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Battery Supply for DSO138 portable Scope

Turning a cheap DSO kit into a cheap automotive scope

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This project is to basically furnish a DSO138 kit with 3D printed case with a portable power supply in the form of a lithium battery pack to turn it into a cheap/poor-mans automotive scope for very little cost.

The Plan

The aim is to assemble my old DSO138 scope into a 3D printed housing, with a power supply and use it primarily as a portable/automotive scope.

The housing was on thingiverse - so many thanks to salvation76 for that!

The supply should be able to meet the input requirements of the scope, which is nominally 9V. The battery supply should last for a while. 9V PP3 cells aren't up to the task (100mAh), AA are equally as weak and would need loads to get a semsible voltage, plus I don't want to keep dismantelling it all the time to change the batteries.

So I plan to use 2x 18650 cells in parallel, bumping the capacity up. The Cells will be out of my salvaged stock from laptop batteries.

The charging/monitoring will be taken care of by a TP4056 based USB charging module. The 3.7-4.2V battery supply will be boosted up to 9V using a Boost converter.

Boost converter

Charge Board

Its a fairly simple project, and should prove quite useful, especially diagnosing things on cars.

Below is a simplified schematic of the circuit layout, demonstrating the simplicity:

Building the Battery Pack

Firstly I sourced some suitable batteries from my stock of 18650's I'd collected and found 4 that were round about the same voltage.

Considering I only needed 2 I thought why not see if I can fit 4 cells in parallel inside the case...

...which they seemed to fit nicely in this configuration. Construction on the pack began based on this layout by carefully soldering some tinned wire onto the battery contacts.

3 of 4 cells soldered together. Got to have duct tape somewhere in your project! the final cell was solder to the other 3 with some heavy-ish gauge wire.

With the pack assembled, it was time to test if the TP4056 board did what it was supposed to

Top meter is current from supply to TP4056 board, bottom one is battery voltage, this seems to be working, but a more lengthy test was needed to determine proper operation. Ideally this charger in this application should provide a constant current up to a battery voltage of ~4.0V and then change mode and allow to charge further to ~4.2V. So the pack was left to charge over a period whilst I monitored it and took readings at random intervals, to obtain the graph below.

As expected the battery had a constant current of ~1A up to a voltage of ~4.2V and then current supply slowly dropped off over a period of about 2 hours after.

Using the Riemann Approximation of integration on the current curve, I determined from the data that my pack had a capacity of approx. 5500mAh (although this was not taking any residual charge left in the batteries into account)

Proceeding with Assembly

Firstly: offering up the parts to ensure fit:

The hole for the switch did require some filing to get it to fit properly. Once fitted and some self tapping screws found, it remained in permanent place.

Marking up the PCB's for inside the battery compartment:

With these in place I was then able to determine roughly where I needed to cut in the side wall to allow the USB connector through to allow charging. This was accomplished using a drill and a file:

Then double checked that the cable could reach through the side wall and mate with the connector:

Before making an unholy mess of hot gluing it into place! Pro-tip: glue in place WITH connector fitted stops you from gumming up the USB contacts!

It was at this point I decided to fit some external charge indicating LEDs. But then my phone died and was unable to take photo's. So I noted the direction the surface mount LED's were fitted to the board, removed them, soldered up some wires to a 3mm bulb LED's (one red and one green), drilled some 3mm holes in the side wall just above the charge port, push the LEDs into the holes and hot glued them in place, then soldered the relevant wires of the pads of the charge board.

Before fitting the DCDC boost converter, I powered it up on the bench, loaded it up with a 100mA load in the form of a 50R resistor, and measured and set the output voltage using the trimmer to 5V

I then wired up the battery pack...

Read more »

  • 1 × TP4056 based USB Lithium-ion charge module Typical and common board found on eBay, Banggood and the like
  • 1 × Boost conveter module Again typically found on eBay and the like
  • 4 × 18650 lithium-ion cells (circa >1000mAh) Matched voltages or as close
  • 1 × Slide switch model for DSO box came with mounting hole for a slide switch
  • 4 × M3x40 Hex socket, cap head screws

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  • Completed!

    Tron900005/17/2017 at 12:08 0 comments

    Well I've finished it so can finally close this project right after I've updated the details on this page.

  • Construction Finished

    Tron900005/04/2017 at 07:57 0 comments

    I've finished the construction. I'll put up some pictures soon and some test data. I was unsuccessful in getting the pictures off my old phone hence why there's been a lull.

    Also I'e decided not to implement the charge indicator in this build as I can't decided on a suitable place to put it and I don't want to drill more holes in the box.

    Stay Tuned

  • When a window opens...

    Tron900004/16/2017 at 23:06 0 comments

    ...A door closes!

    Well after a productive morning on Friday, my phone finally decided to pop its clogs and refuse point blank to charge, thus I was unable to upload any pictures.

    However I have managed to complete the build on this project and it functions very well; I have managed to measure the waveform off my alternator on my landrover and the tachometer output of the alternator too.

    I have parts on order to fix the other phone and hopefully it will accept charge and I can pull the remaining photos off it and show the build details and testing.

    Cheerio

  • More parts

    Tron900004/06/2017 at 20:37 0 comments

    So I got round to ordering a DCDC boost converter and a battery level indicator. Unfortunately i ordered the wrong battery level indicator, I order one for a 5S pack not a 1S, so I have 2 options: 1. order a new one (more expense and another month on a slow boat from china) 2. modify the one I have.

    I briefly looked at the board and it appears to be based on the LM339 comparator, so am expecting just a few resistor values need changing and they are 0603 size, so not completely impossible to rework.

  • More Ideas

    Tron900002/13/2017 at 13:40 0 comments

    Given it'd be nice to know what the remaining charge of my battery pack is, I found these:

    They would fit nicely down the side of the batter compartment and if I removed the button and used a separate panel mount one, it would be nice and flush against the side allowing the LED's to poke cleanly through some well positioned holes.

    They're cheap enough, so hardly going to break the bank when I finally tote up the final bill.

  • Test Charge of Battery Pack

    Tron900002/10/2017 at 14:02 0 comments

    So this is the setup I used to charge my batteries whilst monitoring their progress. The aim is to verify that the batteries can reach a charge round about the 8800mAh. I am assuming that some residual charge would be left in from the previous nights test and from its previous life in a laptop.

    So anything between 4000-7000mAh is acceptable. I know that's a wide range but I'm taking into account of worst case (1000mAh per cell) and best realistic case given the cells age plus residual charge left over (7000mAh).

    So I began charging at 8:45am at work using a linear bench supply and a DMM and basically took values at random points in the day. The setup was next to my Laptop, so some values are quite frequent whilst I've been looking for parts or reading data and quickly glanced over and entered them in the spread sheet, and others are less frequent as I've had be called away.

    During a break I used work's FLIR handheld thermal camera to find out how hot the little charge board was getting. Not too hot. I also used the finger test and it seemed to be hotter than the FLIR was indicating. It was just bearable to touch, meaning it was just shy of 60 degrees Celsius. This is not a worry either, if it was any hotter I'd be considering a different type of charger (hobby charger maybe).

    So After nearly 7 hours, I managed to obtain the above graph. As it can be seen, the battery voltage slowly rises to about 4.1V whilst the current remains constant at 1A-ish. It then slow reduces the current over the next 3 hours or so till it reduced to about 2mA, whilst the battery voltage didn't get above 4.3V. This proves the Charge board is working correctly to charge Li-Ion batteries - not that I ever doubted, but its a good idea to check to make sure you've not got a fake!

    So to determine the rough capacity of the battery, I used the data from my spread sheet and did some rough integration using the Riemann Approximation with the current values and out popped a number: 5565mAh

    This is bang where I expected it to be. So taking into account of some residual charge, a little charge from the previous test and the roughness of my calculations, I guesstimate the capacity to be in the ball park of 6000mAh. I will confirm this with a load test maybe, but to be honest that's plenty capacity for this application!

  • Battery Pack Construction

    Tron900002/09/2017 at 09:12 0 comments

    Made a start on constructing the battery pack. Since I panned to use 2x 18650's in parallell I needed 2 of them that had voltages very close it each other, which meant going through my 18650 hoard:

    Managed to find 3 cells close, 2 of which were bang on 3.79V, which was also ideal as usually anything lower than 3.7V and the cell is usually deemed unusable, but I have seen people still use them anyway.

    Then I checked they could fit into the enclosure and found I could squeeze 4 of them in there! The charge board could also fit in too.

    The reason for this layout was having all 4 in a row fouled the area where the on switch would go.

    At this point I was curious as to what capacity these cells were. So checked part number on internet:

    So according to a Flashlight forum post No. 21 I could identify the capacity of this battery as out of a sony laptop battery pack and had a capacity of 2200mAh! But is also 10 years old so don't know how much of that capacity is left! Anyway here's a table on how to identify similar cells:

    First lineManufacturers dataUS18650GR - which means its an 18650 cell and googling this also shows its made by Sony
    Second line - first 2 digits after letterCapacity identG4 = 1950mAh
    G5 = 2150mAh
    G6, 6A, 6B = 2200mAh
    G7 = 2400mAh
    8A = 2600mAh
    Second line - first of last 5 digitsYear of ManufactureP=2007, Q=2008, R=2009, etc
    Second line - second of last 5 digitsMonth of ManufactureA=Jan, B=Feb, C=Mar, etc
    Second line - third + fourth of last 5 digitsDay of ManufactureSo I'm assuming that 2+6 = 26th day for example
    Second line - final digitElectrode HistorySome internal reference - unknown

    So deciphering the code:

    • 6A - 2200mAh
    • P - 2007
    • F - June
    • 3 + 0 = 30th?

    So each cell is 2200mAh and they are nearly 10 years old! I expect that they will have lost some capacity over the years. But if not then best case scenario is I have a possible 8800mAh battery. Yeh that should do, but I'm not getting my hopes up. Will have to test to verify this.

    I then proceeded to carefully solder wire to the batteries in 4P configuration:

    Quickly measured the starting cell voltage on the DMM:

    So the cells appear to have balanced themselves out to 3.78V, without any drama. I then proceeded to solder the charger board to my batteries:

    And finally gave it a quick test on bench to ensure it was charging:

    Top meter is measure the current into the charger and I monitored the voltage with the lower meter. The lower meter appears to be 0.17V out when compared to the upper meter - the upper meter is more accurate. So the reading is actually 3.77V. I watched this climb to an indicated 3.97 V with a steady 1A current and then turned it off. I plan to continue charging over a working day and monitor its performance. But from this quick test it appears to work as it should!

  • Upgraded Hardware

    Tron900002/08/2017 at 13:32 0 comments

    got some Hex socket cap head M3 screws. They fit well an sit nicely into the bore of the hole. There is still a bit of material to clear out of the bore, which causes the head of the screw to sit proud but it like it lie that.

  • Hardware fitment

    Tron900002/07/2017 at 22:10 0 comments

    I managed to find some nuts and bolts that would fit but not without a bit of fettling.

    The holes formed by the 3D printer are too small for M3 yet the hexagonal holes for captivating the nuts fit M3 perfectly. Confused but undeterred, I drilled out the bolt holes to 3.5mm for good clearance and managed to source some bolts that were just a bit too long.

    However drilling out the holes broke through the side walls of the bolt guides a little, but not so much that structure was compromised nor the bolt became misguided.

    I'm going to see if i can find some that are just long enough from the local hardware supplier, as i don't fancy shortening the bolts with a hacksaw and buggering the thread. M3 bolts are cheap enough and I know my local supplier stocks them...but in the length i need? Will have to find out.

  • Case Constructed

    Tron900002/07/2017 at 15:42 0 comments

    So using a Ultimaker 2+ Extended 3D printer, I managed to print a box found on thingieverse: http://www.thingiverse.com/thing:1708650

    I went with Bronze green cos its a common colour for classic Landrovers (which I am a fan) and Silver buttons which seem to complement....

    ...nah! I'm joking, it's all I could get and just so happened they suited!

    I need to locate some hardware to screw it all together, I' going to have a rummage in my nut & bolt jar to find something close in M2 or M2.5. I am please with the fit and the overall quality.

    Here is the battery compartment

    As you can see: I have a little over 19mm to play with. so I am going to place a piece of insulation between the battery compartment, just to ensure that the batteries don't short on the underside of the DSO PCB.

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