Pen points to transmitter power switch. This is how you turn transmitter on, off. Directly above switch are three colored light-emitting-diode (LED) indicators that tell battery status. If top green LED is glowing, battery is okay. If middle yellow LED lights, recharge or charge batteries soon. If red is glowing, do not fly until you tend to batteries. Sometimes colored indicators are replaced by meter or LCD (Liquid Crystal Display)-screen voltage readout on more sophisticated transmitters.