Sunday, January 13, 2019

T19 Build Guide - Part 7 - Blaster Controller

Previous: Part 6 - Drivetrain Mechanical

This post covers assembling the backplane of the standard Core-compatible blaster controller on perfboard for the T19 application.


  • Soldering iron
  • Flush cutter
  • Needlenose pliers
  • Knife
  • Wire stripper (or use knife)
  • Helping hands fixture. Optional, but makes this a whole lot less tedious.
  • Vector 8029 2.0x3.0" perfboard (Digikey part V2025-ND)
  • 220uF 50V low-ESR electrolytic capacitor (I use Kemet ESY series, Digikey part 399-6123-ND)
  • 22uF 50V electrolytic capacitor
  • Male and female breakaway 0.1" headers see below for pin counts
  • Insulated ~26AWG wire for board wiring
  • Solid uninsulated ~26AWG wire/buss wire (I use wire from scrap Cat5 cable for both this and the previous)
  • 1kOhm resistor x 3
  • 100kOhm resistor x 3
  • Arduino Pro Mini microcontroller development board - ATmega328/328P, 5V, 16MHz version - first source is Sparkfun part DEV-11113 or use generics. I use generics. (Be sure you also have the appropriate serial gear to flash these, such as Sparkfun part DEV-09716 FTDI breakout board)
  • TI DRV8825 2-phase bipolar stepper motor driver IC carrier board - these are a standard, 3D printer style card. First source is Pololu or use generics. I use and have always used generics.
  • Leadfree solder
  • Flux



DRV8825 card.

Processor card.


Not really much to it eh? Let's look at connectors now. Every low-current connector in a DZI blaster is a piece of breakaway 0.1" header, cut to length. This makes things simple - you have exactly 2 parts to order for all connectors. You have been seeing them in past posts as mechatronic things get wired and terminated with them; now here's what all those plug into.

2 x 12 pin female (processor card socket)
2 x 8 pin female (bolt motor driver socket)
1 x 4 pin male (power connector)
2 x 3 pin male (flywheel throttle connectors)
1 x 4 pin female (bolt motor connector)
2 x 3 pin female (trigger connector, bolt limit switch connector)

A generic APM board will generally come with a set of headers:

If not, you need 2 more 12-pin males for the host plug-in pins and a 6-pin right angle male for the flash connector. Here's how you need to assemble this:

Apply flux. Insert. Make sure straight. Solder the end pins first to tack in place, then march on down the rest of the pins.

DRV8825 cards may or may not come preassembled with headers, if not add 2, 8-pin males to it with the IC/component side facing up.

Time to get after it. First refer to the visual schematic from earlier:

This, and the above parts (further reading: Core source, DRV8825 carrier pinout, and Arduino Pro Mini pinout), should help you get your mind around what's what, and start mentally routing things. There is no specific way to lay this out though, and it doesn't really matter. The above image does NOT attempt to suggest layout, connections just have to solidly exist, and this is not a demanding application so as long as a wire is solidly there somewhere, it shall work. My big hints are to align the DRV8825 and APM offset by one pin as so and place them either immediately adjacent or spaced one click apart:

I arbitrarily decide that the cards mount transversely, and that the 8825 card is in front of the blaster.

Important consideration is to allow room (generally UNDER the APM card, as I do it) for the bolt limit switch connector to be in the right position. This connector must be on the BACKSIDE of the board and stick through the access hole in the drive housing, where the bolt limit switch will plug in - so hold up your board and mark where that is. Position the cards accordingly.

Then the other majors:

  • The 4 pin power connector. I put this near the end of the 8825 card. If we look into the drive housing with the muzzle facing left, this is top left on the board.
    • The pinout of the power connector is, from left to right on this one: DC bus, ground, ground, 5V. (Mark it on the board.)
  • The logic power capacitor (22uF) goes near that connector, so as to conveniently connect between 5V and ground.
  • The 220uF low-Z capacitor is the DC link cap for your bolt drive. It will go between DC bus and ground and needs to go as close to the switching devices as possible. The Vmotor pin on the 8825 card is down at the other end next to a ground pin, so place it very near there.
  • The flywheel throttle output connectors: Place these near APM pin 10, which is going to drive pin 1 (top) of both. Pin 2 is non-connected (We don't use BEC outputs for anything). Pin 3, bottom, is ground.
  • The trigger connector: Place this near APM pins 11 and 12, which are the inputs for it. Leave room for wiring the 1k pullups to 5V and 100k input protection resistors between the MCU pin and that node.
Here's what you have after soldering down all the headers:

Note that limit switch connector out the back.

Start adding caps. Observe polarity on electrolytic caps.


N.b. I'm using MG Chemicals SAC305 solder that cools with a matte surface and I haven't cleaned up flux residue, pardon the appearances

First wire example: Power connector DC bus pin, down to DRV8825 Vmotor pin and 220uF capacitor. Run on the topside, soldered on the bottomside

And keep going ad nauseam - use uninsulated tinned wire to solder between close pins (like the adjacent DRV8825 and APM control interface pins, and the DRV8825 and motor connector), use insulated to make longer runs. Starting with running the ground and Vcc to everything that needs those is a good way to go.

Here's after all the wiring is completed and pullup and input protection resistors are added. This is a finished backplane.

The backsides of these, are either something to be proud of that I can do reliably and find fun, or something to be absolutely disgusted that I have done, but either way, it's like we went back to the 1940s for a moment:

NOW IS THE TIME TO CHECK EVERY CONNECTION, and to check between solder joints with a BRIGHT LIGHT and a magnifier for solder blobs, bridges, stray metal or anything that could result in a mishap. Make sure all solder joints are SOLID. Fix anything needing it NOW.

All good? Righty allmighty, let's just forget about that scary shit on the back of that board, flip it around and stick the cards in it...

See? All better.

Test fit.

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