I recently added a small circuit board to delay my car stereo powering up. The idea was to prevent the stereo coming on in the brief period between turning the key from ACC to START and back again. It played havoc with my Bluetooth connectivity.

Anyway, before adding this little board my stereo was more than happy to accomodate temporary voltage drops such as when I turned on the Air Con or a drop in revs.

It seems the new board doesn't like the drops, so when the revs drop (and the voltage from the alternator drops), the board resets and disconnects the output supply and after another 10 seconds the stereo is powered again.

I was wondering if, between the supply and the delay circuit I placed a diode and then a capacitor to ground might help matters.

                      |         |
                    [CAP]       |
                      |         |
                     GND       GND

Thoughts? Thanks, Scott

UPDATE: After checking out a few diagrams, I realised the above diagram is wrong. The DELAY BOARD is actually actuating a 12V relay - ie, the stereo is not powered directly from the board, rather the same 12V supply it was on, except now that 12V is switched by the board. So that should remove the possible problem of a voltage drop induced by the board.

So, currently:

                  |                           |
[SUPPLY] ---------+------ [DELAY BOARD] -- [RELAY]
                                |             |
                                |          [STEREO] 
                                |             |
                               GND           GND

Here's a pic (no schematic, sadly): NB: the pot is to adjust the relay-on delay.

enter image description here

  • 2
    \$\begingroup\$ What does the delay board look like? Do you have a schematic? \$\endgroup\$
    – Dejvid_no1
    Commented Jan 9, 2016 at 13:07
  • 1
    \$\begingroup\$ You may need an unfeasibly large capacitor, although you idea would work in principle. It would be better to make a replacement for the other device. \$\endgroup\$ Commented Jan 9, 2016 at 13:08

2 Answers 2


If things worked before you added this delay board, then the problem is with the delay board. It's not doing its job properly. It is probably dropping too much voltage, or being too aggressive about thinking power is going away and shutting down. Either way, this delay board is not a good fit to your requirements.

If you just bought it, then return it and explain what's wrong. If you designed it, then show us the schematic so we can fix it.

  • \$\begingroup\$ I think your second point is right with regard to shutting down too easily. While the stereo may be tolerant of a voltage drop, the board itself may not be. Hence why I need to keep the voltage at 12V to the board until the alternator resumes deliver 12V (and in doing so keep the relay energised). \$\endgroup\$
    – Scott
    Commented Jan 12, 2016 at 4:55

Probably not a good idea.


simulate this circuit – Schematic created using CircuitLab

It seems as though you already have a voltage drop with 'this little board' at low revs. This can occur for an extended period when idling. Adding the diode will only make the problem worse as you'll drop another 0.7 V.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.