Alimentar arduino

Mi duda es si puedo alimentar arduino con un cargador de laptop que da 19v y 3.5 A?

No hagas eso!

Get a common iPhone charger block that has a USB port, and power it via the USB cable. That would be better.

Y si reduzco ese voltaje con l7805?

Probably yes. Use the circuit with capacitors in the "application circuits" section of the L7805 data sheet.

Thanks to whole