Phototype sensor and IR signals

Hi,
I found the following circuit that reads the skin phototype. I assume that uses the principle of refraction. The amount of signal emitted by the IR transmitter reflected from the light skin is different from that reflected from dark skin. So the IR receiver will give different values ​​depending on whether the skin is lighter or darker. These voltage values ​​appropriately interpreted by a microprocessor, are translated into phototype 1 (light skin) to skin type 5 (dark skin). I got the wiring diagram:



I have the following questions:

- What is the function of OUT2 (suitably amplified and connected to the microprocessor)?

- What kind of IR transmitters and receivers are used? (See photo)



Best Regard

tnnedaboard said:

Hi,
I found the following circuit that reads the skin phototype. I assume that uses the principle of refraction. The amount of signal emitted by the IR transmitter reflected from the light skin is different from that reflected from dark skin. So the IR receiver will give different values ​​depending on whether the skin is lighter or darker. These voltage values ​​appropriately interpreted by a microprocessor, are translated into phototype 1 (light skin) to skin type 5 (dark skin). I got the wiring diagram:



I have the following questions:

- What is the function of OUT2 (suitably amplified and connected to the microprocessor)?

- What kind of IR transmitters and receivers are used? (See photo)

Best Regard

Click to expand...

The circuit in your schematic has two outputs as the output voltage from LM358 opamp is read against an "artificial ground" of ~1.6 V generated by the voltage divider from +5V.
Equally well you can simply connect the IR photodiode from +5V through a ~10 kOhm resistor to ground, and read the voltage from the diode. It will vary but not from the "artificial zero" like in the schematic.
The IR LED and photodiode are apparently taken from a transmit sensor, and as you have no specifications about the LED spectrum, photodiode spectral response and about the skin reflectivity to determine the skin color (in IR), you will have to experiment. You can also try a visible-light LED and photodiode, a photo-resistor, etc.

If you do not know how the circuit should work, start with the basics: You can light up a LED, and use a DVM as an ohmmeter to see the resistivity of a photodiode or a photoresistor varying with LED illumination, and skin reflection. You will see the photodiode blinded by ambient light, and for testing skin reflectivity, you will possibly need a dark "chamber" to remove ambient light.
Instead of such "making darkness" you can modulate the LED source, and extract and process the audio output from your photodiode to find what you need.

Forget about microprocessors, such devices can be used only after you know what your sensor really responds to. Use an analog voltmeter to get good results.