Hello! I have bought the INFRARED DETECTOR 5MM 8MA COLLECTOR PHOTO TRANSISTOR from Sayal. The number is KID-7404-1P. The IR phototransistor looks exactly like an LED. I wanted to make a heartbeat detector with Arduino by shining an IR LED through my finger, and have the IR phototransistor pick up the fluctuations in the IR light caused by the pulse of the blood. I connected the emitter (small leg) of the IR phototransistor to GND, the collector to an analog input pin and a 100k ohm resistor to 5V. However, it cannot detect the 880nm IR LED's light. I made sure that the LED was actually turned on. Am I connecting the IR phototransistor incorrectly? How should I position the IR LED and phototransistor so I can use it to detector my heartbeat with minimal interferance? Thanks!
dkl65: However, it cannot detect the 880nm IR LED's light.
What do you read on the analog input when the phototransistor is a) in dark b) and when it is irradiated by the Sun, a lamp, or your infra LED?
a) To be tested.
b) Actually, it can now detector the IR LED when shining on it directly. If the IR Phototransistor and IR LED are touching, the analog read is about 1013. When farther away (about 5 cm) the value is about 130. When I shine it through my finger, the value is about 35, but I cannot detect fluctuations similar to a heartbeat. All of this was done at around 5pm, with the blinds closed, but some sunlight coming in.
Is the total dark reading > 35? with this low reading the delta between the pulse high and low may be below the threshold of detection.
To get any sort of reading you need to use a differential amplifier. Google for circuits, try this for a start:- http://electronicdesign.com/article/components/build-your-own-optical-heart-rate-sensor6343
just like transistors they are pretty much variable from 0-saturation, even for encoders, a digital situation, you have to add a comparator (or equvilant software), never dealt with a heart rate do-dad but I would take mike's advice as the signal its producing is probably too low for the ADC to properly register