August 1, 2019 posted by

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Actually the measurement fluctuated between about 0 and 15 nanoamps. The basic circuit is exactly the same but choosing the right component values is more difficult. Nor the timing of the reset singnal. There was obviously no connection to that pin.

(Datasheet) 74HC pdf – Quad buffer/line driver 3-state (1-page)

First of all nothing worked at all. While that was really hard to notice on the schematic I should have noticed the problem when I laid out the board. Like in the solar charger designa 74HC quad tri-state buffer is used together with some resistors and capacitors.

The backlight brightness is controlled by the other digipot via an op-amp and a n-channel mosfet. My final solution was as follows: Debouncing something like a pushbutton is quite easy. Another design requirement was to keep this a very low-power design. And as mentioned, the op-amp was not a great choice for the job at hand. And a time constant of a few milliseconds is totally sufficient so you might end up with something like 10ms of delay. So the enable signal 74hc1226 controls the input of that gate.


But I like the look and feel of this universal user interface and think the overall concept is good. So the measurement works adtasheet it seems that the 8 microamps given in the data sheet are just a very conservative worst case.

74HC Quad buffer/ Line driver with 3-state outputs

Ultra Low Power State Another design requirement was to keep this a very low-power design. But getting them initialized with the right settings is always a lengthy trial-and-error process.

When I measured it, this is what I got. Dwtasheet long as the push button is not pressed, no power flows there so the only power consuming device is the 74HC More on that later. So you face the challenge of debouncing the signal while still letting relatively fast transitions pass.

When the enable signal is high, the display is on, when enable is low, the user interface goes into its low-power state. Since there are 4 gates on the 74HC and I only need to debounce 3 signals, there is still one gate left. 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. They make nice, pretty and affordable displays. As so often, google was my friend and others have faced similar problems before so I managed to find some commands that make the display work properly.

Add a dual I2C digipot to control both the reset signal as well as the backlight. This was a bit harder to fix than the previous error.


Tag Archives: 74HC126

I asserted the enable signal and nothing happened. After the two hardware fixes this user interface works quite as intended. Here you find the eagle files as well as PDFs of the schematic and board. There are various contrast settings that must datasueet set properly via I2C commands but there is very little information on those commands in the data sheet.

And the commands for this black-on-white display are different to the otherwise identical datashet display. When in low-power mode only the push button pull-up and the 74HC is supplied with power. Toghether with the positive feedback via the k resistors that works very well.

Testing First of all nothing worked at all. Turning on the backlight basically worked as intended but the load was a bit much for the 74HC That way I get a quite universal, easy to use user interface that I can use for other projects as well. I expected to tweak those values a bit but found that they work so well that I just leave them as they are. Another issue I noticed was that the LM op amp I had chosen is not a rail-to-rail op amp.

The error was soon found.