Electronics > Beginners

Discreet startup delay?

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FlimFlam89:
Hi!

I need to make a delay for a project of mine. The delay has to keep a microcontroller disconnected at startup and after 2s it will make the power go through the micro controller. I’ve tried using a PNP but I don’t know how to make the delay...

Could anybody enlighten me?

The March Hare:
First though would be why can't the micro be powered as opposed to simply delaying it's intended function with code?

Simple R/C delay might work for you (for example, charge a cap connected to a reset pin through a resistor .. though at the threshold voltage of the pin, depending on the pin, sometimes hairy results can happen, possibly requiring a Schmitt solution). The good old 555 can be implemented as well. A jelly bean comarator and two resistors. There are literally hundreds of ways to do this, the solution usually a matter of what's on your bench, board real estate, power requirements, and dollars (The latter, there are two lead thermal delays available - but they're somewhat exotic and expensive).

Jimbo.

FlimFlam89:
The thing is that this is all connected to a 4-20mA current loop and the instrument will not work unless it can start up on its own without having the micro attached. Once it’s booted up it can run without a problem regardless if a micro is in line with it.

Since this is a current loop I want to drag as little power as possible, this is a school project/ one off so cost isn’t an issue but id like to keep it "jelly bean"!

The March Hare:
How many lines on your data loop? And I assume the micro has it's own supply? If you don't mind moving parts you could put a small relay in series with your data line and use one of the micro outputs to hold it open until your delay is satisfied via code - if you happen to have a low powered reed on hand could be a one component solution.

Not really certain what you're up to but keep in mind you're not going to want a chip unpowered with data on I/O pins. Your micro won't tri-state?

Otherwise, if it's a matter of just holding an interrupt I'd probably either charge a capacitor through a resistor on the front of whatever comparator you have at hand configured as a Schmitt trigger - 5 components total I think - or a stand alone 555 which may or may not then need a signal transistor to satisfy the interrupt logic.

MiDi:
If little power is of concern and it does not matter if it is delayed 2s or 5s, a p-channel mosfet in 12V line could fit.

The threshold voltage of particular mosfet is starting point for calculation of RC network rise time.
The maximum resistance should not exceed 1Mohm, depending on particular gate and capacitor leakage current.
Now you can calculate the capacitor out of the results.
The threshold is temperature dependent, so calculate for worst case high temperature of mosfet and capacitor.

The formula to calculate (you have to solve it to RxC):


Where:
Vc is the voltage across the capacitor = threshold of mosfet
Vs is the supply voltage = 12V
t  is the elapsed time since the application of the supply voltage >= 2s
RC is the time constant of the RC charging circuit

source


There is a problem with this circuit when turning off the powersupply, but there is a simple solution  ;)

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