This article is a continuation of the previous one.
Let’s get started to designing a PCB on the KiCad software.
The designed circuit diagram is shown below.
Iluminate Tube PCB circuit
Associate the footprint with reference from the schematic data.
associate the footprint
Next, I will draw the wiring of the board. Need to know the size of the PCB.
Therefore, we designed the case first that wrote by 3D-CAD.
TOP
BOTTOM
You can secure wood by buying wood with a thickness of 45 mm x 45 mm at a home center and cutting it out at 100 mm intervals.
Using a 35mm hole saw, make a hole for mounting the board from the back. Make a hole to insert the test tube with a 25 mm hole saw from the top.
The 2D drawing is shown below.
LA-01図面
Therefore, I decided that the size to put the board in is 35mm in diameter, so I think that the size of the board should be 34mm in diameter.
Let’s get started to design the PCB.
Board is 2 Layer.
The contents of the board are shown in the figure below. Power is supplied from USB-type C.
In addition, power supply, color switching, brightness adjustment, etc. are all integrated into one home button.
After that, I would like to link the terminals for writing software and the same boards when expanding within the same product in the future. For that purpose, we have prepared a signal input terminal and an output terminal. After that, there are two screw holes Φ3.2 for fixing the board.
Detail of PCB
The front and back sides are shown in the figure below. There is a space in the middle of the TOP where you can place the full color LED.
PCB（Bottom）
Board（TOP）
The 2D drawing is shown below.
TOP of PCB
Bottom of PCB
After making the drawing, we will check the actual product.
Print the drawing to full size and actually place the part on top to see if it is the correct size. This completes the design. Next time, I will order the board from a vendor.
Last week. I went to the “Rikashitsu” at the Kiyosumi-Shirakawa in Tokyo. I often purchase a Cork-Glass-Tube from This shop.
The Rikashitsu is directry management shop from Sekiya Rika Co,Ltd a wholesaler of glass for science and medicine. This shop is aims at “physical chemistry + interior”.
This sells science and chemistry products actually used by professionals and Original product processed by Glass Craftmen.
The reason why this shop was opened that the craftsman’s skill could be utilized by getting ideas by using physics and chemistry glass products by the general public.
There was a very stylish space inside the shop. I felt that the science experiment was very photogenic.
I am amaized from new value.
I impulsiverly bought a Cork Glass Tube and Woodern Stand at this shop.
Cork Glass Tube
Woodern Stand
From the moment I saw it at the store, I strongly wanted to make it Iluminate.
As soon as I got home, I remodeled a Woodern Stand.
I made a hole in the back of the stand with a drill and packed artificial flowers in the Glass Tube.
The photo below shows the actually Glass Tube illuminated.
I inserted an LED through the hole to make the Glass Tube Iluminated. This is very Good.
This will quickly make the Print Circuit Board.
I would like to make it possible to Iluminate in any color using a Tiny-Microcomputer and a full-color LED.
size of microcomputer（from Akitsuki-Denshi Home Pages）
The memory capacity of this microcomputer is only 2kB.
Is it about 2000 half-width text characters? Since the capacity is very small, only simple programs can be written.
However, this should be enough if it only illuminates a three-color LED.
Next, the LED specifications are shown below.
For the LED, I adopted a full-color LED containing the three primary colors.
DC Forward Current：150mA Power Dissipation: １W
・VF： Red…2.5V Blue、Green 3.3V
・ΦV：
Red…22lumen
Green…35 lumen
Blue…12 lumen
・λD： Red…624nm Green…525nm Blue …460nm ・2θ1/2：120°
size of LED chip（from Akitsuki-Denshi Home Pages）
Power Dissipation is 1W. The standard current is as high as 150mA. It’s very dazzling.
I wrote the electric circuit diagram below.
LA-01 回路図
The Full color LED used has a very high current. it is difficult to operate it by connecting it directly to the output terminal of the microcomputer.
Therefore, Increase the current by connecting an N-channel MOSFET to each LED.
The brightness of the LED is adjusted by the PWM signal from the microcomputer.
Wrote the calculation of limiting resistance(R5,R8,R11) connected to LED below.
IF the MOSFET is turn ON completely that can be only resistor and LED on the circuit by omitted MOSFET.
The information required for the calculation is the standard current and voltage of the LEDs below.
IF=150mA VF：Red…2.5V Blue Green 3.3V
The value obtained by subtracting the LED Vf from the power supply voltage is the voltage between the terminals of the resistor.
Since a current of 150mA is passed through the resistor and LED, the calculation result of the resistance value of the limiting resistor is as follows.、
Blue； R=V/I = 1.7V / 150mA = 11.33[Ω]
Green； R=V/I = 1.7V / 150mA = 11.33[Ω]
Red； R=V/I = 2.5V / 150mA = 16.66[Ω]
However, the right resistance value is not sold on the market. The resistors sold on the market follow the E24 series, with the closest values is below.
blue； 11.33[Ω]＝10Ω
Green； 11.33[Ω] =10Ω
Red； 16.66[Ω] =15Ω
next, Calculate the actual current when this resistance value is applied.
Blue； Current If [mA] = V/R = 1.7 / 10 =170 [mA]
Green； Current If [mA] = V/R = 1.7 / 10 =170 [mA]
Red； Current If [mA] = V/R = 2.5 / 15 = 166[mA]
Make sure that the calculated current value is less than the rated current value listed in the LED data sheet.
Absolute Maximum Rating on the LED datasheet
Since the DC rating of the LED is 200mA, there is a surplus of about 30mA.In addition, since this unit performs PWM control, the value of “Pluse Forward Current” in the data sheet is applied. Then, the rating becomes 250mA, and there is sufficient margin even if the error of resistance is taken into consideration.
The voltage between the resistor terminals should be low due from the power loss of the FET.
Next, Calculate to the Power Loss of the Resistors below.
Blue； Power Loss of the Resistor(Blue) Pr [W] = V ＊ If = 1.7 V＊170mA = 289 [mW]
Green； Power Loss of the Resistor(Green) Pr [W] = V ＊ If = 1.7 V＊170mA = 289 [mW]
Red； Power Loss of the Resistor(Red) Pr [W] = V ＊ If = 2.5 V＊166mA = 415 [mW]
Only the red color is prominent and the loss is large.
For the above reasons, use a resistor with a rating of 0.5W or more.
Next is the selection of FET.
Since a maximum current of 170mA is flowing between the drain and source of the FET, it is necessary to allow more drain current to flow. I chose BSS138 because of its price and performance.
BSS138 (from Akitsuki Denshi Home pages）
・Construction：MOSFET ・Number of circuits：1 ・channel：N ・Voltage between drain and souce：50V ・ Voltage between gate and souce ：±20V ・Current of Drain(DC)：300mA ・ON resistance between drain souce：1.6Ω ・Tolerance of Power Loss(25℃)：350mW ・Package：SOT-23
Next Calculate the Power loss of FET.
We simulated FET loss with LTSpice. I wish I could use the BSS138 Spice model, but I couldn’t add it successfully. Therefore, we simulated with a FET model with similar values of Rds and Vds.
シミュレーション結果
The Joule integral value of the power obtained by multiplying Ids and Vds from this value was 44.49uJ.
Since the PWM pulse is 500Hz, 500 * 44.49u = 22.24 mW
The thermal resistance θjc of this FET is 350 ° C / W.
When the temperature is 25 ° C, the junction temperature is Tj = 25+ (350 × 0.02224) = 32.78 ° C.
Even with the maximum current, the temperature rises by + 7.78 ° C, so I don’t think it will have much effect.
Ciruit
For the above reasons, it is decided by this circuit diagram. Next, design the board based on this. see you next time.
