This Instructable is offered to those who no longer have the ocular acuity or even mental facility to readily decode the colour coding of the ubiquitous axial-lead resistor for the limiting current of breadboarded LEDs.
It could also be interesting to those who feel most of there breadboarding time involves searching for and retrieving LED’s, resistors and combinations thereof from the floor, shirt front or seat cushion.
Lastly it maybe helpful in avoiding need for a ‘tear down’ of a project when confidence in LED polarity is lost.
My solution to the latter problems has been to fashion a LED/resistor combo where the only rule was “LED pin goes to ground”. The safe use of the apparatus was left to ‘common sense’. Rather sufficient until complicated by the addition of shrink-wrap for its structural strength and insulating qualities. Ultimately even this became moot when ‘common sense’ involved a magnifying glass or multimeter to determine a safe voltage.
Although far from an Eureka moment, as I needed to replenish by appliance (yes they do disappear) it seemed there was a rather simple answer. I could ‘colour code’ the working voltage of the trinkets. As long as the code was simple and readily visible the onset of the ocular condition could be ignored.
For the remainder of the Instructable a more imperious “appliance” will be substituted for “trinket”.
The ambitions (or entertainment) for this Instructable is to show the material and methods for constructing the breadboarding LED/resistor combo and suggesting a method to visually recognize their current limiting characteristic.
The appliances are only constructed for three voltages names 5, 9 & 12V, as these are the practical limits for breadboards, assuming a 5-watt ceiling on any contact. The recognition can be determined by material available or preference, here, red was chosen for 12V/560Ω, yellow for 9V/390Ω and green for 5V/180Ω.
There is no expectation this will provide inspiration or heart warming comfort, it is just sometimes fun to melt solder and bend little pieces of wire.
Step 1: Material
1) Soldering heatsinks
2) 3mm Red LED (older stock 1.6v – 20ma)
3) Resistors of nominal calculated value (e.g. 180Ω, 390Ω & 560Ω)
4) Third hand.
5) Shrink-wrap collection 2mm, 3mm, 5mm (3/32”& 3/16”?)
6) Heat gun
7) Piece of sandpaper
8) Round nose pliers (or at least well worn needle nose)
9) Soldering iron.
My stock is of a certain vintage that has for the most part taken on the patina of age. A gentle scrub with 220grit sandpaper makes the solder flow nicely.
Step 2: Design
To reiterate, the intention is to construct a LED/resistor appliances that provide current limiting appropriate to 5, 9 & 12V. Actually there isn’t any design involved, rather selection of components that satisfy Ohms Law.
It is important that the characteristics of the LED be known to some extent. Unfortunately this is not always assured and best guesses must be made. Getting the parameters very wrong could result in the LED expiring in a brilliant flash or a LED that is only visible in the dark. There are a number of excellent web sites that provide good information on (modern) LED’s and ‘calculators’ for getting a good current limiting R-value including here.
For some completeness, it is the diode’s ‘forward voltage’ and ‘forward current’ that determine the current limiting component. Take the R=V/I formula and a knowledge/guesstimate of the diode and assume I=20ma as the maximum current that should/can flow through the diode. Since LED’s are resistive, described as their forward voltage, the formula’s V is actually (Vs-Vl) or the source voltage minus the voltage drop across the LED. So for example with Vs=5V and Vl=1.6v (my stock) a value for R=(5-1.6)/.02=3.4/.02=170Ω. A common value, on the safer side is 180Ω.
A quick application of the formula for power P=I²R show a mere .068 watts readily handled by your garden variety1/4watt resistor. If this is repeated for the 9V, R=370Ω (used 390Ω here), P=.156watts (safe at 1/4watts) and for 12V, R=520Ω (used 560Ω here), P=.224watts (lots of arguments for this to be 1/2watt resistor).
Step 3: Template
The template for the appliance is somewhat dependent on the physical characteristics of both the axial-resistor and the LED. One lead of the resistor will serve as any male connector pin. The other lead is soldered to the anode(+) leg of the LED (the longer of the two legs). The cathode leg of the LED is the second male pin.
First consider the lead being soldered to the LED leg. The combined length of the lead and leg is probably most useful between 30-35mm. Make a judgement of where you wish/can make the soldering join and trim the resistor lead accordingly or possibly no trim is required. The image shows the shape that the legs of the LED should ultimately assume, however for easy addition of shrink-wrap it is better to only bend the LED leg about 45°. This should be done using the round nose pliers where the leg is bent about 2mm from the plastic LED lens.
Next, using the round nose pliers bend the lead into an “S” such that 10mm protrude past the first bend. The second bend in the lead should not extend past the resistor body. Once the bends ‘look good’, any excess lead should be trimmed. Simply trimming the lead to a suitable length is an alternative but we’re illustrating a method here, not arguing relative merits. The preference for an “S” is the (unfounded?) concern that the protective coating of the resistor should be protected from too much flexing or excess force when the lead is a breadboard pin.
If available, perhaps the best resistors to use are in fact 1/2W carbon film for their usually larger lead diameter (.6mm?). The larger body size also improves the pin insertion.
Step 4: Soldering
My soldering technique is paramount to brute force. The soldering heatsinks are used to protect the LED and resistor, the exposed leads are heated by dragging the iron then a minimum amount of solder is dragged down the join area. This seems to ensure good bonding without leaving any peaks or pinnacles.
In my first constructions, before employing heat-shrink, I rotated the solder joint back-to-front to make a similar solder join. Of course this was achieved in about 50% of the attempts or the first solder liquefied and the leads separated. The single soldering makes a very good join and is strengthened further by the shrink-wrap.
Step 5: Shrink-wrap
The application of shrink-wrap is straightforward but not easily developed as an instruction. The colour, diameter and length are a matter of preference, availability and results from the soldering step.
Maybe the best guideline would be described by “be generous” in the lengths and diameters of shrink-wrap tubes. First cut a piece of shrink-wrap tube to a length that would cover the looped lead of the resistor and the leg of the LED. The image shows a less than desirable outcome that results from leaving a 90° bend in the LED’s leg.
A final touch, add a small (5-7mm) piece of shrink-wrap over the resistor end of the first shrink-wrap. This gives a good finger grip for the appliance. The diameter of the shrink-wrap could be the same as the first or a larger size. Generosity does make for easy application but also choose a size that will make a fairly tight fit at both ends.
Black is also a good colour for the second piece. The contrast is helpful and smudges of peanut butter or sticky buns aren’t as noticeable.
Finally, using finger pressure, reshape the appliance such that the LED leg pin and the resistor lead pin are more or less parallel.
Step 6: Summary
Quick to deploy and ease of storage is a short list of possible benefits. Simple to construct, each unique artwork may be bragging points or ho-hum “as-is” accordingly.
It would be a remiss if at least one shortcoming of this (rather lengthy) Instructable were not acknowledged. Perhaps if that shortcoming were masked as a challenge, it would be less noticeable.
LED’s of course are not restricted to the colour of the lens. Many LED’s have a clear lens and their illuminated colour is revealed with a small electric current or known through some magnificent feat of labelling and organization.
Anyone wish to offer a strategy for clear lens LEDs?