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 Din 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

2N5088 and the 1 k

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.