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Breaktru Forum  |  eCigarette Forum  |  General Discussions  |  Topic: Need help delaying power for a few seconds
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Offline Lee1111

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Need help delaying power for a few seconds
« on: February 16, 2014, 02:15:55 AM »
TL;DR @ bottom

I bought a bluetooth receiver for the car because the stock head unit didn't receive A2DP capabilities until the next year model,  raged: So I bought one knowing the reviews said that you had to keep turning it on when it would "sleep". I thought, no problem, I'll just remove the internal battery, then it will only get power when the car comes on, and "boot". I did some testing still in stock form yesterday when I got it . After it powers off/sleeps, I can hold the button in that turns it on and activates the pairing procedure for as long as I want and it will pair and operate normally. That was encouraging for my plans. Tonight, I de-soldered the battery, and shorted the button/switch with a jumper wire. Plugged the 5v USB cable in, it started up, paired, and played music for 10 mins straight, good enough for me. Put it back together, and of course it starts throwing a fit. I've figured out that if it gets power, even for a second, loses power, then on the 2nd power application - it throws the fit and won't work. Error's out I guess. It takes it 1.5 minutes to reset and work again on the first plug in.

The problem is when the car is starting. It applies the power for a second before the starter engages thus cutting power, then after it's running it gives power again.  This trips something in the circuit causing it to need the 1.5 min timeout again.


I've been trying to brush up on capacitors and resistors for the past hour, but I'm getting myself nowhere. Would it be possible to add a resistor and capacitor to the positive and negative pins on the input to delay it? Or would that require a huge capacitor? I'm only needing to delay it for maybe 5-10 seconds max. I know I could always configure the 5v source in the car to always be on, but that's probably not ideal. Plus, this little thing is easier to disassemble lol I've seen some 5v time delay relays but I would believe I can modify the existing circuit somehow. I hope I'm not way off in left field here, and you guys can understand what I'm trying to accomplish.



TL;DR - I need to delay power from 5v source for 5-10s going to the input of the circuit by modifying the existing circuit.

Any comments or suggestions are appreciated. Thanks for reading!

Offline CraigHB

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Re: Need help delaying power for a few seconds
« Reply #1 on: February 16, 2014, 03:11:15 PM »
Although OT for this forum, it's an interesting and somewhat relevent question so I'll comment on it.

A capacitor and resistor, more formally known as an RC circuit, can provide a delay for you.  However, you're right, you would need a large capacitor since you are dealing with a larger current demand from the BT receiver (probably around 50mA). 

In general there's something called a time constant with an RC circuit that is a function of both the resistance and capacitance.  It describes how RC circuits respond to step inputs.  Since you need a rather large time constant and resistance is low, you would need a very large capacitor.

To get around this issue, you can use an N-channel MOSFET as a switch and use an RC circuit to drive the gate.  In that case, you can get a long delay with a lower value capacitor and a high value resistor.  Though, this will result in a fairly slow ramp up of power as voltage at the gate of the MOSFET slowly crosses the the gate-source threshold voltage.  This may or may not be an issue for the BT receiver. 

To get a fast ramp-up of power, you could build a more sophisticated circuit that switches a logic output from high to low for a P-channel MOSFET.  You can use a logic inverter chip driven by an RC circuit to provide this function.  I've not had to look for a part like that, but you should be able to find one.  Though, discrete logic chips like that are getting less common now with the predominant use of ucontrollers and logic arrays.

There's a good forum on the All About Circuits web site where you can post this question and get a range of solutions.

Offline Lee1111

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Re: Need help delaying power for a few seconds
« Reply #2 on: February 16, 2014, 11:26:17 PM »
Thanks for the reply Craig. I was hoping I could add a few simple components to the bluetooth board to accomplish this.. I'll go bug the all about circuits guys. Thanks for the tip.

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