On Tue, 27 Dec 2016 00:12:04 -0000, Uncle Monster wrote:

On Monday, December 26, 2016 at 5:53:53 PM UTC-6, James Wilkinson Sword wrote:
On Mon, 26 Dec 2016 23:44:34 -0000, Uncle Monster wrote:

On Monday, December 26, 2016 at 12:01:49 PM UTC-6, James Wilkinson Sword wrote:
On Mon, 26 Dec 2016 14:22:02 -0000, FromTheRafters wrote:

Mark Lloyd explained on 12/25/2016 :
On 12/25/2016 03:26 PM, James Wilkinson Sword wrote:

[snip]

Hearing a 50Hz mains hum isn't that bad, but imagine hearing a higher
frequency.

Once I had a TV where the horizontal frequency (almost 16KHz) was audible. It
was unpleasant to be around for very long. IIRC, I sold it to someone with
hearing loss who couldn't hear it.

I could hear most of them.

What year?

I could also hear the detectors in stores
back when they were sonic. Not anymore though, and not because the
technology has changed, but because my hearing has changed.

Detectors?
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Back in the 1970's, ultrasonic motion detectors for security systems were quite common in businesses. The last of the old Polaroid instant cameras used an ultrasonic transducer to measure distance to the subject of a picture and focus the camera. There were some ultrasonic motion detectors at one time that detected a shopper in the isle at a retailer so an electronic advertising display could start playing music and the voice of a pitchman. I haven't seen one for a while but I could hear the harmonics and clicks. ヽ(€¢€¿€¢)ノ

What I think's strange is motion detectors stay on when the alarm is deactivated. Why don't the PIRs only run at night?
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The PIR motion detectors have and indicator light for a "walk test" to indicate that the unit is operating correctly and covering the area it's meant to. There is a jumper on the circuit board inside the PIR that can be moved to disable the walk test light but most security system installers just leave the jumper in place. ヽ(ヅ)ノ

[8~{} Uncle Infrared Monster

I know, but surely the main control unit should switch all the detectors off unless you either have the alarm activated or you select test mode. What a waste of electricity, wearing out the PIRs, and annoying people with lights and clicks.

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A great way to lose weight is to eat naked in front of a mirror. Restaurants will almost always throw you out before you can eat too much.