On 6/4/2014 10:13 AM, Jim Thompson wrote:
On Wed, 04 Jun 2014 02:03:47 -0400, ehsjr wrote:

On 6/2/2014 7:43 PM, Jim Thompson wrote:
Anyone make an off-the-shelf timer that I can trigger and it'll run
for X-hours then operate a relay?

Or do I have to roll my own?

...Jim Thompson


Start with something like this for $5.00:
http://www.harborfreight.com/lamp-an...mer-40148.html
and add whatever roll your own trigger you want. Plug a wall
wart into it to power the relay.

Ed

How do you _retrigger_ a mechanical timer ?:-)

...Jim Thompson


A flip flop and a one shot:

Modify it to stop after 24 hours. Your roll your own trigger
circuit (FF + triac, relay, whatever) latches on, but you'll
need to add something mechanical to the appliance timer to reset
your circuit when the timer reaches 24 on the dial.
Functional diagram:

-----------
input pulse---|Jim circuit| -----
| | |Timer|---timer ac out---relay
AC in---|ACin ACout|------| |
| | | |---+
----------- ----- |
| |
+---------Reset--------+

Input pulse to flip flop, one shot to reset it comes to mind,
but I'll call it your circuit as it will include more than
a flip flop.

The input pulse turns your circuit on, which sends ac to the
timer, starting it. After the timer has produced AC out to
the relay for the amount of time you set, it drops timer ac out,
but keeps turning, because your circuit has not been reset.

If you can mod the timer to trigger a one shot when the dial
hits 24 to produce a reset signal to turn your circuit off, that
will leave the timer at 24, awaiting the next trigger pulse.

Your original post mentioned a duration of up to 23 hours,
implying an interval between trigger pulses of greater than one
day. If that interval is less than a day or you can't mod the
timer, then this approach is no good for you.

Ed