>Before starting the design I would like to see a couple of keyboards
>as an example... Can anyone address me to some product using infrared
>led and transistors as buttons?
When someone asks about non-contact keyboard entry, the first question
should be, is the operator using gloves (or mitten) or not.
If the special signal generated by a signal source is actually
reflected back by the finger, then you could determine, if the finger
is close to the button or not.
But what about a glove with low reflectivity ?
Paul
Reply by Tom●September 9, 20092009-09-09
In article <9534d794-2aca-4c16-96ce-680eb5b6e432@p10g2000prm.googlegroups.com>, linnix <me@linnix.info-for.us> wrote:
>On Sep 9, 3:11=A0pm, Enrico <zig...@libero.it> wrote:
>> Hi,
>>
>> > This is a confusing question, you cannot use led/trx as buttons ?
>>
>> yes, that's exactly what I would like to do.
>>
>
>I misunderstood it as well. Yes, IR detectors (look like LED, but not
>really emitter) can sense IR from fingers. Some come with integrated
>transitor pre-amp. Even so, you need additional amplifier before
>wiring it to your logic block, whatever that may be.
The way I understood it was that he wants each button to have one infrared LED
and one infrared phototransistor. Both of them point upwards at the finger.
When the finger is absent, the LED shines up at the ceiling and very little is
reflected back. When the finger is present, a lot of light is reflected back
and this triggers the phototransistor.
There's no reason why this shouldn't work, however there are a few pitfalls.
Most importantly, what happens when the sensor gets dirty? If it doesn't get
cleaned, too little will get reflected back and the sensor won't work. Next,
there might be 60 Hz interference from ambient lighting which generates false
triggering. This can be avoided if the LED output is modulated at several kHz
and the phototransistor is tuned only to that frequency. Modulating each of
the five keys at different frequencies will also reduce possible crosstalk
between adjacent keys if they're physically close together.
Reply by Paul Keinanen●September 9, 20092009-09-09
>On Sep 10, 2:28�am, Paul Keinanen <keina...@sci.fi> wrote:
>> On Wed, 9 Sep 2009 01:22:22 -0700 (PDT), linnix
>>
>> <m...@linnix.info-for.us> wrote:
>> >On Sep 9, 3:11�pm, Enrico <zig...@libero.it> wrote:
>> >> Hi,
>>
>> >> > This is a confusing question, you cannot use led/trx as buttons ?
>>
>> >> yes, that's exactly what I would like to do.
>>
>> >I misunderstood it as well. �Yes, IR detectors (look like LED, but not
>> >really emitter) can sense IR from fingers. �Some come with integrated
>> >transitor pre-amp. �Even so, you need additional amplifier before
>> >wiring it to your logic block, whatever that may be.
>>
>> The finger at about 300 K will have a black body radiation peak at
>> about 10 um.
>
>Not sure what this 10um is? Distance?
Wavelength.
10 �m = 30 THz
Sorry for using um instead of �m to help news readers that are not 8
bit compatible.
>
>>Are you really saying that some cheap detectors will
>> detect the presence of the finger without some cryogenic cooling of
>> the detector ?
>
>Yes, certain IR detector tuned to around 600nm will change around 10%
>to 20% in resistance, if I recall it properly.
600 nm is orange (i.e in the middle of the visible spectrum, not IR),
how well would such detector work with a non-white population ?
Paul
Reply by linnix●September 9, 20092009-09-09
On Sep 10, 2:28=A0am, Paul Keinanen <keina...@sci.fi> wrote:
> On Wed, 9 Sep 2009 01:22:22 -0700 (PDT), linnix
>
> <m...@linnix.info-for.us> wrote:
> >On Sep 9, 3:11=A0pm, Enrico <zig...@libero.it> wrote:
> >> Hi,
>
> >> > This is a confusing question, you cannot use led/trx as buttons ?
>
> >> yes, that's exactly what I would like to do.
>
> >I misunderstood it as well. =A0Yes, IR detectors (look like LED, but not
> >really emitter) can sense IR from fingers. =A0Some come with integrated
> >transitor pre-amp. =A0Even so, you need additional amplifier before
> >wiring it to your logic block, whatever that may be.
>
> The finger at about 300 K will have a black body radiation peak at
> about 10 um.
Not sure what this 10um is? Distance?
>Are you really saying that some cheap detectors will
> detect the presence of the finger without some cryogenic cooling of
> the detector ?
Yes, certain IR detector tuned to around 600nm will change around 10%
to 20% in resistance, if I recall it properly. But it might not be
just the finger, but the hand and arm behind it. In theory, a single
button IR detector might work, but not for 5 buttons.
Again, capacitive sensors would work better for multiple buttons.
>
> Paul
Reply by Paul Carpenter●September 9, 20092009-09-09
In article <DKWdnYfBXLBWvzXXnZ2dnUVZ_vOdnZ2d@forethought.net>, don <don>
says...
> Paul Carpenter wrote:
> > In article <8c1acec3-88ef-4819-a34b-
> > 31e68498e23f@m11g2000yqf.googlegroups.com>, zwsdotcom@gmail.com says...
> >> On Sep 8, 10:23 pm, -jg <jim.granvi...@gmail.com> wrote:
> >>
> >>> * reflective sensor, that looks for fingers - this needs a surface
> >>> above the sensor for natural operation.
> >> Why does it need a surface above the sensor? It can detect the
> >> difference between "nothing in proximity" and "finger in proximity".
> >
> > And you would never see ANY other IR source that might trigger it
> > like the sun, fires, bodies close by........
> >
> How hot does a body need to be for an IR diode to detect it ??
>
> How close does a fire have to be for an IR diode to detect it ??
Considering
a) as other posters have said the low level to datect a finger
the tip being about a couple of square cms, we are talking
of a very sensisitive sensor.
b) this thread started with the requirement for 4 to 5 such IR
'buttons'
c) finger tips are connected to fingers to a hand, a VERY much larger
surface area, how far apart do all the IR 'buttons' to avoid
incorrect operation of the other IR 'buttons'?
d) At the levels of sensitivities required without suitable shielding
I am sure the IR from overhead office/home lighting would give false
triggers. Due to its relative closeness and amount of the source.
e) when set to a low sensitivity level for a finger tip it will easily
overloaded by power levels from extraneous sources.
Closeness to a prticular temperature is not a simple figure, as it
is based on what the IR response to frequencies and area of sensor
with sensitivity to amount of power received. Similar to visible light
photodiodes. Then if you amplify it for your lowest level signal
you have a lot of processing to sort out false readings and
effective sensor overload.
This is more suited to proximity detectors like Qprox and others..
If doing IR you really need to do a lot of shielding to control
which IR light is being detected and preferably by some form
of beam interuption, where the beam is modulated in some form
to determine the beam is there compared to background.
--
Paul Carpenter | paul@pcserviceselectronics.co.uk
<http://www.pcserviceselectronics.co.uk/> PC Services
<http://www.pcserviceselectronics.co.uk/fonts/> Timing Diagram Font
<http://www.gnuh8.org.uk/> GNU H8 - compiler & Renesas H8/H8S/H8 Tiny
<http://www.badweb.org.uk/> For those web sites you hate
Reply by don●September 9, 20092009-09-09
Paul Carpenter wrote:
> In article <8c1acec3-88ef-4819-a34b-
> 31e68498e23f@m11g2000yqf.googlegroups.com>, zwsdotcom@gmail.com says...
>> On Sep 8, 10:23 pm, -jg <jim.granvi...@gmail.com> wrote:
>>
>>> * reflective sensor, that looks for fingers - this needs a surface
>>> above the sensor for natural operation.
>> Why does it need a surface above the sensor? It can detect the
>> difference between "nothing in proximity" and "finger in proximity".
>
> And you would never see ANY other IR source that might trigger it
> like the sun, fires, bodies close by........
>
How hot does a body need to be for an IR diode to detect it ??
How close does a fire have to be for an IR diode to detect it ??
don
Reply by Paul Keinanen●September 9, 20092009-09-09
>On Sep 9, 3:11�pm, Enrico <zig...@libero.it> wrote:
>> Hi,
>>
>> > This is a confusing question, you cannot use led/trx as buttons ?
>>
>> yes, that's exactly what I would like to do.
>>
>
>I misunderstood it as well. Yes, IR detectors (look like LED, but not
>really emitter) can sense IR from fingers. Some come with integrated
>transitor pre-amp. Even so, you need additional amplifier before
>wiring it to your logic block, whatever that may be.
The finger at about 300 K will have a black body radiation peak at
about 10 um. Are you really saying that some cheap detectors will
detect the presence of the finger without some cryogenic cooling of
the detector ?
Paul
Reply by linnix●September 9, 20092009-09-09
On Sep 9, 6:35=A0pm, Enrico <zig...@libero.it> wrote:
> Hi,
>
> > I misunderstood it as well. =A0Yes, IR detectors (look like LED, but no=
t
> > really emitter) can sense IR from fingers.
>
> Ok.
>
> > =A0Some come with integrated
> > transitor pre-amp. =A0Even so, you need additional amplifier before
> > wiring it to your logic block, whatever that may be.
>
> Got it, thanks.
>
> I read a bit what the literature says about IR detectors:
>
> basically they are bipolar transistors without the Base terminal: on
> receiving IR light they drain some current from the Collector
> terminal.
>
> Now, the problem is to understand whether or not, the signal shown at
> the Collector terminal is digital or not.
>
> My guess is:
>
> 1) If the injected IR light in the Base is intense, the transistor
> saturates and the voltage at the Collector terminal should goes down
> to 0.3 Volts.
>
> 2) If the injected IR light in the Base is NOT intense, the transistor
> DOESN'T saturate and the voltage at the Collector terminal is
> unpredictable.
>
> Enrico
The IR from a finger is very weak but detectable. It will not
saturate the transistor. You must add amplifying circuits. This
might not be any cheaper or better than capacitive sensing, for
example.