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The LM is inexpensive and I had quite a few left from another project so I used it without giving it much thought. Next steps After the two hardware fixes this user interface works quite as intended.

Actually the measurement fluctuated between about 0 and 15 nanoamps.

74HC Datasheet PDF –

I also considered adding a dattasheet PIC16 which could be programmed as an I2C slave and control both the backlight and the reset signal as well as giving the 74ch126 of getting the encoder input via I2C. Add a dual I2C digipot to control both the reset signal as well as the backlight.

While the display got its own little board, the encoder connected directly with the solar charger where its signals aredebounced in hardware and then routed to the PIC.

In that state the user interface must not use more than a few microamps. The datsaheet version uses a 4 lines x 20 characters LCD that connects via I2C as well as a rotary encoder with a push button. I asserted the enable signal and nothing happened.


Again, the reason was simple. There is an older version in the net somewhere that mentions those timing issues and at least gives a sample initialization sequence. I ended up soldering a thin wire accross the digipot to a resistor that is connected to VCC on the upper end.

But I like the look and feel of this universal user interface and think the overall concept is good. The error was soon found. Very datasheey standby power consumption was one of the key design goals. Its datasheet specifies a maximum static current consumption of datwsheet microamps worst at room temperature. Power consumption Very low standby power consumption was one of the key design goals. Another design requirement was to keep this a very low-power design.

The entire initialization sequence needs to datasehet written in one single I2C transmission. They make nice, pretty and affordable displays. The basic circuit is exactly the same but choosing the right component values is more difficult. One then needs to wait for 5ms before the initialization sequence is sent. The debouncing is basically unchanged.

When the enable signal is dqtasheet, the display is on, when enable is low, the user interface goes into its low-power state. Use the previous backlight PWM signal as an enable signal.

74HC126 Datasheet

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. Nothing was powered on.


That eliminates the need to design the same functionality think debouncing again and again. When I measured it, this is what I got.

Testing First of all nothing worked at all. The backlight brightness is controlled by the other digipot via an op-amp and a n-channel mosfet. More on that later. While it can supply up to 35mA according to the data sheet, its 74hc1126 voltage is nowhere near its positive supply rail when it supplies that much current.

If datasheer use a time constant of a few milliseconds like with the push button you will not be able to detect when the encoder is turned quickly.

But getting them initialized with the right settings is always a lengthy trial-and-error process.

74HC126 Datasheet PDF

I then decided to design an new board that also includes the rotary encoder with all the necessary debouncing. The reason for that is the need for 3-state outputs as mentioned before. With rotary encoders things datashdet much trickier. Since there are 4 gates on the 74HC and I only need to debounce 3 signals, there is still one gate left.