The audio traffic light is a straightforward circuit which tells you whether an audio signal is dangerous for your ears or not.

At its core it’s a basic VU-meter, but instead of having multiple LEDs representing the audio level, the audio traffic light represents it with only 2 LEDs: an orange (or amber) LED and a red LED. Because of that it’s easy to see to see if the volume is too high. You can also adjust the trigger level at which the circuit tells you the volume is too high.

The audio traffic light has a total of three LEDs. A green LED, to tell you the circuit has a power supply, an orange LED, to tell you that it has an audio input and a red LED, to tell you whether the volume of the audio input is too high. It also has two connectors, a USB-B connector for its power supply and a 3.5 mm stereo socket for the audio input. And finally it has 4 potentiometers, two to determine when the orange LED emits light and the other two to determine when the red LED emits light.

When there is an audio signal, it goes to a JFET op-amp so the audio level gets amplified. After the amplification the signal gets send to a monostable multivibrator which sets or resets its output. When its output is high, it goes to a transistor and the orange LED emits light. When the input audio exceeds a certain level, which is adjustable with two potentiometers, the signal goes through the same process, but triggers a second monostable multivibrator in conjunction with the first monostabel multivibrator and powers the orange and red LED.

Except for the monostable multivibrator, every component is taken from the Elektor.Labs Preferred Parts (ELPP) library where the RS and Farnell order codes can be found.