1, if the current is only about 200MA, you can use LM7805. LM7805 is a linear power supply IC (the limit input voltage is 36V), the main problem is that the loss is relatively large when 24V to 5V, the heat is relatively serious, you can add heat sink, the peripheral circuit is simple. 2, if there is no requirement for the cost, it is best to use LM2596-5 (switching power supply chip, small loss), this peripheral circuit is relatively simple, a fast recovery diode, a 100UH inductor, 2 electrolytic capacitors and 2 104 porcelain chip capacitors. 3,MC34063 do not have to consider, the first two are 5V dedicated, and this IC can boost the voltage can be reduced, the circuit is more troublesome.