IR detector behaviour query

[Bruce Varley]

Trying to get a RPM7100 IR detector going (Jaycar cat no ZD-1952). I have
the IR source emitting a 38 KHz square wave, as specified, but the device
appears to only respond to sudden changes in the received light signal, ie.
if I hold it steadily in front of the continuously emitting diode, the
output stays high. The data sheet is fairly vague (increasingly common
problem?), but it does specify maximum, typical and minimum values for a
600uS 'subcarrier' (my term), suggesting that the device is designed to
activate if it receives a 38 KHz carrier that is pulsing on and off every
600 uS.

Checked some other sources, they all bring up the same datasheet.

Can anyone confirm what the behaviour of this device is? TIA

 

[Phil Allison]

"Bruce Varley"

Trying to get a RPM7100 IR detector going (Jaycar cat no ZD-1952). I have
the IR source emitting a 38 KHz square wave, as specified, but the device
appears to only respond to sudden changes in the received light signal,
ie. if I hold it steadily in front of the continuously emitting diode, the
output stays high. The data sheet is fairly vague (increasingly common
problem?), but it does specify maximum, typical and minimum values for a
600uS 'subcarrier' (my term), suggesting that the device is designed to
activate if it receives a 38 KHz carrier that is pulsing on and off every
600 uS.

Checked some other sources, they all bring up the same datasheet.

Can anyone confirm what the behaviour of this device is?

** The datasheet makes it clear the device is designed to sense IR light
with a 38 kHz intensity modulation rate further modulated at approx 833Hz
with a 50% duty cycle.

It then outputs an 833 Hz square wave.

Any more complex modulation pattern is then up to the designer.

Seems it should be inherently very immune to ambient IR light and various
other IR sources with 38kHz intensity modulation.

Did I hear CFLs being mentioned ??



...... Phil

 

[Alan]

On Fri, 7 Aug 2009 12:38:16 +0800, "Bruce Varley"
<bxvarley@weastnet.com.au> wrote:

Trying to get a RPM7100 IR detector going (Jaycar cat no ZD-1952). I have
the IR source emitting a 38 KHz square wave, as specified, but the device
appears to only respond to sudden changes in the received light signal, ie.
if I hold it steadily in front of the continuously emitting diode, the
output stays high. The data sheet is fairly vague (increasingly common
problem?), but it does specify maximum, typical and minimum values for a
600uS 'subcarrier' (my term), suggesting that the device is designed to
activate if it receives a 38 KHz carrier that is pulsing on and off every
600 uS.

Checked some other sources, they all bring up the same datasheet.

Can anyone confirm what the behaviour of this device is? TIA

A lot of the IR detectors require you to switch the carrier (38Khz in

your case) on and off to represent the ones and zeros that you want to
send. The spec sheet will probably tell you the maximum length of
carrier on and the minimum length of carrier off times that you need
to work with.

Perhaps look at: http://www.sbprojects.com/knowledge/ir/ir.htm

If you don't want to use a carrier then just get an IR photodetector
(diode or transitor) and use that.

HTH
Alan
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