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Holtek HT12E and HT12D
The HT12E encodes the 12 bit data on it's 12 data input lines (A0 to A11), and then serially transmits it when the transmitt enable pin is taken low. The data output appears on the D-OUT pin. The data is transmitted four times in succession. The data consists of differing length positive going pulses for 1 and 0, the pulse for 0 being twice the width of the pulse for 1.

The HT12D receives the data from the HT12E on its D-IN pin. If the data received matches the levels on the A0 to A7 pins four times in succession, the valid transmission (VT) pin is taken high. The data on pins A8 to A11 of the HT12E appears on pins D0 to D3 of the HT12D. Thus the device acts a receiver of 4 bit data (16 possible codes) with 8 bit addressing (256 possible channels).

The third device in this family (not used in this design) is the HT12F. This is similar to the HT12D, except that the levels on all 12 data pins must match those on the HT12E for the VT line to be taken high. This device acts as a single channel receiver with a 12 bit address (4096 possible channels).

The devices contain an internal clock, the frequency of which is set by a single external resistor connected between the OSC1 and OSC2 pins. The frequency of the receiver (HT12D/HT12F) clock should be approximately 50 times the transmitter (HT12E) clock. The relationship of frequency to resistance is not linear, and also depends upon the supply voltage. The clock resistor values used in this design were obtained with reference to the graphs in the device data sheets.

The devices operate over a supply voltage range of 2.4V to 12V, with a typical standby current of 0.1uA at 5V and 1uA at 10V. The operating current is under 1mA.

Transmission Problems
The Infra-red transmission medium works best with very short pulses, and is ideally suited to coding systems that rely on the presence or relative timing of pulses. This is the type of coding used in most commercial pieces of equipment, as well as by the obsolete GEC-Plessey devices mentioned earlier.

Infra-red does not lend itself so well to pulse width based systems such as the Holtek devices used in this design. This is partly because the transmitting LEDs must be operated at currents of around 1.5A to give a reasonable range, and this sort of current can only be sustained for very brief periods to avoid damaging the LEDs and to keep the average current consumption to a reasonable figure. Also the receiving photo-diode and amplifier IC are designed for very short pulses, to prevent interference from lower frequency light variations such as mains fluorescent lighting at 50Hz.

In this design the variable width pulses from the HT12E are used to control a higher frequency pulsing circuit which in turn drives the LEDs. At the receiving end the groups of pulses are recombined to give the variable width pulses required by the HT12D.

This system works fairly well in practice although, as mentioned earlier, the range may not be as great as some commercially made remote control systems. The prototype operates reliably up to about 5 metres (16 feet), which should be ample for an average sized room.

Remote Control Handset Circuit Operation
The handset circuit diagram is shown in figure 1. The HT12E data lines A0 to A7 are taken high. This is because the IC outputs shorter pulses for a logic 1 than for a logic 0. Thus the output LEDs are lit for less time, giving a reduction in current consumption from the battery.

If this coding causes problems with other pieces of equipment, some of the lines may be taken low by cutting the appropriate PCB track and linking pins 8 and 9 of the IC. Obviously the tracking on the motherboard must be modified in a similar manner. The PCB tracking of both boards have been designed to make this modification easy.

The remaining four data lines are connected to the four channel push buttons, so that one line goes low when the appropriate button is pressed. The four push buttons are also gated to the active low transmit enable (TE) line via diodes D1 to D4 and R6.

U2:A (4093), and surrounding components form an oscillator running at about 30KHz. This oscillator only operates while one of the push buttons is held, due to U2:B. D5 and R7 modify the mark-space ratio such that the output on pin 3 is low for about 20% of the time. This is inverted by U2:D and then gated with the data output (D-OUT) from U1 by U2:C.

The output LEDs (D6, D7 and D8) are driven by Q2, which is in turn driven by Q1. R13 limits the LED current to about 1.5A, although the value could be reduced at the expense of battery life if extra range is needed. C2 provides the power for these brief high current pulses, while D9 protects the circuit if the battery is connected incorrectly.