Help with Vehicle Audio Sound Board Design

Discussion in 'The Projects Forum' started by Ser Brynden, Mar 12, 2015.

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  1. Ser Brynden

    Thread Starter New Member

    Mar 11, 2015
    Hello everyone!

    I want to connect an audio sound board to my car’s stereo system that will play a short audio file through the speakers immediately after ignition. I will use an Adafruit Audio FX Sound Board (, which has a 2.5mm headphone jack, a micro-USB port for power, and 11 triggers (I’ll only need one). I already designed and bench-tested a simple circuit that would probably do the trick (see Figure 1).

    Figure 1. Vehicle Audio Sound Board v0​

    I’ll tap into the vehicle’s fuse box to get +12 VDC from either the accessory power or radio power (or splice into the power wires behind the stereo). Switch S1 is there simply to let me disable the circuit so no audio plays when I take the vehicle in for maintenance, for example. I use a micro-USB direct hardwire car charger cable kit from Amazon ( to power the sound board (labeled as voltage regulator). To play the audio, I just have to ground pin 0 for a split second. If pin 0 were to remain grounded, the audio would loop over and over. To trigger pin 0 once, I use a constant-to-momentary relay design that I found here ( The capacitor C1 allows the coil of the relay to be energized until the C1 stores a charge, thus de-energizing the coil. The resistor R1 bleeds off the charge of C1 when positive voltage is removed from the other side of the coil. I had to increase the capacitor value from the referenced design because the sound board takes a little less than a second to boot up first. Since I have an aftermarket stereo with a separate wiring module for providing warning chimes (among other things), I thought I would simply splice the audio output from the sound board with the wires that provide the chimes to the vehicle amp/speakers. I want my audio file to play immediately after ignition, but before the stereo boots up (which takes about 30 seconds). Since the vehicle warning chimes can be heard after ignition, regardless of the state of the stereo, I thought this would be a good way to get my audio to the speakers.

    Here’s where I could use some assistance:

    While the above design might work, I want to make it more robust with the following enhancements:

    1. Since I only have to ground pin 0, why not remove the relay and just connect pin 0 straight to the capacitor C1? Wouldn’t that work too?
    2. When I turn the key in the ignition, there is a position where it provides power to the stereo and other electrical systems before the startup position. Then, power is cutoff to these systems during engine start, before being restored. How do I avoid having my sound board play before the engine starts? I don’t want it to chirp or play partial sound then get cutoff. I also don’t want to risk damaging the sound board or corrupting the file, though I feel it’s pretty robust.
    3. How can I automatically cutoff power to the circuit after about a minute? It only needs power for a few seconds after ignition and there is no need to have it powered for long hours on the road, wearing down on the chip’s life expectancy. I did a web search and found a few auto-off designs, some with 555 IC timers, but wasn’t sure how to adapt them to my circuit.
    4. For now, I’m okay with having my audio play at the same time as the vehicle’s warning chimes that come over the speakers after engine start. But I may also want to include a relay that will switch the vehicle speaker wires between the chime wires and my sound board wires. This would need to switch while the sound board is playing, and then switch back after. Thoughts on how to control this relay switch? If I use an auto-off timer, like in Item 3, this is a no-brainer.

    I drafted up a design that may include solutions to Items 1 and 2 above (see Figure 2). To avoid having my sound board play before the engine starts, I thought of using a delay circuit that I found here:

    I’ve also seen delay circuits with two transistors, though I don’t fully understand why. I haven’t tinkered with the resistor or capacitor values to change the delay time, yet. I would only need a 1-2 second delay. This way, as long as I turn the key to the start position in less than 1-2 seconds, nothing will play. Then it will only be 1-2 seconds after the engine starts that my audio plays. However, the current design only delays the audio trigger and not power to the whole circuit. I would rather delay power to the whole circuit so the sound board doesn’t cycle needlessly. Is this delay circuit the only way to resolve Item 2? Did I do it correctly or is there a better way?

    Figure 2. Diagram of Vehicle Audio Sound Board v0.1​

    I will greatly appreciate any help I can get in completing this project. Please let me know if you have any further suggestions or if there are any “don’ts” I should take into account. I’m open to constructive criticism and advice. If you can show me circuit diagrams of your ideas, that would be awesome! I drew my diagrams using TinyCAD (, a free open-source software, and links to my files are below. Also, please explain how your circuit works and what the purpose of each component is, because I barely understand the reason for the different resistors and their values in the delay circuit of Figure 2.

    Thank you for your time!

    Vehicle Audio Sound Board v0
    Vehicle Audio Sound Board v0.1
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    Welcome to AAC
    Tapping into the fuse box is a bad word Mate.
    Not allowed in AAC. You did not read the forum regulation
  3. bertus


    Apr 5, 2008

    I am closing this thread as it violates AAC policy and/or safety issues.

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