Heart Rate Monitor

John Beale  Dec. 2010   bealecorner.com

Here is a battery-powered heartrate monitor project I did back in April 1992. Below are a few photos of my log book.  It used a three-lead connection to get an electrocardiac signal (EKG) and then detected the QRS peak to send out a short range FM-band radio signal, once for each heartbeat.   A separate microcontroller connected to a FM radio logged the signal.  The main challenge was preventing the RF output signal from interfering with the high gain instrumentation op amp reading the low level (millivolt) EKG signal. I used separate shielding and RF chokes to make it work.

In the circuit as built, I  included an audio tone generator around 1 kHz to modulate the RF signal, and had a toneburst detector on the FM receiver to detect the signal.  You can see that circuit (through epoxy encapsulation) next to the RF transmitter circuit in the photo of the completed device.  Not shown are the body leads, three electrodes connected to a stereo miniplug going into the "op amp" section.

DC op-amp circuits for EKG

RF transmitter for EKG

complete EKG circuit

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