Introduction: (written 5-26-2007)

I designed this circuit for my graduation project at State College High School.  A few months after completing this project, I was ready to design an improved version that was better engineered and with greatly expanded functional capabilities.  See the improved version 2.0.     

Bass-Activated LED Circuit - [version 1.0] - 2003 High School Graduation Project

Devin R Ott
Design - February 2003
Final Assembly - February 2003

view my final report: Senior Project - (Feb 2003)


Uses low frequencies (<100Hz) in a preamp audio signal from my car stereo to trigger (ON & OFF) an array of blue/red LEDs mounted under the seats of my 1999 Pontiac Grand AM.


User Operation:

Trigger Sensitivity - 5kΩ POT - adjusts the LED bass sensitivity over a wide range of volume levels; LEDs can be triggered by very loud or very quit audio.

*LEDs always ON - rotate sensitivity knob to its maximum (clockwise) position.

*LEDs always OFF - when sensitivity knob is at minimum position, LEDs are never triggered.

Brightness Sensitivity - 5kΩ POT - adjusts the brightness of both blue & red LEDs together.

Color Switch - DPDT (center OFF) - selects which LEDs will turn ON when triggered.

UP position: Red LEDs only 

CENTER position: both Red & Blue

DOWN position: Blue LEDs only 

Circuit Operationrefer to senior project.


    Featured Devices:

741  op-amp - compensated

LM301  op-amp - general purpose

1458  dual op-amp - compensated

LM339  quad comparator

4066  quad CMOS bilateral switch

4069  hex CMOS inverter

LM317  adjustable voltage regulator

2N2222A  npn transistor


Final Assembly: refer to senior project.


   <- final PCB layout (computer sketch)


   <- final PCB layout (pencil sketch)



Schematics:  11 figures from senior project.


Sallen-Key 100Hz Low-Pass Filter
Buffer / Virtual Ground Generator: provides (+VS) supply ground reference (+6V) for audio processing circuitry
Inverting AC Amplifier
Noninverting AC Amplifier

Audio Amplitude (Peak) Detector: outputs peak value of AC signal as DC voltage (6V to +12V)
Voltage Comparator: triggers LEDs when audio amplitude is above the DC reference provided by trigger sensitivity potentiometer
4066 Quad CMOS Bilateral Switch: internal device schematic
Color Switch: uses remotely controlled 4066 to interrupt on-board LED trigger signals from a remote dash-mounted
Adjustable Voltage Regulator:  LED Brightness Control
Blue LED Transistor Switch
Red LED Transistor Switch


Other Schematics & Layouts:


PCB layout of earlier version of circuit. 

It was unsuccessful because I could not get the JFET input op-amps in the LF347 to generate the 6V ground reference function. 

I foolishly assumed that what worked for one op-amp would also work for another op-amp.  This is when I learned a valuable lesson:

ALWAYS breadboard circuits before you build them!!!  Otherwise, you might build a circuit that doesn't work. 
one of many PCB layout brainstorms.

all layouts were designed by hand.
another one of my many PCB layout brainstorms.
an early schematic of the audio processing circuitry


Improvements: refer to senior project.


During my Junior and Senior years in High School, my knowledge of fundamental electronic theory was expanding so rapidly that each time when I'd finally finish constructing a project, there were already a dozen ways I could think of to improve it.

This circuit functioned exactly the way I designed it, and seemed quite advanced at the time, but its electronic design needs much improvement and its overall functionality could be easily expanded to give the operator more options to maximize the experience.  For this reason, I designed a much better Bass-Activated LED circuit.

See my improved Bass-Activated LED Circuit [v2.0]


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2007, Devin R Ott