Power-on delay circuit

[dotm]

Yesterday I designed this (in a hurry) for an old drumbox.
The goal was that the CPU will start a few seconds after the drumbox is powered on (reset is inverted).
The secondary goal was that even if you switch the box on and off quickly, the delay will start again, so a simple RC won't do it.
This schematic actually works, but I'm wondering if there is a more simple solution.
My idea was that C1 can discharge faster through R2 which is smaller than R1, because D1 will be conductive when the power supply is missing.
The problem is that the smaller R2 is, the faster C1 can be discharged but the more supply current the circuit needs.

[UncleBog]

Hello dotm,

Why don't you just use the RC (R1, C1) with a diode across R1 (cathode to +ve rail). Then as the +ve rails falls the capacitor will discharge into it.

So in your schematic:

Remove D2 (and connect R1 directly to C1)
Remove R2

[dotm]

Then as the +ve rails falls the capacitor will discharge into it.

you mean that the attached load will discharge the capacitor..
thats clever. thanks.

[MTong]

I usually used a 555 configured as in here
http://clarkson-uk.com/555-timer/operation/frames3.html

[UncleBog]

I think dotm wants simple. A 555 timer can be used but if you're going to use an IC then why not use a dedicated reset / supervisor IC such as those produced by Linear Technology, Maxim, TI etc. Then you get a well defined reset period plus some combination of options such as integrated buffer, voltage monitoring, watchdog etc.

[dotm]

I usually used a 555 configured as in here
http://clarkson-uk.com/555-timer/operation/frames3.html

there can be a case with this circuit that when the RC is not fully loaded, and the device is switched on and off again, then the delay will be too short.
lets say it's 2 seconds delay, I switch the device on for 1,5 seconds, the cpu wont start then switch off and on again, the RC will continue to load and the delay will be then 500msec.

[pickit2]

try looking for oneshot smitt trigger, it will do what you want, and you could do it with parts from your original post.

but using a 40hc93 is better option.

[CocaCola]

I have become so lazy as I get older (and cost has come down so much) that I would just use a small micro, that and the fact that I always have micros on hand, can't say the same for many other components...

[zac]

There are also reset ICs:

http://www.maximintegrated.com/products/supervisors/reset_ics/

and some voltage regulators have reset outputs:

http://www.maximintegrated.com/products/supervisors/regulators/

[Gallymimu]

RC with single diode across the R is a pretty typically asymmetric charge/discharge circuit.  It's the best way in my humble moron opinion.

[dotm]

I just did not see the most obvious solution.
I designed the circuit above without load on the power rails. That's why.

[picavr]

you do use lm2675 with delay start up (rc circuit on pin N°5)