One day I noticed, that arduino pro mini are quite cheap for everyday stuff.
It’s very small, and to keep lower price, this Arduino does not have USB interface.
On ebay it’s ~3$:


For writing sketches to this Arduino, bought CP2102 USB to UART brige:

It’s easy to connect. Arduino has few pins with the same name, so You may choose any of them.
I personally didn’t even solder that additional part of arduino mini pro in front, as same pins were on side anyway.

CP2102        Arduino mini pro
--------------------------------
GND            GND (green wire)
VCC            VCC (blue wire)
TX0            RX1 (grey wire)
RX1            TX0 (violet wire)
DTR    +||-    RST (capacitor between pins 0.1-1uF) (orange, capacitor, then red)

Also both ends on arduino: VCC with RST via 10-12Kohms resistor. (just resistor between pins)

Mine, easy to setup for code uploading: