This instructable is an enhancement of my old project https://www.instructables.com/id/Simple-7-Segment-... , the RTC module seemed not quite real time at all and drifted day by bay even I changed new battery, I reference to some other instructable makers would like to try use ESP8266 to connect the Internet clock source to get the real time.
The old 74HC595 decoder board still usable, I built an ESP8266 program fixture to connect the Arduino, many ESP8266 players should known it must be connected to 3.3V voltage source, but when I connect it to the decoder board, I accidentally connected the 5V source to decoder board and fired the ESP8266 even it connected to 3.3V source, find out later that TTL IC can also use 3.3V source.
After the first success I want it can be displayed as an wall clock, so I try to source bigger 7-segment module, this is not quite easy, finally I found some 2.25" 7-segment modules but are common cathode type (the LED used in the decoder board is common anode type), at first I think there should be no any problem, may be I can change the polarity to fix but I was wrong.
Except the common cathode type, the larger LED needs higher voltage minimal 8V to light it up, obviously it cannot be driven by the decoder board at TTL voltage, however the decimal point at the lower right doesn't high volt, I fired up one module when I test to light up the segments, but no problem, I only need to light up one decimal point.
The problem of common anode and common cathode is significant with the decoder board, the outputs from 74HC595 drive the digit lines and segment lines respectively, in order to work properly, the digit lines should connected the anode of LED, while the segment lines connected the cathode of LED, change the polarity the LED still lit, yes lit forever.