Hi i have 3904 and 3906 transistors, leds and spdt switch. I tried to use the 3904 so that when voltage is applied to base, it flow from power source into the led and then to ground. But now i have difficulty to use another transistor so that when 0v or ground is applied to the base of the transistor, the led will turn on. How would i do this? In other words the spdt will toggle 2 leds, one led will show the on status and the other led the off status. Schematic i drawn below is working but i need to add another led that will switch on if voltage is 0 or ground. Thanks a lot, very noob here.
A basic solution would be to just make a copy of this circuit and invert the driving signal which is being applied to the base of the second transistor. Here is a small schematic:
You could also spare one transistor and use just a standard diode instead. Here is an example:
Since the driving signal is an audio signal, the following circuit can be used to detect the presence of an audio. It basically consists of amplifying the audio signal and using a peak detector to store keep the output
DET high if the audio signal is present. You could use this detector to drive one of the circuits above. Maybe a buffer withh be required between both circuits in order to not overload the detected signal.