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Ledificador

Pedro Jesús García García

pedro2g@correo.ugr.es

First at all, before describe the main integrated project, different activities made throughout the course will be presented.

1. Desoldering techniques

In some practical courses, we have learned to desoldering electrical components from real circuits. The components desoldered were both Through-hole Technology (THT) and Surface Mount Technology (SMT) as shown in next pictures. The process will be explained below.

· THT: as we know, THT components pins go through all the PCB layers. In previous years, THT components were desoldered melting the tin with a soldering iron and removing molten tin with a desoldering pump. But this year a soldering/desoldering machine can be used, so components could be easily desoldered with a Desoldering Gun tool which, connected to the machine, can melt and suck away the melten tin. But sometimes, some component pins have no enough tin or you have not sucked away all of it so we have to add more tin with a soldering iron before can remove all the metal. For this process we have used a temperature of 350ºC. In next picture we can observe some capacitors, diodes and a resistor desoldered.

· SMT: these components have their pins soldered on the surface of the PCB, whether in the top layer or the bottom layer. This technology is new for us thus we have learned to desolder it with the Hot Air tool of the soldering/desoldering machine. With this tool, a hot air flow is blown which we can heat the entire SMT component and melt the metal under it. Therefore, with tin molten, we can remove the component with tweezers. In the picture we can see a lot of components desoldered with a flow of 350ºC too and an air pressure of 85 P, like resistors, capacitors, diodes, arrays of resistors, and others SMT packages (SOP).

2. Copper Clad project

Falta por hacer!

3. Integrated Project: Ledificador

3.1 Introduction

In this project, a frontal bike light named Ledificador will be developed. This light will be composed by high power LEDs, which are more efficient and luminous than standard high power lights. With this, all the knowledge acquired at Printed Circuit Technology course will be put into practise.

3.2 Design flow

To explain the design flow, a graphic diagram is going to be used.

First, the project was chosen searching among those in Elektor magazine. I chose this one because of its complexity and because it was a product useful for me since I don’t have a powerful bike light and it is interesting for me to learn how to do it. The Ledificador was found July-August 2009 magazine.

Next, a circuit study was made in order to understand what is going to be fabricated.

Once the circuit was understood, two tasks were developed in parallel at same time: the box model design and the schematics in Altium. The first phase is very important in the product design since the PCB design (with all its component) depend on the box. To do the schematics, the project schematics in Elektor magazine was used, but a circuit simulation was made in order to can calculate the power that components have to support, and can choose them properly.

So now, the box design can be built into SolidWorks. Once built, improvements were added into the design until the box is totally finished and we have then the final box product.

With this, we are ready to design and make the PCB. For this, the size of our box is used to describe the boarder outline. In this step all the components chosen have to be placed a connected, so if there are problems of space we can redesign the box or change some components with others smaller.

When the PCB is well designed, the assembly between PCB and the box is made. If everything fits there is no problem, but if there is any inconvenient yet, we can redesign the box model and start again.

Finally, the outputs such as images, videos or profiles can be extracted, and the report can be written.

3.3 Mechanical design

In this important step of flow design, the box of our frontal bike light has to be conceived. We had to consider a lot of constraints and problems such as:

- technology of our 3D printer

- the way to place the light on the bike

- price

- design attractive

- the way to open the box in order to change the batteries

- the way to close the box (and reopen) in order to assembly the PCB

- components size

A first basic model thought appears in above picture. One of the main characteristics is its oval shape with the semi-circular form in the back. This oval shape seemed to be attractive, but it wasted a lot of plastic in the sides of the box, so a new basic shape was thought. With respect to the semi-circular form in the back, it is thought to fit into the bike frame (based on my own circular bike frame). For this, the way to hold together the light and the bike is with an external not included trap, which passes into the box through two holes located in the back size. This closing system will be maintained in all mechanical designs. The right hole is thought for a generic ON/OFF switch and the frontal one, for three power LEDs. The problem with this last hole is that dust can entry into the box, so other idea had to be elaborated.

For this, a second and final model was created. First, the model has now a rounded rectangular shape, in order to reduce the plastic. In my opinion, this shape can be a little less attractive, but it is made up for with the price reduction. The switch hole was changed to fit the switch component chosen, and in order to avoid dust, three different holes were opened for each power LED. For this reason, a slot was created in frontal side to put in a small PCB containing the LEDs, as we can observe in the pictures.

The box was divided by the middle in two different parts in order to can introduce the PCB and the battery holder. To close the box, a lips and hook closing system was created. To change the batteries, a cover was made in the down side with other lips and hook closing system. The size of the cover was adapted to the battery holder. Finally, the way of maintain the PCB and battery holder together and still was using the two holes of the battery holder: two screw sink were built in the opposite side of the battery cover, so the PCB and battery holder are fixed with two screws. With this, the box is finished.

A last problem happened when battery holder didn’t fit into the box, so we have to make it bigger. This was a flow design mistake since this problem should have been expected before build the box.

Some section pictures:

Next, we can observe the views of the different parts of the box, with their dimensions.

Insertar vistas

3.4 Electrical design

Explicación del circuito, resultados de simulación, enlaces a datasheets.

First at all, we are going to explain the circuit of the project chosen. The schematics appears in the image below.

The circuit has a 4.5V source with a switch that feeds a DC-DC boost converter LM2577T-ADJ, which supplies the rest of the circuit with 12V, necessary to feed the three power LEDs. The IC2A is used as a current source for the LEDs, while the second amplifier operational avoids the efficiency lost since disconnect the circuit when the voltage supplied by the batteries are below 4V. Hence, the LM2577 provides the necessary voltage and the LM358 the necessary current to the 1W LEDs. All these components were through-hole in the original Elektor project, but we have chosen the SMT ones when possible, in order to decrease the PCB surface needed.

Normally, a complete circuit simulation should have been carry out, but it was impossible to find the LM2577T-ADJ PSpice model, so only a part circuit simulation was made in order to choose the size of the SMT components properly since, depending on the package size, the component may support more or less power. The simulation was made in Proteus, where current and voltage in some components were measured. In all components the power was less than 0.1W, so we could use the 0603 package. If you click at the picture below, you can access to the Proteus project.

3.5 PCB Design

Procedimiento seguido en el diseño de la PCB y su relación con el diseño mecánico.

To do the PCB design, the board outline of the PCB was imported from the box of SolidWorks, with the two holes for the screws, as we can see in the image below. After have the PCB shape, the 3D components and their footprints were imported from schematics, and were placed on the board in an organized way with the help of aligning tools, using both top and bottom layers.

One inconvenient was the placement of the switch, since the board outline imported from SolidWorks there was not a hole for it. For this reason, a board cutout was defined for the switch. Also, some 3D components had to be changed, as the BD139 transistor in order to get a lied down component.

After that, the routing was made with the Auto Route tool of Altium that place the routes and the vias automatically.

However, the PCB was not finished, because there was a lot of free copper space wasted, copper that had to be etched. Because of this, in the second version of our PCB, a ground plane was placed on the top layer. This way, there is no need to have a ground route throughout the PCB, the ground is more stable and the etch copper is minimum.

With all this, the final PCB can be observed in the pictures and video below.

INSERTAR VIDEO!

3.6 Assembly of PCB and box

Finally, the PCB can be exported from Altium to SolidWorks in a .step file. In SolidWorks, an assembly with the PCB and the box can be built and we can observe the final product in the next videos.

INSERTAR VIDEOS!

3.7 BOM

Here we can observe all the components of the project, some information and the price. If you click in component name you can access to its schematics. Also, the schematics Altium library with these components can be found here and the PCB library here.

Component	Designator	Supplier	Unit price (€)	Amount	Description
1 Ω	R12, R13, R14, R15	RS	0,018	4	Resistor ±1%, 0,1W, 0603
560 Ω	R11	RS	0,008	1	Resistor ±5%, 0,1W, Carcasa 603
1 KΩ	R4, R5, R6	RS	0,008	3	Resistor ±1%, 0,1W, Carcasa 603
2K2 Ω	R1,R3	RS	0,007	2	Resistor ±1%, 0,1W, Carcasa 603
4k7 Ω	R10	RS	0,007	1	Resistor ±5%, 0,1W, Carcasa 603
22 KΩ	R2	RS	0,008	1	Resistor ±5%, 0,1W, Carcasa 603
100 KΩ	R7, R9	RS	0,008	2	Resistor ±5%, 0,1W, Carcasa 603
3M9 Ω	R8	RS	0,011	1	Resistor ±1%, 0,1W, Carcasa 603
5K	P3	RS	0,288	1	Potentiometer ±25%, ±250ppm/°C, 0,1W, SMD
10K	P1, P2	RS	0,13	2	Potenciometer ±30%, ±500ppm/°C, 0,1W, SMD
1N4148	D3	RS	0,033	1	Diode 300mA, 75V, SOD-323
LM2577T-ADJ	IC1	Mouser	5,78	1	DC-DC boost converter
LM358	IC2	RS	0,11	1	Dual Operational Amplifier
LED 1W	LED1, LED2, LED3	Farnell	2,51	3	Power LED
Zener 2V7	D2	RS	0,028	1	Diode zener 2.7V, SOD-123, 2-Pines
BD139	T1	RS	0,246	1	Through hole transistor
STPS340U	D1	Farnell	0,572	1	Diode smd, DO-214AA
330 nF	C1	Mouser	0,043	1	Capacitor 603
100 nF	C3, C4, C9	Mouser	0,052	3	Capacitor 603
100 pF	C8	Mouser	0,052	1	Capacitor 603
47 uF	C2	Mouser	0,363	1	Capacitor 2917, 16V
470 uF	C5, C6	RS	0,055	2	Capacitor ±20%, 16 V
10 uF	C7	Mouser	0,13	1	Capacitor 1206, 16V
100 uH	L1	Fastrongroup	1,02	1	Inductor smd
		Total price =	16,881

3.8 Conclusion

With this work we could approach the actual process of PCB manufacture with all its design flow, from taking the idea and requirements (this time from an electronics magazine), through the box model, to the design of the PCB, including all feedback processes every time in appeared a mistake or a problem.

Perhaps now, with the project already implemented, if I started a new work, I would do things differently, and some mistakes and subsequent modification by feedback could be avoided.

Also, I think I have managed to make a complicated project with many components and many decisions, and surely other engineer with more experience could have made it in a better way, but I am glad with the results.

3.9 Improvements

The project can be improved in many ways, but only box and PCB improvements are going to be mentioned, not electronics ones.

With respect to the box, if the 3D printer allows, another system gripping could have been done instead of a not included trap. Current bike lights have two different parts: the first one is to grip to the bike frame, and the second one is the light itself, that is assembled to the gripper. Also, another improvement, besides the shape, can be adding a slot to the batteries cover for easy opening.

With respect to the PCB, perhaps a better location of the components could have been done in order to reduce the number of vias, or a lied down package for BD139 transistor could have been found in order to increase the thermal dissipation and ground connection.

3.10 Valuation of the course

In this work, the difficulty was not the electronics since the project was taken from an electronics magazine, but the box and PCB design. We had to learn about design software (SolidWorks) and PCB design software (Altium) among others. We have learned the design flow of a PCB and that it is not a sequential flow, but it has feedbacks in order to improve the characteristics or solve an error.

In addition, as personal opinion, I prefer courses like this since the student can implement all the concepts learned during the course in a real project, which brings you to the real world of engineering. And also, as the course is in English, we could practise the main language used in engineering, which prepares us for our future.