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funelectronicstoday/ projects/ scorpion 3.0/

 

†† Scorpion 3.0

 

Scorpion 3_0_concept color drawing_PCB.png

Challenge:

†† I'm a former Romanian HW engineer and I recently moved to Papua New Guinea for a few years. While being involved here in some technical works, I had the opportunity to travel a lot into the remote jungle sides of this country. In the rural areas, people are trying to develop better living conditions here and to begin to use some small solar panels and some lead acid batteries to power a few lights and maybe in some cases, charge their mobile phones. The technology is many times too expensive for these places and people are ending up connecting their refurbished solar panels directly to some old car batteries. This dangerous practice for a desperate cause to light up some 3W light bulbs at night, leads many times to setting the batteries or the houses on fire.

 

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Solution:

A fairly technical and good solution would be to design a low price adjustable and multi-purpose DC to DC converter power supply that is very simple to use. My idea is to have a small 40W custom built solar charger that has the possibility of converting into a battery to device power supply that can allow multiple voltages at it's output. This multi-purpose tool could have an AGM battery charger implemented in it's menu and it could also have a Li-ion battery charger.

The aim would be to create this tool for the masses at a low price and keep it under 10$ so the people of these places could afford to buy it.

 

 

Potential benefits:

The potential benefits of having such a device are enormous. Some of them would be:

 

-          The possibility of having a very cheap solar charger for some AGM batteries between 7Ah and 55aH (in some cases)

-          The possibility of having a mobile phone charger. In some areas, there is mobile phone signal, but people donít have the possibility of charging mobile phones.

-          The possibility of having access to other devices as mp3 players and audiobooks.

-          Access to radio (local news, information and other media content)(where local radio stations are available)

-          Small indoor illumination system.

-          Increasing the chance of access to education having stated all the above benefits.

 

 

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Tools to be implemented within this device:

 

Solar USB charger (SP voltages up to 40V)††††

Aiming to build something that is able to convert the voltage of the solar panel (SP) down to 5V for directly charging a mobile phone

 

Solar AGM 12V battery charger (maximum charging current of @ 3A)

Planning to add a sub menu in the software that will be able to act like an AGM solar charger. The Scorpion 3.0 should be able to provide the floating voltages of 13.6V - 13.8V

 

Intelligent Solar Li-ion battery charger (configurable number of cells)

Planning to be able to set from the menu a Li-ion battery charger. This function could allow the user to configure the number of cells or the nominal voltage of the battery they want to charge and to be able to input the C parameter of the battery. Scorpion 3.0 should know what to do with these numbers.

 

USB charger using the power stored in the batteries by connecting the device in reverse

Scorpion 3.0 should be able to support other voltage (power) input sources like an AGM battery. In these conditions, it should be able to still function as an USB charger and as an universal variable power supply

 

Universal variable power supply that can be connected either to the SP or the charged battery

This leaves me with the need of implementing four sub-menus in the graphical interface of the display, as follows:

 

- Power supply

 

- AGM charger

 

- Li-ion charger

 

- USB 5V

 

 

 

Power concerns, humidity concerns and Ömore concerns about the Buck idea.

 

†† I realize that this design must be water proof. In these areas, the measured humidity exceeds 80% almost everyday. I realize that having a piece of electronic - whatever thing - inside of a coconut leaves roof house, can lead to water being splashed over the case. After I will have a mature design, I plan to cover the PBG with a varnish or a can sprayed laquer or something. I plan to 3D print the first cases and to keep in mind that I have to make them as tight as possible. Iím concerned that the display will have to come out of the case and this will lead to some gaps between the case and the edges of the display. I will see about this.

 

†† Iím not planning to add any fan or heatsink to the converter, so this automatically means that I will have to aim for an efficiency factor of over 90%. Iím planning to be able to build this box to switch about 40W and this still means that in some cases I will have to dissipate 4W on my PCB inside of a PLA plastic box. Wish me luck! Maybe I can use the frame of the LCD to take some heat out. By the way: the air temperature here is around 32C indoors (in average during a normal day).

 

†† The Buck itself is a cheap and simple design, but is has some problems and shortages. It is a well known fact that usually, the mosfets are likely to end up short-circuited if they fail due to high current or/and junction temperature. A short circuit between the input and the output of the Buck converter means automatically that I will toast my mobile phone if my design ends up in this situation. Having the OL voltage of the solar panel directly connected to my output device is not a nice thought. I must definitely have a thermal protection and a current sensor. Iím concerned because Iím not very sure how fast I can make the circuit shut down if something bad happens. Will this mean that I will have to add a secondary mosfet to the input to switch the Frankenstein off? This remains to be investigated. If I fry my phone in a couple of weeks from now, itís all for a good cause. :)

 

Video of the PCB:

 

 

Final presentation video:

 

 

Topic about the production cost:

After I posted the component list and uploaded a full excel sheet with the component prices, you are gonna ask me where do I buy my components from. The answer is Chinese market (alibaba, ebay and others). Probably you are worried about the quality of the components but I can tell you that so far it's been going good from this point of view. Keep in mind that I'm creating this design for the 3rd world countries and it is supposed to be low cost and affordable. Trolling with a chunked smile on the corner of my mouth, I can say that that's the art and beauty of creating electronics from scratch: being able to make a circuit last even if you use low quality capacitors. Just kidding guys! Don't take me too serious. :P

 

†† The truth is that it's a pretty robust design and I'm personally satisfied by the way it evolves. After all, I'm planning to use it myself if I get stuck in a village somewhere. I always say "be careful what electronics you create, cause one day, they might save your life. We had a few examples here on hackaday.com.

 

††† So I'm happy that it actually turned out to be cheaper than 9$ to produce. This is just like I initially estimated and it's a good thing. Of course, I haven't got into the added cost of the actual labor, packing, accessory cables and shipping. And above all, the selling share will add to the final price.

 

 

 

†† PWM resolution discussion (as promised):

 

†† I implemented the following code for controlling my buck converter using this output PMW (as you probably saw)

 

#define pwm_period 700††††††††††††††††† //PWM Period example 700*(1/16,000,000) = 43.75us / 22.857 KHz

...

//--------------------config PWM

P1SEL |= BIT2;†††††††††††††††††††††††††††††††††††† // P1.2 Select as TA0 output

TA0CCR0 = pwm_period;††††††††††††††††††††

TA0CCTL1 = OUTMOD_7;††††††††††††††††† // TA0CCR1 reset/set output mode

TA0CCR1 = 0;††††††††††††††††††††††††††††††††††††† // TA0CCR1 PWM duty cycle

TA0CTL = TASSEL_2 + MC_1;††††††††††† // SMCLK, up mode

 

 

†† The equation for determining the output frequency of the PWM is the following:

pwm_period*(1/16000000). After this, you come up with the period and you have to divide 1/period again, in order to establish the frequency. 16000000 is the number of oscillator cycles per second that is then internally divided and dealt with. Now, in my case I used the PWM period of 700 (correct me, I know, is actually 701), because I want a frequency that is more than the audio band. I really donít want it to pollute my audio devices when is functioning and I really donít want it to make audible noise. It would be awkward. Now the internal timer module, based on the values of TA0CCR0 and TA0CCR1 in this case, actually starts to count until it reaches the value of the TA0CCR0. Which means I have an internalcounting of 701 oscillations and then it starts all over again.

Based on the PWM filtered period, my external optocoupler will have a different internal resistance being inserted in the respective voltage divider and it will act like a potentiometer (a non-linear one). The graph of this PWM potentiometer looks like this:

 

non linear PWM pot.png

 

†† So the LED of the optocoupler is actually having an external filtering capacitor that filters the PWM duty cycle in a continuous variable voltage. This means that is actually normal to see the optocpupler not opening at values of the PWM that are lower than 200, because the LED has a threshold forward voltage itself. Now, the problem is that, as you can see from the graph, I must set all my voltages from 10V to 25Ö30V with only 160 leftover units of my PWM scale. I want to be able to set the output voltages from 100 to 100mV and that leaves me with a necessary of at least 200 different PWM values, which I donít have.

 

PWM pot.png

 

†† I implemented this piece of code in my A3 firmware and the entire system was oscillating. It was trying to set a certain voltage value using a PWM duty cycle of 615 units and then the read output voltage was too high and when it was lowering the duty cycle to 614, the output reading was too low and it was oscillating like this.

 

†† A decent solution to the problem would be increasing the resolution by lowering the frequency on the LED. This has itís drawbacks. Besides the fact that the parallel capacitor wonít be able to filter the LED voltage so good, I have a lot of steps to increment the PWM duty cycle register before I hit the right spot using the A3 software and this would take a few seconds to reach the desired output voltage after setting it up.

 

†† At this stage I wonít modify the hardware (except the value of the capacitor). I think I will go with a lower frequency and I will have to implement an algorithm that will make the duty cycle to jump to the right spot according to the desired output voltage.

 

Component parts of the Scorpion 3.0:

case components.png

 

Accessories of Scorpion 3.0

†† I was thinking that these could be the necessary cables needed in some situation. I haven't decided yet witch of them shall be included in the final box of the product the customer will buy. I have to decide over the price, versus needed functions. It might sale with one cable of another at user's choice.

accessories.png

 

†† How to connect - Instruction set

†† Depending on what function of the device is desired by the user, the functionality modes can be achieved in the following configurations. I also placed a note of the used software sub-menu for the designated function

 

How to connect.png

 

 

 

††† Files to download:

 

         B1 schematic

         B2 layout

         A4 firmware

         Stl case files

         Connection instructions and others

 

 

 

 

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