DC time delay circuit

[Rekarp]

Need some help on this one. I need to delay a power on for 4 seconds. Now I know I can do this with a simple 555 timer and a relay but I would like to not have to use a relay. The voltage line will be 5V at around .5 amps.

Thanks

Oh and a RC circuit won't work. This is to help prevent power cycling if the user happens to turn the system off and on really fast. ie there is enough wire to cause inductance in this thng that prevents it from booting correctly.

[GonzoMPM-1]

Need some help on this one. I need to delay a power on for 4 seconds. Now I know I can do this with a simple 555 timer and a relay but I would like to not have to use a relay. The voltage line will be 5V at around .5 amps.

Thanks

Oh and a RC circuit won't work. This is to help prevent power cycling if the user happens to turn the system off and on really fast. ie there is enough wire to cause inductance in this thng that prevents it from booting correctly.

Well, you can use a solid state relay. They make really small ones that control about a half an amp for burglar alarm applications.

You could also use a really simple transistor circuit.

Here's an example of one embodiment. It should work just as well if you replace the darlington / relay combo with a nice FET.



A first stage different transistor will be necessary, as you'll need one that has a reverse emitter/base breakdown of far less than a 3904, particularly at the 5v source you're using. (that may be tough -- obviously, zenier diode would work in a pinch -- try a 3.3v zenier).

You'll have to monkey with it. That diagram results in 7 seconds using a 12 volt source and a 3904 which breaks down at about 7 or 8 volts. Assuming you're using a zenier 3.3 instead of the 3904 and 5 volt source, you'll probably have to raise both the resistor and capacitor values to get the delay you want.

You can see, I think, how a FET would be a delicious alternative to the transistor/relay combo in the output stage.

EDIT: Just noticed the last part of your post. I'll give it some more thought and post later.

[Rekarp]

I just cobbled together a 555 and a relay. I had the stuff on hand and didn't feel like ordering parts.