In this paper, we present hardware-software methodology to measure and calculate heart and respiratory rates by using single, low-cost microcontroller chip of limited computation and memory performances, as ATTINY85. Sensors and analog front-end are very simple, directly interfaced to microcontroller pins. Implemented time and frequency domain signal processing algorithms are optimized for low-bits, low-memory architectures and allow fast reading of rates, provide satisfactory accuracy, noise immunity and low power consumption. The same methodology can be used in similar applications to determine dominant spectral frequency of slow signal, from a smaller number of points.