to replace the redirect, and the cache, as they were found.
Hints:
- Don't forget that when looking at the bottom of the board, the image you see is a mirror image of what it would be looking at it from the top of the board.
- Unless you have great spacial perception, it is suggested that you create a schematic from the PCB and Silkscreen views provided.
- Resistors can be inserted upside down; They would still have the same value...
- Assume each resistor is exactly the value indicated by its colour bands, regardless of the tolerance indicated.
In case you want to double check your answer for the redirect before heading out, click here
Congratulations to MythicLionMan on his fifth FTF of this series 
Notes:
- If you take the correct route, there are trails that take you pretty well to the redirect and within about 45m of the cache, Some minor bushwhacking required in those last 45m; Minor if you pick the right way...
- Depending on the season, the trails can be icy, snow covered or muddy - Use caution.