Arduino Pro mini (5V)

With an Arduino Pro min (5V), can I: 1. Use a RTC module such as DS3231 module 2. Read a LDR to automatically turn lights ON at dusk and turn them OFF at dawn (thru a relay board) 3. Shut down the light from midnight to 4:00am (I don’t want to keep them on all night long – this is why I would need the RTC) 4. Drive a 4x20 LCD or even better a 320x240 TFT to show current date, time and status of light (ON or OFF) 5. And why not display the temperature since I think the DS3231 includes a temperature sensor

I have no doubt about points (1), (2) and (3)...not sure about (4) LCD or TFT


You should be able to do what you want. You need to make sure that the libraries for the modules that you want to use do not conflict by wanting to use the same pins.


Normaly it should work to use all things together on Arduino pro mini. The most critical thing is the memory on the Arduino. In this case there will be some big libarys but an small program. So there is no more programm funktions it should work. I tried to program an watch an an arduino pro mini and it worked with some spase left for other libarys.


  1. TFT drivers can be memory hogs. That might not leave very much room for the other stuff. 4x20 LCD, no problem.

TFT LCD's are memory hogs, pin hogs, and they don't even end up looking that good because you can't generate graphics that take advantage of it.

I vote 20x4 LCDs. They're very readable.

How will you be turning the light on and off? You will need an external piece of hardware to actually switch power to the light, depending on what kind of light it is - the Arduino itself can only supply 20mA (40mA tops) per pin. Typically you might use a MOSFET (for switching a DC device) or a relay (for switching AC or DC - but you need a transistor or mosfet to drive the relay too)

Those are my only concerns - this is an entirely doable project, pretty simple, the kind of thing a lot of us have done ourselves.

I was planning to use a relay board, such as the Sainsmart with 2 solid state relays Can switch 2A @ 240V = 480W per channel My outdoor lights are 110V/9W LED, and I have 4 for I should be good I agree that a 20x4 LCD will be perfect, and probably easier !!! Which one would you recommend, decent and cheap? But if the Pro mini is too difficult to use, I have 2 UNO I can also use.....But I thought the Pro Mini should suffice

Whether a display is decent, depends on what you display on it. Almost all the 2 and 4 line LCD's are extremely economical.

For the LDR, I will need 1 analog input For the relay board, at least 1 digital output (2 if I want to drive the 2 relay from the board in the link) For the DS3231, I will use A4 and A5 How many I/O do I need for a 4x20 LCD ? Which library should I use?


It's better to get an LCD with an I2C backpack PCB. They eat up available pins otherwise.

It’s better to get an LCD with an I2C backpack PCB. They eat up available pins otherwise.

But am I not already using I2C with the DS3132 on pin A4 and A5?
How would this work then if they both need the same pins ?

That's the beauty of I2C - it's a bus. Every device has an address, and only responds when its address is called.

Yeah...I've just found out So both boards (RTC and LCD) should be daisy chained? Or can it be cabled in parallel?

Parallel. SCL goes to all devices, SDA goes to all devices, Gnd goes to all devices. Ideally, 4.7K pullup from SCL and SDA to 5V, or 2.7K to 3.3V, at the end of the line.