siliconchip.com.au Australia’s electronics magazine April 2019 25MRVssVddLCK DSQ0Q1Q2Q3Q4Q5Q6Q7Q7'OE1 2 3 4 5 6 78912 11 14 13 1015
16SRCKIC1
7H 4 C5 95IC1
7H 4 C5 95MRVssVddLCK DSQ0Q1Q2Q3Q4Q5Q6Q7Q7'OE1 2 3 4 5 6 78912 11 14 13 1015
16SRCKIC2
7H 4 C595IC2
7H 4 C5951 2
3 41 2
3 41 2
3 41 2
1 3 42 34 57 616 815 14121310 11
9VCC1 VCC21,2EN3,4EN1A2A3A4A1Y2Y3Y4Y12 34 57 616 815 14121310 11
9VCC1 VCC21,2EN3,4EN1A2A3A4A1Y2Y3Y4Y12 34 57 616 815 14121310 11
9VCC1 VCC21,2EN3,4EN1A2A3A4A1Y2Y3Y4Y12 34 57 616 815 14121310 11
9VCC1 VCC21,2EN3,4EN1A2A3A4A1Y2Y3Y4YCON3
1000 mF+12V +12V
GND GND+12V +12V +12V +12V+3.3/5V +3.3/5V +3.3/5V +3.3/5VIC3 L293DIC3 L293DCON5IC4 L293DIC4 L293DCON6+3.3/5V +3.3/5V
GND GND GND GND
DIN
DOUT DINDOUT
LT LT LT LT
CK CK CK CK
EN EN EN EN33 mF 1kW+3.3/5VCON1 CON2CON4CON7IC5 L293DIC5 L293DCON8IC6 L293DIC6 L293DÓSC (^20 1 9) FLIPDOT DISPLAY DRIVER
P1 P2 P3
P4 P5
P7 P6
P8 P9
P10
P11
P12
P13 P14 P15
COM
TO CONNECT
WITH FURTHER
DRIVERS
TO 12V
POWER
SUPPLY
TO CONNECT
WITH MCU OR
PREVIOUS
DRIVER
end of all the coils, which are joined
together (common or COM).
So to flip a single pixel, the common
(COM) output goes either low or high,
and one of the other fifteen outputs
(P1-P15) is driven with the opposite
polarity. This causes current to flow
through that one coil in a direction
determined by the output polarities.
The direction of current flow de-
termines whether the coil produces a
North or South magnetic pole in prox-
imity to the permanent magnet.
The software needs to ensure that
only one coil is driven at a time, be-
cause all the coil currents return to the
same common driver pin. While this
pin may be capable of sourcing/sink-
ing enough current to flip more than
one pixel at a time, we’ve found it to
be a bit marginal, and it results in IC6
Fig.1: the circuit of the driver for one 3 x 5 pixel Flip-dot display. The control signals and logic supply from CON1 are fed
to IC1 & IC2, two 8-bit serial-to-parallel latch ICs. These drive the 16 control inputs of L293D dual H-bridge motor drivers
IC3-IC6. Here, they are driving 15 coils etched in a separate PCB, shown in Fig.2.
(which drives the COM pin) getting
rather hot. So our software flips one
pixel at a time.
To achieve this, all outputs are set
high or low, except for one, which is
set to the opposite polarity. Any out-
put that is set the same polarity as the
COM pin will cause no current to flow
through the connected coil. Only the
single coil that is driven with a differ-
ent polarity will receive current.
The instantaneous current require-
ment of the coils is around 1A with a
12V supply, which is above the con-
tinuous rating of the L293D. But the
coils only need to be pulsed briefly, so
the average current is much less than
the peak current. The microcontroller
pauses briefly between updating each
pixel, to keep the average current un-
der the thermal limit and to allow the
pixel time to finish its flip manoeuver.
Since the display holds its state with
no power applied, the circuit’s average
operating current is not usually terri-
bly high. Note that no more than two
of the four drivers on any IC should
be active at a time.
The enable pins of the four L293Ds
(pin 1 of IC3-IC6) are joined together
and held low by a 1kΩ pull-down re-
sistor, so that the default state of all the
outputs is off (high-impedance). It isn’t
until the microcontroller pulls the en-
able lines high, via pin 6 of CON1, that
IC3-IC6 are activated and that is only
done once the control data has been
shifted through IC1-IC2 and latched
at their outputs.
The enable pins are only pulled
high for 100ms at a time, to limit the
current pulse duration, as explained