On Tue, 20 Nov 2012 13:08:02 -0800, mike wrote:

On 11/20/2012 12:31 PM, Jim Thompson wrote:
On Tue, 20 Nov 2012 05:15:02 -0800 (PST),
wrote:

Hi all,

I seem to recall from distant memory that it's possible to make a pulse detector out of two of transistors and a few resistors, but I can't find the circuit anywhere. Searches keep throwing up stuff about astable multivibrators and nand gates and that's not what I'm looking for.

Just need to be able to detect a 1 second pulse of 5v on an otherwise 0v line. The pulse comes only after several days of inactivity and should turn an LED on permanently. No reset required and any subsequent pulses must *not* turn the LED off again, so it's operating like a bistable latch (if that's the right term).

Can anyone assist?

The 1 second requirement significantly complicates the solution.
If any old pulse will do, use a latching relay...either magnetic latch
or feedback from the contacts.

... or just a resistor (eg. 4K7) to drive the gate of a 2N5064, in turn
drive an LED with series resistor (4 parts). It will detect a positive
pulse of any length greater than a couple microseconds by latching on,
and thence cease to care about the state of the input, so the LED will
stay on until the power is cycled.

As always, the devil is in the details.
How much current can you get out of the 5V pulse?
What voltages are available when the pulse is not present?
How do you expect to reset the thing?
What are the specific details of the 1 second requirement?
Timing accuracy and boundary conditions?
Assume you wouldn't have mentioned it if it weren't important.


Tia.

[snip]

Along these lines, what would the group recommend for a weatherproof
switch to detect that a yard gate is not closed all the way?
magnet/reed switch from a burglar alarm door/window ajar sensor.

Should be battery powered sensors with RF interface available for
security.

...Jim Thompson