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Encoder and decoder applications

The radio frequency spectrum is filled with noise and other signals, especially those frequencies where unlicensed transmitter operation under FCC part 15 rules is allowed. When using a wireless remote control system it is desirable to have a way of filtering out or ignoring those unwanted signals to prevent false data from being received.

A simple way to accomplish this is to use an encoder IC at the transmitter and a decoder IC at the receiver. The encoder  generates serial codes that are automatically sent three times and must be received at least twice before data is accepted as valid by the decoder circuit.

In the early days of  "radio control", before these coding ICs were available, radio controlled garage doors sometimes opened themselves when they received transmissions from a plane passing overhead or a two-way radio operating in the area. Encoding and decoding is now used in most wireless control systems to prevent this type of interference.

A GL-104 IC in an 18 pin DIP package available from Glolab can be used as an either an encoder or decoder just by changing the connection to one pin. These devices also offer more flexibility than the usual encoder and decoder ICs. One GL-104 that is configured as an encoder is needed for each transmitter and one that is configured as a decoder for each receiver. You will also need one 4 MHz ceramic resonator (CR) and one voltage detector/reset (VDR) device for use with each GL-104.  The GL-104 will be used in the following application examples. These devices have four data channels. See GL-104 for a description of pin functions.

 

Active low push button transmitter

Figures 7 is a complete push button transmitter that operates in the input active low mode which means that the circuit becomes active and transmits data when any data input is connected low (to Vss).

gl-104-7.jpg (30131 bytes)

The circuit operates from a 9 volt battery through a 5 volt low dropout micropower regulator and uses only 3 microamperes of current when no buttons are being pressed. This design needs no resistors and only one capacitor. All four address pins 2-3, 17-18 are shown connected to Vss but addresses may be changed by connecting some or all to Vdd. If only one transmitter - receiver pair will be used in an area, all pins can be as shown.

See figures 10 and 11 below for receiver application circuits. Pressing transmitter button 0 will activate the valid data pin in a receiver. Pressing 1-4 will activate valid data and one or more data out pins in a receiver. If the receiver has momentary outputs its data pins will remain active for as long as a transmitter button is pressed. If the receiver has latched outputs valid data output will be momentary but data out pins will latch on. If multiple buttons are pressed then multiple receiver outputs will latch. Latched outputs can be reset by pressing transmitter button 0.

A 5 volt low dropout micropower regulator such as the Telcom TC55 allows operation from a 9 volt battery while maintaining very low standby current.

 

Active high push button transmitter

Figures 8 is a complete push button transmitter that operates in the active high input mode. In this mode data is preset by DIP switches or other means and is transmitted only when transmit enable is connected high (Vdd).

gl-104-8.jpg (30872 bytes)

This design also uses only three microamperes of standby current even though it has pulldown resistors on its input pins to allow the use of single throw DIP switches to set the input data pattern. This low standby current is achieved by connecting the pulldown resistors to pin 1 which goes high during standby and pulls down to Vss only when the circuit is active. All address pins 2-3, 17-18 are shown connected to Vss but addresses may be changed by connecting some or all to Vdd. Input data is set by DIP or other switches and the data is transmitted when the transmit enable button is pressed.

 

Latched output receiver

Figures 10 is a complete receiver circuit having cumulatively latched outputs.

 gl-104-10.jpg (26308 bytes)

LEDs can be driven directly from this decoder since it is capable of sourcing up to 25 milliamperes at each output. The LEDs can be latched on individually or simultaneously from one transmitter or individually from up to four transmitters. They will remain latched on until remotely reset by an all zero (low) transmission or until they are reset by the reset button at the receiver.

This is useful to record up to four separate events that occur while the LED display is not being viewed. An application example is a transmitter located at a mailbox to signal that the postman has opened the box. A Piezo buzzer sounds an alarm whenever data is being received to alert someone that an event is occurring. The responsible transmitter or transmitters can then be identified by the LEDs that are on. The LEDs can be replaced by solid state relays to control 120 volt AC loads such as a coffee pot, toaster and radio,

All address pins 2-3, 17-18 are shown connected to Vss but addresses may be changed by connecting some or all to Vdd. Power is supplied by a 12 volt DC wall transformer and regulated down to 5 volts by a 7805 regulator.

 

Momentary output receiver

Figures 11 is a complete receiver circuit having momentary outputs. 

gl-104-11.jpg (31187 bytes)

The decoder drives IRL510 power FETs in TO220 packages that can sink 5 amperes to power relays or loads directly. Higher power FETs may be used for heavier loads if desired. The momentary function can be used with the transmitter in figure 7 above for wireless control of machinery or any device that has to be energized while a remote control button is being pressed. Output 0 will be on whenever any other data pin is on. The other outputs may be energized either individually or simultaneously.

All address pins 2-3, 17-18 are shown connected to Vss but addresses may be changed by connecting some or all to Vdd. Power is supplied by a 12 volt DC wall transformer and regulated down to 5 volts by a 7805 regulator.

 

These encoder/decoder devices do not provide error correction. They just accept valid data and reject corrupted data. This means that a control signal that is corrupted will not be processed through the decoder and must be sent again.

See GL-104 for more data on the encoder/decoder or

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Updated August
2013