Switching low voltage AC with a microcontroller

This circuit can be used to switch a low voltage (12V) AC load with an Arduino or other microcontroller
A positive signal on the entrance will bring T1 into conduction, triggering the Triac via emitter resistor R2. D1 and C1 are just there to provide DC-power to the transistor.
R3 will pull the base low. It can be left out, but it avoids accidental switching when the microcontroller port is undefined (like at start up). I would really advise to put it there.
The Triac should be cooled with a metal profile. remember that the Gate is galvanically connected with the metal housing of the Triac.

WARNING: This is for LOW AC voltage only. Switching a high voltage AC requires use of opto couplers to isolate the High Voltage from the microcontroller.

Notes. I have used an SC141B Triac rather than a TIC206 as I had that around and it is 110 V. max. In a 220 V. country I am not likely going to use that soon for another project.
The picture of the mounted PCB does not show the heatsink yet. It is advisable to use a heatsink.
The printdesign can be downloaded here. There is space for various size capacitors. Mine seems very big but it is a very old one I still had lying around. You may want to move the position of D1 to make it a bit easier to mount a heatsink.
The Diode D1 does not really need to be a 1N4007. The DC part of the circuit does not really need much power. a 1n4148 would be OK, but they have the same price and I had a 1n4007 lying around

Print mounted with components

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Another Simple Arduino

In its simplest state the Atmega328 can be made to work with a 16 MHz Crystal and 2 small capacitors or even just with a ceramic resonator. Load that with an Arduino bootloader and you have a small arduino compatible microcontroller.
This article describes the building of such a device, on strip board, that can be put together for just a few Euro:
Needed:

1x Atmega 328 at € 3.40
3x 100nF ceramic capacitors € 0.09
2x 22pF ceramic capacitor € 0.12
1x 16 MHz Crystal € 0.40
1x 100uF 10 V electrolytic capacitor  € 0.12
1x 100uF 25 V electrolytic capacitor € 0.11
1x 1kOhm Resistor € 0.10
1x 10kOhm Resistor € 0.10
1x 330Ohm Resistor € 0.10
1x LED Red € 0.12
1x LED Green € 0.12
pinheaders ca 33 pins € 0.45
1x 7805 € 0.30
stripboard 20x 8
2x 14 pins IC foot € 0.15

The stripboard seen from the TOP/Componentside

The power connections on the componentside of the stripboard

Placing of the 28 IC foot pins on the componentside

Placing of resistors, capacitors, LEDs and jumper. The grey capacitors are 100nF, the brown capacitors are 22pF

Finalised Arduino compatible on stripboard

Simple Arduino