If the ATMEGA chip with low voltage works at 5V, there is no problem, but the voltage is higher than 5.5V, it is possible to burn out the chip. First of all, the circuit output is positive voltage, so L7912 must be replaced by L7812. Secondly, the output end of the L7812 to add a 47uF 25V capacitor, the performance will be better. Again, the transformer is 12V, the voltage after rectification and filtering is 12*1.4-1.6 = 15.2V, the lowest voltage of the 78 series regulator drops to 2V, in order to output 12V voltage, that is to say, the lowest voltage of the rectifier and filtering circuit is 12 2 = 14V close to 15.2V, so the circuit can barely work. However, if the 220V voltage is slightly reduced, the circuit may not be able to stably output 12V voltage, and 13V-15V transformers are recommended. If the output current is large, also add a heat sink. The 7805 does not require a circuit diagram. 1 pin power supply, 2 pin ground, 3 pin output is 5V. Two pieces can be used in parallel, or an indirect resistance of 200-300 ohms \/1W at 1-3 feet can play a role in expanding the flow, and it is no problem to reach 1.7A, and the voltage regulation effect is slightly worse, but it can work normally.