IR Phototransistor and phototransistors

[arivalagan13]

Hello all,
Can I use L14G3 phototransistor to receive IR light from IR LED source?

Is there special IR phototransistors?

Regards
Arivalagan M

 

[FvM]

Every silicon photo sensor has it's maximum sensitivity in the near IR. If you read the datasheet thoroughly, you'll notice that some specifications are for a 940 nm IR source.

There are special IR sensors, also photo transistors with daylight filter (black plastic case). It can be advantageous in some situations.

 

[arivalagan13]

(1)I use the following circuit:
IR LED in series with 100ohm resistor at transistor.
IR detector with 10kiloohm resistor in reverse bias.

I shouldnt get any output in the absence of IR and when the signal is present i should get a positive voltage.right?

But, even in the absence of the IR am getting a positive voltage..

I donno whether the detector am using have daylight blocking filter or not..as the local vendor don have the part number.

How do I solve this problem?

(2) Am using the above set up for measuring the heart-rate using the near infra-red spectroscopy technique.(Passing IR rays through ring finger tip veins and receiving the reflected signal and processing)

Now, I've problems on how to go about mounting the sensors.Any idea or suggestions to do that would be of great help to me.

Thanks and regards
Arivalagan M

 

[Audioguru]

You said you have an "IR detector" with reverse bias. Is it a photo-diode or is it an LDR (light-dependent-resistor)?
If the 10k resistor is feeding the cathode of a photo-diode with a positive voltage then in the absence of IR it does not conduct and should give a positive output voltage. When IR is present then it conducts and its output voltage should be less than the supply voltage to the 10k resistor.

An IR photo-diode that blocks visible light has a black coating. A clear one also receives visible light.

Here is a project called HeMon:

 

[schmitt trigger]

(1)I use the following circuit:
IR LED in series with 100ohm resistor at transistor.
IR detector with 10kiloohm resistor in reverse bias.



Thanks and regards
Arivalagan M

The IR "detector" in reverse bias.....are you using the phototransistor as a diode? Do you have a spec sheet?

If it is used as a diode, and operated in the photocurrent mode (reverse bias), the actual photocurrent is very small. You require a transimpedance amplifier to boost its to a voltage of significant value.

Having said that, reverse bias photodiodes are the fastest detectors (with proper techniques, in the ten of microsecond range), and I don't think your application requires that much speed. Use it as a proper phototransistor. You'll trade off speed for higher gain.

 

[arivalagan13]

To answer you both:
1.First I tried with a photo transistor(part no.:L14G3) as IR receiver. Then I tried with a IR receiver( claimed by the vendor so). Probably it is a photo diode.
2.Audioguru, regarding Ur second point I get a similar response as I connected the way you suggested. Also it looks black and not a clear one. Probably one with day light blocking filter. I’ll do a rigorous test again and share the results with U.


Regards
Arivalagan M

 

[D.A.(Tony)Stewart]

Here is a typical IR heart rate design with 2 stage high gain LPF and 0.7Hz HPF.
**broken link removed**
In order to maximize signal to noise ratio, optical crosstalk from emitter to detector must be blocked by design with appropriate black apertures and geometry

Some crosstalk is expected so the 0.7Hz filter is essential. I can't vouch if this is optimal, but sounds right. Normally the receiver filter is matched to the spectrum of the source signal of interest for best performance.

 

[Audioguru]

First I tried with a photo transistor(part no.:L14G3) as IR receiver. Then I tried with a IR receiver( claimed by the vendor so).

The phototransistor detects visible and IR light. When its collector has a 10k resistor to a positive supply and its emitter is at 0V then its collector voltage is the positive supply when it is dark and is about +0.2V with bright light.

An IR receiver has an amplifier inside that is tuned to the remote-control frequency of about 38kHz. It does not detect DC light.

 

[D.A.(Tony)Stewart]

The IR Receiver is very sensitive but has AGC which may negate the modulation effect of blood flow, so it is not a good choice, unless you have access to the internal AGC voltage, which is not avail on 3 pin Rx.

So Using Photo transistors or photo diodes with black daylight filter and high gain with DC blocking is needed.