Remote Control using IR codec chip ST12

Here to make a IR remote control I am using an ASIC codec chip ST12. Codec means coder+ decoder. That means same chip is used as encoder at transmitter side and as decoder at receiver side. It’s very nice chip and some of the major features of the chip are:

Encode / Decode on single chip
Built in Oscillator
Minimum External Components
Wide operating voltage range. (2.0 – 5.5V)
Single chip Encoding Decoding Mode
40kHz carrier for infrared transmission medium
18 pin DIP package

The major applications of this chip

Burglar alarm system
Smoke and fire alarm system
Garage door controllers
Car door controllers
Car alarm system
Security system
Cordless telephones
Other remote control systems

Because both transmitter and receiver made up of only ST12 codec chip, let us first understand pin diagram, pin function and working of this chip and then go for transmitter and receiver.

Pin diagram: –

ST 112

As shown in above figure, it is 18 pin chips with DIP. Pals refer the table given below for details

Pin no	Pin name	Description	Function
1 to 4	A0 – A3	address inputs	to setup an address for the system. each pin can either be connected to 0V or Vcc. cannot be left unconnected
5	gnd	0v input	connected to circuit ground
6 to 13	D0 – D7	data pins	at transmitter side in encoder mode works as input (connected with switches) and at receiver side in decoder mode works as output (connected to external circuit)
14	Vcc	+V input	connected with +V of 2 to 5 V
15	ENC-DEC	encoder or decoder	High or low logic on this pin decides its operation as encoder or decoder. if connected to Vcc works as encoder and if connected with Gnd works as decoder
16	Data Tx-Rx	transmit or receive data	in encoder mode it transmit data and in decoder mode receives data
17	Lat – Mom	latching or momentary	In decoder mode the output will be either latching or just momentary. if connected with Vcc output will toggle every time and if tied to ground a low-high-low pulse on pins
18	IR – RF	infra red or radio freq	selects transmission via either IR or RF. if tied to Vcc transmits 40 KHz signals via IR led and if tied to Gnd sends data via suitable RF transmitter

So now after going through above table one should have complete idea about the operation of this chip as transmitter (encoder) and as receiver (decoder).

Transmitter & Receiver

Transmitter: – As written above, the major advantage of this chip is that there is least requirement of external components

As shown in figure Circuit Diagram Tab1, it’s a single chip remote control with very few external components like switches, resistors, transistors and LEDs.

Connection: – all four address bits A0 – A3 are shorted to ground to setup an address 0000. 8 push button type switches are connected with data pins D0 – D7 as shown. The transmit data pin drives IR LED and red LED through Darlington configuration of Q1 & Q2. IR-RF pin is connected to Vcc to configure the chip in IR mode. enc-dec pin is also connected with Vcc to configure it as transmitter. A 3V battery (or 2 dry cells) provides power to the circuit.

Operation: – when any of the key is pressed data is transmitted via transmit data pin at 40KHz freq through IR LED. The address and data together modulated at 40 KHz freq with the help of built in oscillator. This data transmission can be seen on red LED as blinking o/p.

Receiver: –

As shown in Circuit Diagram Tab2 along with St12 we need current driver chip uLN2003A to drive relay coil.

Connections: – All four address bits A0-A3 are connected to ground to setup same address 0000. Keeping first data o/p D0 unconnected, rest all D1 – D7 are connected to input of ULN chip. Each one output of ULN chip drives one relay and one LED. So total there are seven relays that can switch on / off 7 different devices (instead of ULN2003 if one uses ULN2803 then all 8 data outputs can be used). The enc-dec pin is connected to ground to configure the chip as receiver. The lat-mom pin is tied to Vcc to get toggle output every time. The output of IR sensor TSOP 1738 is connected to receive data pin of ST12. Two different regulated supplies 5V and 12 V are used to give power to complete circuit.

Operation: – As a 40 KHz burst of data and address is detected by TSOP it will demodulate it and gives address and data to decoder chip. On getting valid address and data st12 will latch the data on D0-D7(inverted). Because its toggle output every time when same switch is pressed from TX, the relay will switch on / off. Also relay on / off is indicated by respective LED.