I would like to have a pushbutton switch to turn a circuit on, but have an on-delay of about 5 to 10 seconds before the circuit "turns on". I know I can use a resistor and capacitor with a transistor to ramp up the voltage, but I would like the circuit to go from off to on rapidly (a small ramp is ok) when the time interval has elapsed. I.e., one would have to continue pressing the button for the entire time before the circuit turns on.

If the pushbutton were released before the time elapses, I would like it to stop, but pushing the button again a short time later is not that important if it starts (almost) where it left off or starts the time completely over. If there were a few minutes delay before pushing the button again, I would like the time to start from the beginning. The voltage is 24VDC and will be a very small current. On-delay relays are much, much too large and expensive for this application.

Thanks,
Dale

 

Here is a sugestion

Attached Images

Delay 1.gif (4.7 KB, 20 views)

__________________
Len

 

Len,
Thanks for the help. I think I understand your circuit.

I'm not great in circuit design, so I would like to make sure I understand a couple of things--correct me if I am wrong anywhere. I believe that Z1 and the 5.6k resistor are there only to provide a supply voltage to the Schmitt trigger. The 1M resistor and C1 provide the time constant.

Actually, this is not the complete problem, but I think I can adapt this to my needs. I had a different post for another part of my problem (Ramping Up DC Power), but have not received a complete answer yet--still don't know component values to use. After I wrote that post, I got the bright idea that if I held in the power button for several seconds (like on my notebook computer), I could also turn the system off with the same momentary push button.

I'll try to create the combined circuit to provide the power on/off functionality I need.

Thanks,
Dale

 

Yes, the Schmitt supply must be 15V or less. Yes re time constant.
To do an on/off circuit, you could add a Flip Flop (eg. the 4027) that is set/reset by the button. You would need to use one of the unused Schmitt sections as a "power on reset" to reset the FF when the power is turned on.

However, you would need to hold the button for 5 ~ 10 sec to turn off also. If this is undesirable, it could be avoided by installing some extra circuitry.

As for the ramp up circuit, I would take an output from the FF and use a transistor in lieu of the relay contact. BTW, the relay contact would burn out fairly rapidly due to the large discharge current when it closes since there is no resistor to limit the discharge current.

__________________
Len

 

Len,
Yes, I do want to hold the button for 5-10 seconds to turn off the power and a momentary press (when it is off) to turn it on. I plan on having the computer set an output to turn off the power after the shutdown proceedure has run. The 5 second push button power-off is in case there is a problem with the automatic power-off and would be wired in parallel with the computer power-off output. A momemtary button press while the power was on will be ignored to prevent an accidential power-off.

I was already planning on eliminating the relay as part of the merged design. This On-delay question (the subject of this post) was actually to provide control for the power-off part of the circuit.

Now that I have the pieces, I will spend some time later this week to put everything together.
Thanks again,
Dale

 

Dale,
I you post the circuit you devise, I'll look at it and make suggestions if necessary.

__________________
Len

 

I think you some words out. LOL.

If you said what I think you said, yes, I will post my solution. I am looking at using a 14093 to combine all the functions for my complete circuit. I would appreciate some advice with some of the support components when posted. It will be next week before I will be able to post it though.

Thanks,
Dale

 

Yes the first word should have been "If" not "I"

I'll tune in next week.

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Len