MC145026•MC145027•MC145028•SC41343•SC41344 MOTOROLA
16
APPLICATIONS INFORMATION
INFRARED TRANSMITTER
In Figure 18, the MC145026 encoder is set to run at an os-
cillator frequency of about 4 to 9 kHz. Thus, the time required
for a complete two–word encoding sequence is about 20 to
40 ms. The data output from the encoder gates an RC oscilla-
tor running at 50 kHz; the oscillator shown starts rapidly
enough to be used in this application. When the “send” button
is not depressed, both the MC145026 and oscillator are in a
low–power standby state. The RC oscillator has to be
trimmed for 50 kHz and has some drawbacks for frequency
stability. A superior system uses a ceramic resonator oscilla-
tor running at 400 kHz. This oscillator feeds a divider as
shown in Figure 19. The unused inputs of the MC14011UB
must be grounded.
The MLED81 IRED is driven with the 50 kHz square wave
at about 200 to 300 mA to generate the carrier. If desired, two
IREDs wired in series can be used (see Application Note
AN1016 for more information). The bipolar IRED switch,
shown in Figure 18, offers two advantages over a FET. First,
a logic FET has too much gate capacitance for the
MC14011UB to drive without waveform distortion. Second,
the bipolar drive permits lower supply voltages, which are an
advantage in portable battery–powered applications.
The configuration shown in Figure 18 operates over a
supply range of 4.5 to 18 V. A low–voltage system which
operates down to 2.5 V could be realized if the oscillator sec-
tion of a MC74HC4060 is used in place of the MC14011UB.
The data output of the MC145026 is inverted and fed to the
RESET pin of the MC74HC4060. Alternately, the
MC74HCU04 could be used for the oscillator.
Information on the MC14011UB is in book number
DL131/D. The MC74HCU04 and MC74HC4060 are found in
book number DL129/D.
INFRARED RECEIVER
The receiver in Figure 20 couples an IR–sensitive diode to
input preamp A1, followed by band–pass amplifier A2 with a
gain of about 10. Limiting stage A3 follows, with an output of
about 800 mV p–p. The limited 50 kHz burst is detected by
comparator A4 that passes only positive pulses, and peak–
detected and filtered by a diode/RC network to extract the
data envelope from the burst. Comparator A5 boosts the sig-
nal to logic levels compatible with the MC145027/28 data
input. The D
in
pin of these decoders is a standard CMOS
high–impedance input which must
not
be allowed to float.
Therefore, direct coupling from A5 to the decoder input is
utilized.
Shielding should be used on at least A1 and A2, with good
ground and high–sensitivity circuit layout techniques applied.
For operation with supplies higher than + 5 V, limiter A4’s
positive output swing needs to be limited to 3 to 5 V. This is
accomplished via adding a zener diode in the negative feed-
back path, thus avoiding excessive system noise. The bias-
ing resistor stack should be adjusted such that V3 is 1.25 to
1.5 V.
This system works up to a range of about 10 meters. The
gains of the system may be adjusted to suit the individual
design needs. The 100
Ω resistor in the emitter of the first
2N5088 and the 1 k
Ω resistor feeding A2 may be altered if
different gain is required. In general, more gain does not nec-
essarily result in increased range. This is due to noise floor
limitations. The designer should increase transmitter power
and/or increase receiver aperature with Fresnal lensing to
greatly improve range. See Application Note AN1016 for
additional information.
Information on the MC34074 is in data book DL128/D.
TRINARY SWITCH MANUFACTURERS
Midland Ross–Electronic Connector Div.
Greyhill
Augat/Alcoswitch
Aries Electronics
The above companies may not have the switches in a DIP.
For more information, call them or consult
eem Electronic En-
gineers Master Catalog
or the
Gold Book
.
Ask for SPDT with
center OFF.
Alternative: An SPST can be placed in series between a
SPDT and the Encoder or Decoder to achieve trinary action.
Motorola cannot recommend one supplier over another
and in no way suggests that this is a complete listing of trinary
switch manufacturers.
