Silicon Chip – April 2019

26 Silicon chip Australia’s electronics magazine siliconchip.com.au

1 2

3 4

1 2

3 4

1 2

3 4

1 2

3 4

CON5

CON6

CON7

CON8

P1 COIL

P4 COIL

P7 COIL

P 10 COIL

P 13 COIL

P2 COIL

P5 COIL

P8 COIL

P11 COIL

P14 COIL

P3 COIL

P6 COIL

P9 COIL

P12 COIL

P15 COIL

COM

COM

COM COM

COM COM

ÓSC 20 1 9 FLIPDOT COIL PCB CIRCUIT

P1 P2 P3

P4 P5

P6

P7

P8 P9

P10

P11

P12

P13 P14 P15

ALL COILS ARE COMPOSED

OF TRACKS ON THE PCB

Fig.2: the fifteen coils on this PCB are driven by the circuit of Fig.1 and either

attract or repel permanent rare-earth magnets mounted in pixel flaps on top of

them. Because those rare-earth magnets have a North pole on one side and a

South pole on the other side, depending on the direction of current flow

through a coil, the flap flips to one side or the other, exposing a different colour.

above. Due to this relatively long drive
time, the extra time taken to shift con-
trol data from the micro through IC1-
IC2 is negligible.
As required by the L293D, the logic
ground and power ground are com-
mon. Separate connections for 12V
power and 3.3V/5V logic supply are
available, via CON3 and CON1 re-
spectively.

Construction

Being a mechanical design with
moving parts, a fair degree of preci-
sion in the construction is required
to ensure proper operation. The pri-
mary requirement is that all the parts
are put together squarely and lined up
correctly before fixing them in place.
The first step is to glue the magnets
in the pixel flaps. We highly recom-
mend that the flaps be left in the PCB
frame during this step, to avoid pieces
getting lost. The flaps are spread out
enough that interaction between the
magnets is minimal.
We do this step first to allow time for
the glue to cure. We used epoxy resin
as it has a bit of resilience and is quite
strong; cyanoacrylate-type glue (super-
glue) is probably too brittle and might
causing the magnets to come loose af-
ter some use.

To make this process easier, you

need a disposable, flat plastic surface.

The lid from an ice-cream tub or take-

away container is ideal, as epoxy will

not stick to this. Another helpful item

is a flat sheet of ferrous material (some-

thing that a magnet would stick to,

such as plain steel). This can be used

to help hold the magnets in place. We

used a steel case, but you could also

use the lid of a Milo tin.

Place the ice-cream tub or takeaway

lid over the ferrous material, then sit

Fig.3: this PCB can be cut

apart into eight separate

frame pieces - enough to

make one 3 x 5 pixel flip-

dot display with two pieces

left over. The holes form the

‘hinges’ for the pixel flaps

to rotate about, while the

exposed copper is soldered

to the coil PCB to hold the

frame in place. Cut carefully

where shown using a side-

cutter to separate the pieces.

The frame pieces are quite

thin and could be damaged if

handled roughly.

the PCB frame on this. Once you in-

sert the magnets in their holes, they

should be held in place by their attrac-

tion to the steel, but the ice cream lid

will allow them to be removed with-

out too much force. The most critical

point of this step is that all the mag-

nets’ poles line up.

To achieve this, take the stack of

magnets (they’ll form into a stack of

their own accord), and push the mag-

net at the end of the stack into one of

the holes in the pixels. Then detach it

from the stack by sliding the stack to

the side, leaving a single magnet sit-

ting in the hole. The PCBs are 1.6mm

thick, so the magnets should sit just

below the surface of the PCB.

You will see that there are 16 pixel

flaps in the frame, but we only need

15, so there is a spare if needed.

Then repeat for the other 14 or 15 pix-

els, without changing the orientation of

the stack. When you’ve finished, you

may want to check the magnetic polarity

by moving another magnet nearby (but

not so close that it pulls them out). You

should feel that all the magnets are at-

tracted to the magnet in your hand with-

out changing its orientation.

Mix up a small amount of epoxy res-

in, and apply a film to the top of each

magnet in its hole. Try to work it down

the sides if possible. The rough edges

of the PCB will provide good purchase

on the glue. Finally, wipe down any

excess. Any extra glue may foul and

unbalance the mechanism.

You should also ensure that the PCB

panel is still flush with the plastic be-

low, as if it is sitting up, the magnets

may end up protruding slightly.

Allow the resin to harden. We rec-

19111183 Flipdot Display Pixel Frame

(2)

(3)

(4)

(6)

(7)

(8)

(5)

(1)

ÓSC^20 1 9