The circuit can be used to give a visible or an audible warning that the headlights are on. It uses a 2N1305
transistor as a switch to turn on a Sonalert tone generator or a small 12-V lamp. Operating current for the transistor is supplied from the wire that feeds the headlights. When the ignition is on, the transistor is biased off and the alarm is not activated. Turning off the ignition while the lights are on sets off the alarm.