#025 Sherman Firefly

Practicing finishing techniques on the Airfix 1:72 Sherman Firefly, then animating the turret with some ATtiny85 magic.

Build

Here’s a demo..

Notes

I started the kit as a quick build to practice some weathering techniques (it is great for that) but then .. scope creep: animate the turret!

The Kit

The Sherman Firefly Vc Airfix No. A02341 1:72 is a nice little weekender - can be built in a few hours (depending on options chosen) and then spend as long as one likes finishing it;-)

kit_parts

Paint

I’m going to base the finish around scheme A from the kit - but without the unconfirmed black camo pattern. I’ll be using this kit as a “finishing techniques” test subject, so everything after the initial base coat will start to go its own way!

I am using Uncle Night Shift build series on the Char B1 bis as a perfect tutorial for the techniques I’ll be trying/learning here.

kit_scheme_a

I’m using the Vallejo Model Color equivalents:

Humbrol	Vallejo
155	70.887 US Olive Drab
33	70.950 Black
53	70.863 Gunmetal Grey
110	70.875 Beige Brown

The Build

I kept the build as simple as possible and went with the pre-molded tracks instead of the link-and-length option also included in the kit. This saves a lot of time in the build - interested to see if they hold up to final scrutiny.

build_01

Primed in black so I don’t need to worry too much about unpainted nooks and crannies.

build_02_primed

Made some progress on the finishing - base coat, some color modulation by mixing degrees of 70.917 Beige, 70.953 Flat Yellow and/or 70.989 Sky Grey.

Panel line highlighting with Tamiya Black Panel Line Accent Color.

build_03a

build_03b

ready for next round of effects - some oils, dust and chipping

build_04a build_04b build_04c

Time for some Animation

The weathering is going well, but now I’m thinking: how about an animated turret?

Circuit Design

To be packed inside the body, with only a 5V power supply (through the bottom of the hull):

ATtiny85 microcontroller, programmed with the Firefly sketch
an electret microphone and TL072 opamp to amplify the incoming sound
a resistor/diode pair, used to set the sound threshold voltage to around 0.4V (an input sound above that level will trigger the animation)
a “firing pin” GPIO out - am still thinking about finding/making a suitable cannon-fire sound effect, however for now it just blinks an LED for diagnostic purposes
a Micro Linear Servo GS-1502, used to rotate the turret

Breadboard

Schematic

Testing the circuit on a breadboard:

Breadboard Build

About the Program

The ATtiny85 is configured to run at 8MHz, and programmed with the Firefly sketch. The sketch:

sets up an interrupt handler on the analog comparator:
- when voltage in from the electret microphone goes above the diode-regulated reference voltage of about 0.4V, it triggers the ANA_COMP_vect
- ANA_COMP_vect sets a “triggered” flag
the main look checks for the “triggered” flag, an initiates the animation if fired.
the animate traverses the turret by driving a linear servo using the SoftwareServo library
- it provides some “realistic” behaviour by stepping through a series of animation sequences
- the animations sequences basically define a series of waypoints (degrees of arc) and how fast to traverse to the waypoint (in degrees per increment)
- it could randomise the animation but I haven’t done that yet

Animated Turret Construction

After confirming the circuit on a breadboard, I transferred to a small piece of protoboard:

build_05b_module

And inserted into the hull. the power connection is provided by two pins that poke through the bottom of the hull. Since the circuit runs fine on 5V, I’m using a USB adapter for power over USB cable.

build_05a_electronics_inside