LCR-T4 Transistor Tester Display Upgrade

I ended up with a  transistor tester lcr t4 with a damaged LCD display.  We'll try to replace the display with a 1.8" 128x160 TFT color LCD (which can be purchased for about $4).  We used a version of the display previously.  This version has a different (better) pinouts that separates out the SD card connections.  The display uses ST7735S controller chip that is supported by the tester software.

The first issue is the supply voltage.  The tester uses 5V and the display uses 3V.  The display has a regulator to generate 3V from 5V, but the I/Os have to be compatible.  There is no evidence that the input signals to the display are 5V tolerant.  So we'll insert 1-2K resistors in series to limit the current to about 1mA.  Then these is the issue of the backlight.  The display has the cathode grounded, incompatible with the tester connections.  The backlight LED has a forward voltage about 3V@20mA; the tester circuit seems to operate at about 5mA (2.6V).  We'll drive it with a 330-Ohm current limiting resistor from 5V.  The tester is turned on with a push of a button, but kept on with the current through the backlight LED.  So we'll insert a 4.7K resistor at the original LED connection, which would provide about 1mA base current.

We traced the tester's LCD pads,

• SCK - PD2
• SDA - PD3
• A0 - PD1
• RESET - PD0
• CS - GND

1.8" TFT 128x160 LCD Module

LCR-T4 with 1.8" LCD

We updated the firmware.  The Flash is very close to full.  It might be hard to add any new features.

AVR Memory Usage
----------------
Device: atmega328p
Program:   32060 bytes (97.8% Full)
(.text + .data + .bootloader)
Data:        202 bytes (9.9% Full)
(.data + .bss + .noinit)
EEPROM:      882 bytes (86.1% Full)
(.eeprom)

When operating, the tester draws about 23mA.  It works to about 7V.