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HEATER UNIT FOR LCD DISPLAY Disclosure Number: IPCOM000005808D
Original Publication Date: 1989-Aug-01
Included in the Prior Art Database: 2001-Nov-08
Document File: 4 page(s) / 146K

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Fook Teng Fam: AUTHOR


This heater unit is meant for use in communications handie talkie whose controller uses a micro-processor or micro-computer with built-in A/D Converter in it.

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MOTOROLA Technical Developments Volume 9 August 1989


by Fook Teng Fam

This heater unit is meant for use in communications handie talkie whose controller uses a micro-processor or micro-computer with built-in A/D Converter in it.

   This ideaof a heater unit struck me while I was working on a handie talkie with a LCD display to indicate the channel that it is operating on. At cold temperatures, I noticed that when I changed from one channel to another, the LCD display response was very slow in changing its channel status. At -30°C the display took about 3 seconds to response. At -10°C the response time was about 1.5 Sec. Furthermore at the temperature, the contrast of the display was very poor.

   The reason for the poor response at cold is because of the nature of the LCD liquid itself. Liquid displays work on the principle that if an electrical potential difference applied to the 2 plates containing the liquid, the liquid will be forced to rotate and align in such a way as to create a dark segment. See Fig. 1 for LCD operation details.

   How fast the crystals are able to rotate and align determines the response time of the LCD. At room temp. (25°C) the response time is about 30m sec. At cold temp the liquid becomes viscious and this makes the crystals movements slow and sluggish. The contrast ratio degrades because of the shift in the saturation voltage (Vsat), which is a property of the liquid crystal. See Fig. 2 for typical Vsat characteristics over temperatures.

   One way to tackle this problem is to incorporate a heater unit in the handie talkie. A handie talkie which has an in-built A/D Converter in the micro-processor (e.g. MC68HCll micro-computer) can be used as part of this simple heater unit.

   Figure 3 shows the schematic of this unit. Figure 4 shows how this unit could look like on the LCD. R4 is the heater element to heat up the LCD display and R2 is a thermistor which forms a voltage divider to the micro-processor.


The following sequence of events describes its operations:

1. The thermistor R2 which is placed near the LCD display will sense the temperature of the LCD and is tied to the Controller Board via line Jl. The thermistor will form a voltage divider with resistor Rl which is supplied by 5V from the Controller.

2. As the temperature changes the resistance of R2 will change. With this the voltage to PEO (the A/D input port) to the A/D Converter in the micro-computer will change and the port will detect the DCvoltage level. A software could be written in the micro-computer so that when port PEO senses adifferent DC level it will output pulses at port PA3 as indicated in Figure 5.

3. The width of the pulse W will be determined by thevoltage...