Hi,

this blog is connected with my recent one, where HM-TRP was discussed. Why? Well, I wanted to see data comming from HM-TRP on an LCD but as such it displays data only at 5V power supply voltage, while HM-TRP operates at max. 3.6 V power supply voltage.

Sollution for that problem was found at this web page, where author was generating low frequency with a microcontroller or an oscillator. I did try a microcontroller, but it has turned out that this sollution uses one pin, which in "pin critical" applications is not what I wanted. Read more if interested...

So I have used one Schmitt trigger NAND gate that comes packed in a small TSOT-23 5-pin package; just what I needed. I have connected to it one resistor and one capacitor and oscillator was ringing nicely.

Two diodes and two capacitors are rectifying and filtering AC voltage to a negative voltage that is sufficient for a 5V LCD to display data at 3.3 V power supply.

As you can see schematic is pretty simple, this module has sockets for 16x2 and 8x2 types of LCD, it has also a MOSFET that can be used to drive backlight with SW if needed.

That's how it looks LCD module from the soldering side.

and this is how it looks like from the component side. You will notice that it's prepared for the 16x2 LCD. The PCB can be ordered here.

I am very pleased with such a simple modification, thanks to ChaN and his web page that presented original idea.