Inexpensive/compact "Electron Hourglass" ?

[petemoore]

 http://www.apogeekits.com/start_stop_timer_vm141.htm
  100ma. not so good for battery use, expesive in a few ways.
  There's probably a simple mousetrap for battery current, after a time a switch lifts...]like the bathroom fan timer switches, also too large and expensive], I've just never heard of one or seen one..
  Something like a rubber band snap switch with a wet leather timer...when the leather dries out, it pulls the stop that held the %^&*ed popsickle-stick-throw away from the pole, lifting the power connection...something a little stupider or with a fancy spring might be better.
  1008 electronic switches, old style stove and bathroom fan timers for ACwall uses, other timers...no simple little wind-up-and-it-breaks-after-a-while affair for small voltages that I could find.
  Seems like it'd make one fine and handy gadget if it could be set for say a little over 4 hours.
   

[R.G.]

http://www.apogeekits.com/start_stop_timer_vm141.htm
  100ma. not so good for battery use, expesive in a few ways.
  There's probably a simple mousetrap for battery current, after a time a switch lifts...]like the bathroom fan timer switches, also too large and expensive], I've just never heard of one or seen one..
  Something like a rubber band snap switch with a wet leather timer...when the leather dries out, it pulls the stop that held the %^&*ed popsickle-stick-throw away from the pole, lifting the power connection...something a little stupider or with a fancy spring might be better.
  1008 electronic switches, old style stove and bathroom fan timers for ACwall uses, other timers...no simple little wind-up-and-it-breaks-after-a-while affair for small voltages that I could find.
  Seems like it'd make one fine and handy gadget if it could be set for say a little over 4 hours.
 

You want fries with that? 

The obvious, one-chip answer is one of the eight-pin PICs. For $1.00, you get a chip which accepts a logic signal (like from a pushbutton switch) and then stays on for X time, producing a logic signal out that turns on a transistor or relay to power something for X time.

The other way, more complicated and difficult, is with a CMOS 555 or a CD4060.

[petemoore]

  How about a Mosfet chosen to handle sufficient current through D/S, and the gate biased in such a way that a capacitor charged to 'X' voltage, drains slowly through a resistor, when the capacitor-and-gate drop to a certain voltage, the D/S current is shut off.
  By varying the capacitor drain resistor value or the starting 'timer-voltage' of the mosfet, various current-pass times could be created, timed and charted [roughly].
  At least thats the unchecked theory so far.

[R.G.]

  How about a Mosfet chosen to handle sufficient current through D/S, and the gate biased in such a way that a capacitor charged to 'X' voltage, drains slowly through a resistor, when the capacitor-and-gate drop to a certain voltage, the D/S current is shut off.
  By varying the capacitor drain resistor value or the starting 'timer-voltage' of the mosfet, various current-pass times could be created, timed and charted [roughly].
  At least thats the unchecked theory so far.

The MOSFET would go through a linear region as the gate drained down, which might be OK, depending on the transconductance of the MOSFET. The hard part with that circuit is getting the voltage to only drain down after some hours. Leakage is a problem in all sample-hold circuits - which is an elaboration of what this is - and it gets severe for hold times of seconds if you need accuracy.

It's a worthwhile place to experiment, certainly, but put some thought into how long the "hold" capacitor on the gate will keep its charge up.

I did some thinking about the CMOS version. You can build it with a CD4060 oscillator/divider, a CD4013 dual flipflop, and a MOSFET or transistor pass device to actually do the switching. In this version, you run the oscillator of the CD4060 and the divider section divides by 8192 or 16384. One section of the 4013 is used as a set-reset flop. You set it by a switch or some other action, and this lets the oscillator and divider run. When the divider gets to full count, it resets the 4013 and everything goes to sleep again until retriggered. You get either polarity of signal out of the 4013 to turn on your switch. A MOSFET will work directly from the 4013 output, a bipolar will need buffering. For a four hour timeout, you need the oscillator running at 1.14Hz, which can be done. You also get most of the powers-of-two divisions of the timing out of this; four hours max, two hours, one hour, 30 minutes, etc., by connecting the reset to a different divider output pin. You can also vary the value of the timing resistor to get shorted delays in smooth variations.

It can be rigged to trigger by application of DC power. It uses microamps when it's not running, so it could also be left on all the time and only use power when it's letting the effects be powered.