Case Study – Replacement Character LCD Modules
Designing and building an equivalent LCD module to replace a discontinued 40×2 character display.
We received a call asking us to design a replacement LCD for a 40×2 character display
The customer was purchasing a 40×2 character display from the same supplier for 10+ years, when that supplier discontinued the LCD. The customer had estimated the total number of displays needed to complete all future production orders and placed a last time buy, but their sales increased, and their inventory ran out.
The customer planned to continue production on this unit for five more years and then support repairs for an additional five years. There were too many units in the field, and they couldn’t redesign their product for a different display. They needed a 100%, equivalent, drop in LCD replacement.
Not only did the replacement LCD have to match the dimensions of their previous unit, but their current firmware had to operate on the equivalent display.
40×2 FSTN, Positive Mode, LED backlight
- The displays had to be drop-in compatible for future production and for repairs of past units.
- They could not modify their firmware/software, so the controller/drivers had to be compatible.
- Replacement LCD controllers are not 100% compatible with each other.
- Their EAU was 500 displays; the glass supplier would need higher quantities requiring someone to hold their inventory.
- On the previous LCD, the customer was bending a standard header to fit into the backlight mounting holes. They asked if the hole pattern on the PCB could be modified to provide the correct spacing for a standard header.
The LCD glass and bezel had to be custom built to match the discontinued displays.
The ITO (Indium tin oxide) glass was cut to the exact dimensions of the previous LCD.
A mold had to be built to form the bezel; and the color of the previous bezel had to be matched. It is possible to order the bezel in several different colors, including: silver, grey and black (most popular)
Metal Bezel that holds the LCD glass to the PCB
The PCB (Printed Circuit Board) was redesigned to allow a standard header to be placed into the board without being bent. Also, the four mounting holes were placed to match the customer’s current mounting bracket.
Below is a photo showing the controller drivers used to match the customer’s firm ware and the new spacing (2.54mm) to match the standard header in use by the customer.
- Drawings were completed in four business days
- Samples delivered in seven weeks
- Production units completed in eight weeks after sample approval
The customer was using a STN (Super Twisted Nematic) type fluid; it was recommend upgrading this to a FSTN (Film Super Twisted Nematic) to allow a sharper contrast and wider viewing angle.
Below is a breakdown of the different types of fluids.
TN Fluid Type
Twisted Nematic fluid is the least expensive and has the narrowest viewing angle.
STN Fluid Type
Super Twisted Nematic fluid is more expensive than TN and offers a wider viewing angle than TN.
FSTN Fluid Type
Film Super Twisted Nematic fluid is more expensive than STN and offers a sharper contrast than TN and STN.
Half-Life of the Backlight
The customer’s previous LCD was using an EL (Electro Luminescent) type of backlight. EL backlights are thinner than LED (light-emitting diode) backlights and provide a more uniform discernment of light, but they have a much shorter half-life.
Note: Half-life is the amount of time in hours before the display or backlight is ½ (50%) as bright as when it was first turned on. This is not when the backlight burns out.
EL backlights maintain an estimated half-life of 3K hours Vs. a LED backlight which averages a half-life of 50K to 70K.
El backlights are driven by AC, where LED backlights are DC driven. It was not possible for the customer to switch their product from AC to DC, so the replacement display required an EL backlight.
Future designs of this product may switch to a DC backlight.