Last post covered the design considerations for the Voltage Shifter required for the Mixed Scale DRO Controller. As promised, in this post I will provide detailed build instructions for this circuit. The board is designed to work with up to four scales and has three power rails. One provide 3.3V power supply to the comparators and [potentially] iGaging scales; the other two can be set to any voltage between 1V and about 18V. Although the firmware doesn’t [yet] support glass scales or tachometer input, this board is ready for them.
|3"x4" 1200 hole stripboard|
with a track cutting tool
Note: iGaging scales don’t require level shifting, and can be connected directly to the Launchpad. If you’re planning to use only one or two 1.5V scales, only half of this circuit is needed.
While designing the DRO controllers I try to stick with commonly available parts [as much as possible] that can be found at a Radio Shack. This time, though, I opted for a stripboard rather than the standard proto board. Unlike the standard proto board, stripboard has copper strips that run the whole length of the board. Although it’s a bit to find locally, the trouble of getting it online is more than offset by the ease of assembly.
|Unlike the standard prototyping board,|
a stripboard has strips of copper running for the whole width of the board
The table below list the required parts with links to online suppliers that sell them. The first three items are likely unavailable locally but the rest can be found at a Radio Shack.
|MSP430 LaunchPad with MSP430G2553||1||TI Store|
|3”x4” 1200 Hole Stripboard||1||Amazon|
|LM339 Comparator||2||Radio Shack or Amazon|
|10 Kohm resistor||4||Radio Shack or Amazon*|
|20 Kohm resistor||4||Radio Shack|
|4.7 Kohm resistor||8||Radio Shack|
|Battery Holder**||1||Radio Shack|
*Sparkfun Resistor Kit (sold by Amazon) covers all of the required values and then some.
**Any size between AA and D will do, so chose one that is common around your house.
|0.1 uF ceramic capacitor||3||Radio Shack|
|1000 uF or larger electrolytic capacitor*||1||Radio Shack|
* This is needed only for the 5V rail; if you don’t plan to use this with glass scales, it can be skipped
In addition to the parts listed above you will need some solid wire for the jumpers, such as this 24 AWG Pretinned Bus Wire.
|Spool of solid wire, good pair of cutters and needle-nose pliers|
make the job a bit easier.
Before you start the build lightly sand the copper side of the strip board with 400 grit sandpaper and wipe with alcohol. This will make soldering much easier and the solder will stick better.
The 1200 hole stripboard is just large enough to accommodate the circuit, without much wiggle room left. When installing the parts and jumper wires, please make sure they are in the locations shown in the pictures. I notched the upper-left corner of the board so its easier to see which way it’s oriented in the picture.
|I notched the upper-left corner |
so it's easier to see which way the board is turner
Step 1: Cut the traces as shown in the picture.
|All of the necessary cuts are done|
The easiest way to do so is to either use a special “circuit cutter” tool [as shown in the first picture of this post], or an old drill bit that is slightly larger than the trace. Simply insert it into the hole and twist until the trace is completely cut. Alternatively, you can use a sharp hobby knife to cut out a sliver of copper.
|Using an old drill bit or a specialized circuit cutter |
makes cutting traces much easier
Step 2: Install the ICs and Resistors
|ICs and resistors help locate the rest of the features|
|Different angle should help see the locations better|
Having the parts inserted will create “landmark” for jumper wires. The following three pictures show the locations of the parts. To keep the parts from falling out, bend the leads outward before cutting them. That way you can insert all of the components first and then solder them one swoop.
|Bend the lead outwards before cutting them, so the parts/wires wont fall out|
Please note, 10 Kohm resistors look like 100 Ohm, since the orange strip is almost brown on this batch. The resistor values in the picture above are as follows:
|Closer look at the pull-up and voltage splitter resistors|
- Three resistors in the upper left corner should be 20 Kohm (I used 22 Kohm since that’s what Radio Shack carries).
- Three resistors just below are 10 Kohm
- Eight resistors on the right are all 4.7 Kohm (5 Kohm will work just fine as well)
The three ceramic 0.1 uF bypass capacitors should help remove noise from the virtual ground lines. They are entirely optional, though.
|Optional bypass capacitors help to remove glitches on the virtual grounds|
Once the parts are inserted (and locations double-checked) they can be soldered. The bottom of the board should look like the picture below
|The bottom of the board will look similar to this |
once the components are soldered
Please note: on this particular board I’m using 4-pin Molex connectors for inputs and male/female 0.1” headers for power rails and outputs respectively. Those are completely optional, so you can use whatever connection scheme suits your needs.
Step 3: Install the Jumper Wires
|The next step is to solder the jump wires|
The easiest way to deal with the jumpers is to thread some wire through the holes (without cutting it off the spool), bend one end and then firmly pull on the other end. It will straighten out the jumper, reducing the risk of touching the neighboring jumpers. Once the jumper looks straight, bend the end and cut off the excess wire. The jumpers won’t fall off, so you can do all of them first, double check the result and then solder the whole thing at once.
|Gently pulling on the end of the jumper wire will straighten it out|
At this point the board is mostly done, except scales’ power supplies, virtual ground for the comparators and [optionally] clock for the iGaging scales. The board is designed to work with three different power supply voltages: the 3.3V required for the comparators can be shared with 3V scales; the other two rails can be any value between 1V and about 18V. Depending on your particular setup those connections will be different, so in the next post I will try to cover most common scenarios.