Re: Simple LED Alarm Bell for CCTV recorder

["Jackcsg" <nospam@xxxxxxxxxx>]

<me@xxxxxxxxxxx> wrote in message
news:lr9c31t6t1qgvqiijck5m2v8fmslnst55b@xxxxxxxxxx
> On Mon, 14 Mar 2005 17:35:52 -0500, "Jackcsg" <nospam@xxxxxxxxxx>
> wrote:
>
> >It was simple until you said you wanted the LED to flash. Are you good
with
> >a solder gun? It is easily done, but it's going to be a little intricate
if
> >you're only a DIY'er. Do you want the buzzer to be intermittent with the
> >LED, or a small steady buzz? The switches are easy because you can use a
> >DPDT switch to "shunt" the alarm device.
>
> I think a small steady buzz.
> Note that the CCTV DVR box has 8 wires all 500ma, 12volts each.
> All 8 wires from CCTV DVR would go into one black box that is drilled
> on the front, 8 LEDs, 8 switches (1 per sensor), and buzzer inside.
>
> There should be some way to handle the amp and voltage so it doesn't
> go back and blow up the CCTV box.
>
> I've forgotten everything they taught me in science classes. If I had
> a diagram I could show it to the people in Maplin.

The thing to keep in mind is that it is possible to have multiple alarms at
once. I'd be keeping it simple, and with the least amount of draw as
possible. The simplest way would be to come off the positive side of each
LED, with a diode gowing to the buzzer, (piezo) positive, and the other side
going to ground. This will prevent any voltage feedback to the DVR from the
current draw.
I'd keep the LED steady, along with the sound. If your dead set on
"flashing" an LED(s), I'd be using an external power supply and isolating
the voltages so you don't damage the DVR. Stay away from any
electro-mechanical piezo's, as they draw more power. The ceramic's draw
between 3 ma to 8 ma. You have more than enough power. Keep it simple.
K.I.S.S.
I would imagine all the grounds on the DVR outputs are common, unless the
unit switches to ground. Remember, and LED is a diode....
If you need a drawing, I'm sure I could put one together for you.