Using infrared leds and transistors to make a keyboard

TV

Seriously, the transmiter can be a simple PWM port pin from a micro (you are in c.a.e.) into the base of a transistor buffer. The receiver would need a few amplificating transistors feeding ito the timer input of the micro.

This is a confusing question, you cannot use led/trx as buttons ?

- but you can set up a light sense path, which can be either

• Opto interrupter, with a beam-break button/plunger eg Digikey 425-1964-5-ND or

• reflective sensor, that looks for fingers - this needs a surface above the sensor for natural operation.

eg Digikey GP2S60 PHOTOINTERRUPTER REFLEC 1MM SMD 34c/100+

-jg

Hi,

yes, that's exactly what I would like to do.

I understand what you mean about, but that's not suitable for my needs.

That's interesting!

Thanks a lot pal for your suggestion.

Enrico

In message , Enrico writes

OK..

Possibly, but how about this,

Far more interesting to use the LED as indicator and sensor.

--
Clint Sharp

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.

Hi,

Ok.

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

Why does it need a surface above the sensor? It can detect the difference between "nothing in proximity" and "finger in proximity".

If you get an ordinary transistor in a metal case, something common like a 2n2222, and carefully cut the top of the case off, you have a phototransistor... oh course not one optimized for the task.

Additionally, many LEDs are at least weakly photodiodes. Try putting an ohm meter on one and holding it close to a lightbulb, then in the dark.

And you would never see ANY other IR source that might trigger it like the sun, fires, bodies close by........

--=20 Paul Carpenter | snipped-for-privacy@pcserviceselectronics.co.uk PC Services Timing Diagram Font GNU H8 - compiler & Renesas H8/H8S/H8 Tiny For those web sites you hate

t

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.

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

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

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
    PC Services

Not sure what this 10um is? Distance?

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.

Wavelength.

10 µm = 30 THz

Sorry for using um instead of µm to help news readers that are not 8 bit compatible.

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

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.

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

... would be a catastrophic misnomer. 600 nm is way off from being infrared.

I don't know about that sensor specifically but I've not had great difficulty with ambient. Mostly in CCFL-lit rooms though now I think of it.