Re: Delay on a mains-powered relay?

From: Rob Gaddi (rgaddi_at_bcm.YUMMYSPAMtmc.edu)
Date: 01/05/05 

Date: Wed, 05 Jan 2005 11:08:04 -0600

Cheap microcontroller circuit, grounded to the low side of the wireless
sensor's sense contacts. Use the existing relay to signal the uC, and a
mosfet to open the connection. Depending on how much power the wireless
is providing to monitor the sense you may even be able to power just off
that, but if not, decently careful design on a ultra-low power
microcontroller should allow you to run for practically the shelf life
of the battery.

DaveC wrote:
> So the answer is... no.
>
> If power fails at the remote site, so does the control circuitry and its
> power. So a traditional delay relay won't work.
>
> The relay, powered by mains, simply opens the wireless sensor's sense circuit
> when power goes away. I just want it to do so after waiting a few minutes to
> see if power comes back on.
>
> The alarm isn't for security purposes, just for letting those responsible
> know that certain things have occurred, one of which is power fail. The
> sensor is a wireless device run on a lithium battery, and is a packaged deal
> so has no spare power available. The relay's not near the alarm CPU, so that
> power isn't available, either.
>
> Suggestions for a simple mains-powered circuit that will open a pair of
> contacts about 5 minutes after de-energizing? If it's to be battery-powered,
> it needs to be super-low drain. I rather keep battery replacements down to
> every 2 years, at most.
>
> An idea that just struck me: the presence of mains power could keep this
> circuit de-energized. When power fails, the battery is connected to the
> circuit which starts the countdown. When zero is reached (ie, 5 minutes have
> passed), a pair of contacts would open. And the energizing circuit
> (triggering these contacts) would only have to be a one-shot; once the alarm
> is triggered, the circuit could de-energize. That way the battery is utilized
> only briefly, during power-fail situations. How might I construct such a
> circuit?
>
> Thanks,

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