I made a simple ultrasonic project with Arduino. Used an LCD to show the distance. Also I want to turn a LED on with a 5V relay with a switch for some reason. I'm using external power supply 7.5V 2A and regulate it with 7805 and feed the 5V to LCD back light and to relay(the LCD supply pins are connected to Arduino). Also turning on the back light with a NPN that the base is connected to 5V of Arduino.(I also common the GNDs). So the problem is here: when the circuit is running, it works good and measure distance but when I push the switch and driving relay to turn on the LED the back light of LCD gets weak and even almost turns off. Also LCD and serial print get noise and show junk characters. what should I do to solve this huge voltage drop and noise of relay?