Arduino Nano has no INPUT_PULLUP on A6 and A7

I discovered it the hard way: Arduino Nano has no INPUT_PULLUP on A6 and A7. That's because the ATmega328 in tiny SMD package has two extra pins (vs. the DIP processor), which are tied directly to the A/D converter MUX without the digital I/O logic.

I discovered it once I received and assembled PCBs for a TIL311-based clock, where I used most - if not all - I/O and analog functions were left to "pure analog input" pins A6 and A7.

Well, wiring an LDR between A6 and ground lead no readings. Few measurements later, and a quick Internet search, revealed this hard truth.

Fortunately I have an easily accessible 5V point near A6, so I could throw an external pull-up resistor.

Voltage multiplier

It is all right at IK1ZYW's Labs, it's just that I had no opportunity to update the blog.

Last experiment has been a voltage multiplier as Cockcroft-Walton designed it in mid 1930's. Why? Apart my quest for a HV PSU (in the order of 300 Vdc), Xmas lights came with powerful 24 Vac 20VA transformers that should be enough to power some Nixies, once voltage-multiplied.

Electro-party!

The hardest part has been to locate high value capacitors with high voltage rating in and around the junk box. While 2-300V rating is not hard to come by, the high capacitance is required in order to keep low its AC resistance at 50 Hz. Diodes need to sustain at least 60V reverse polarity, so 1N4003 and above.


I started with 220 uF capacitor, but it got too warm. Then I replaced it with 2200uF 63V in the first stage and everything got better. With 4 diodes I reached more than 160 Vdc which happily light up an IN-14 Nixie without dropping more than a Volt.

Nice experiment so far. A final circuit would allow to pick different output voltages stepped at (24*1,41) Volts, which can be useful.

Or just put back-to-back two of these transformers, rectify the output and find a way to drop it to the desired value. Thinking ...