Turn off delay circuit

"vic"

** Google " NE555" .

Gets about half a million hits.

... Phil

Depending on the volatge and current involved, you could be talking about a CMOS 555 timer or as thermal delay relay.

What voltage?

How much current?

Batteries or mains power?

John

Take a look at the recent discussion on SEB on 24th Feb. The link is at

e2f9392cdea86c89#

3 versions of solutions using transistors, NE555 and PIC are available.

Allen

s s e

Further down on post #88, the problem can be sloved by op-amp as well as suggested by John Field.

The PIC software posted by nobody for the delay timer was for something else and has to be reworked.

Allen

Look for the 555 timer, It has 2 inputs, trigger and threshold that are part of a RS (Reset/Set) flip flop circuit. Pulling the trigger low will latch the output on, pulling the threshold high will unlatch the output. etc.

It's smaller and more versatile than employing a flip flip chip when you only need one switch. On top of that, you can also place some timing activity in that same lay out.

"

Thanks for your precious advice. I was not clear enough. Let me explain again : at first, the circuit is completely powered off. The user pushes a button, which injects power into the circuit. Then a mechanism maintains the power active, even if the switch has been released. At a later undetermined moment, when the circuit does no longer need power, it cuts its own power and sleeps until the user chooses to push the button again. I'm pretty sure a 555 can't do this.

vic.

** YOU are bout as clear as mud - f*****ad.

And that situation is certainly congenital.

** Fuck knows WHAT THE HELL you are blathering about.

Whatever it is - you are way off topic here.

This NG is only about BASIC electronic concepts, hardware and practices.

Try " alt electronic.gadgets-wankers " instead.

..... Phil

Sorry I really did not phrase my question well it seems. By "power on" and "power off", I meant really the power to the circuit, maintaining power while needed and cutting its own power after the desired action has been performed. It is intended to save as much power as possible when the circuit doesn't do anything.

A 555 circuit would not do since it would need to be powered on permanently.

That's why I said a FET would do. Probably any other transistor, but the FET is closer to my energy saving goal. Here's what I imagined :

.------------------------. VCC o-+------+ +----+ | | --- | "Slave Circuit" | | ^ | | | | | | | / | | Something here keeps | +--/ o--+----+ the FET conducting | | | as long as needed. | .-. | | | | | | | | | | '-' | | | | | | | | GND o----------+----+ | '------------------------'

When the user presses the switch, the circuit is powered and in turn pulls the FET gate up. The user can release the switch at this time. At some other time in the future, the circuit decides it does no longer need power and lets the gate go down, cutting power and hopefully consuming zero current. I need to figure out if this would work and what kind of transistor to use.

vic.

On Fri, 27 Mar 2009 03:19:56 +0100, vic wrote:

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My microwave oven does that. It uses no power when off. The timer is mechanical.

Here's a generic circuit to show the idea:

N/O N/O +12 ---+---o--- Rly1-1 Rly1-2 ---o---+ | ^---o------+ +---^ | o | | +--- To your o | +-------------- circuit | | +----->|----------+ | ___|___ +---[R]---+---| 555 | | | | |---+----+ | | | | | | | | ------- | |k | /c | [Rly1] [D1] +--[R]---| | | | \\e | | | | | | | Gnd -------------+-------+-------+----+

Use a 555 as diagrammed (not a schematic, just to present the idea) above. The pushbutton causes the npn to put a negative on the 555 trigger, and provides plus to the

555 while it is pressed. The 555 output goes high, and operates the relay, which takes over the role of providing

• to the 555. The 555 stays high for however long you set it, then drops, removing power. No reason you have to use a relay - you can use the idea with a fet or bipolar if you want.

More important: the asumption is that you want a timed delay. However, that may not be the case. You might, for example, have a motor that opens and closes a window, and you want the circuit to run until the window is fully open (or fully closed), then shut off. Or a circuit that is indended to bring temperature up to some point, then shut off. The problem is that the subject of your post says "delay", but the requirement specifies that some action has been completed. So to make it a bit more generic: S1 or Sensor +12 ---+---+ N/C | | | V | -------o---- Rly1-1 N/O | o---^ | | +----o o---+ | [Rly1] | Gnd --------------+

S1 is a microswitch or a sensor that interrupts the circuit when the action is complete.

Ed

"vic the prick "

** So it's a toaster - right ??

.... Phil

Use a PMOS fet as a pass element. Connect the source to the input voltage, and the gate to the input voltage through a 1MEG transistor. Use the drain as the supply for your circuit.

Have two connections to the gate of the FET, one to a normally open switch connected to ground, and the other to a port on your microcontroller.

Push the button to power up. When the uc initializes, have it set the port to 0 output.

When you want the circuit to turn off, have the uc set the port to high impedance.

Regards, Bob Monsen

PNP or PMOSFET Vin--+---- ---------- ! e\\ / ! ---- ! ! -/\\/\\-+ ! \\ / \\ ! N-MOSFET ----+-!! ! !!--------------Port on micro PB ! O -!! --! ! ! O GND ! GND