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I asserted the enable signal and nothing dataseet. So the measurement works and it seems that the 8 microamps given in the data sheet are just a very conservative worst case. The LM is inexpensive and I had quite a few left from another project so I used 74c126 without giving it much thought. I expected to tweak those values a bit but found that they work so well that I just leave them as they are. Very low standby power consumption was one of the key design goals.

And it was not very elegant that the display was powered off by just cutting the entire power supply. The latest version uses a 4 lines x 20 characters LCD that connects via I2C as well as a rotary datzsheet with a push button.

(PDF) 74HC126 Datasheet download

Add a dual I2C digipot to control both the reset signal as well as the backlight. The error was soon found. Turning on the backlight basically worked as intended but the load was a bit much for the 74HC Nothing was powered on. And a time constant of a few milliseconds is totally sufficient so you might end up with something like 10ms of delay. Use the previous backlight PWM signal as an enable signal.

While it can supply up to 35mA according to the data sheet, its output voltage is nowhere near its positive supply rail when it supplies that much current. The logic is pretty much the same here except that I use different non-inverting gates here. So you face the challenge of debouncing the signal while still letting relatively fast transitions pass. The mistake was corrected by cutting the ground connection and conecting the respective pin to the enable singal itself which was available from the pin right next to it.


In that state the user interface must not use more than a few microamps.

As long as the push button is not pressed, no power flows there so the only power consuming device is the 74HC When in low-power mode only the push button pull-up and the 74HC is supplied with power. That will introduce some delay in the output signal but as a rule of thumb anything below 50ms is not noticable. The reset signal needs to be pulled low for 10ms. My final solution was as follows: Not pretty see below but problem solved. Actually the measurement fluctuated between about 0 and 15 nanoamps.

Next steps After the two hardware fixes this user interface works quite as intended. And as mentioned, the op-amp was not a great choice for the job at hand. With rotary encoders things get much trickier.

74HC 데이터시트(PDF) – NXP Semiconductors

Even less pretty but problem solved anyway. And the commands for this black-on-white display are different to the otherwise identical white-on-blue display. The reason for that is the need for 3-state outputs as mentioned before. I made a design error with the result that the respective gate was permanently in ddatasheet high-impedance state. As a result, the display as well as all the other components only get to see about 2.

I also considered adding a sub-dollar PIC16 which could be programmed as an I2C slave and control both the backlight and the reset signal as well as giving the option of getting the encoder input via I2C.


The size of the board is mainly given by the size of the display so there is plenty of board space. Again, the reason was simple.

74HC126 Datasheet PDF

I then decided to design an new board that also includes the rotary encoder with all the necessary debouncing. Power can now always stay on. This was a bit harder to fix than the previous error. They make nice, pretty and affordable displays.

Here dxtasheet find the eagle files as well as PDFs of the schematic and board. But getting them initialized with the right settings is always a lengthy trial-and-error process. Ultra Low Power State Another design requirement was to keep this a very low-power design. Then ok, power was there but when I tried to turn off the reset singal i.

There are various datashheet settings that must be set properly via I2C commands but there is very little information on those commands in the data sheet. I wrote an email to their customer support asking if there is a list of commands the display supports.

I still need a way to control both the backight as well as the reset signal without needing any more singnals. Since the display connects via I2C it was quite straight forward to use I2C communication for those purposes as well. Debouncing something like a pushbutton is quite easy. After the two hardware fixes this user interface works quite as intended.