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Showing posts from February, 2011

555 timer could reduce number of pins in keypad interfacing

A 4x3 matrix keypad requires 7 I/O pins of microcontroller for interfacing. But you could reduce the required number of connections to two by using a 555 timer IC. If you want to know how, visit Embedded Lab's new post "2-Wire Keypad Interfacing using 555 Timer".

Heart rate measurement through optical sensors

Resting heart rate is an very important health parameter that is directly related to the soundness of human cardiovascular system. This project describes a digital method of measuring heart rate through fingertip. The blood volume inside the finger artery fluctuates with heartbeats. This fluctuation can be measured by transmitting an IR light through the finger. A portion of this light is reflected back. The amount of light reflected back depends upon the blood volume. This small change in the reflected light is amplified through proper signal conditioning circuit and converted into a pulse. Later, a PIC16F628A microcontroller is used to count the pulses at the output of the signal conditioner and display the heart rate on seven segment LEDs.


The signal conditioning circuit uses two operational amplifiers to build a two-stage active low-pass filter with a gain of about 10000, and a cut-off frequency close to 2.5 Hz.