digital heart beat monitor

7-96 elektor july/august 1980 LS 1501) 2 12 V R3 5 R4 R6 12 V0 ioo= R11 ..-012 V 2 fir ;* 16 80071 80145 Recently we published a circuit for a digital heart beat monitor that was sent in to us by Mr. P. Leah. We have since improved on it in places, brought it up to date and designed a printed circuit board for it. The AND gate N6 and the three NOR gates N7 ... N9 of the original circuit have been replaced by one NAND gate and a separate (4528) IC. In addition, the circuit around IC8 has been modified to such an extent that a preset pulse is now generated at every zero crossing (pin 15 con- nected to pin 14). In addition to this, the circuit of the sensor amplifier has been completely revised. The first section is now self-adjusting within a large intensity range and the second opamp is connected as a schmitt-trigger with self-adjusting threshold levels. The display now uses the more modern 7760 type of seven segment indicators from Litronix and HP. Finally, a loud- speaker has bee...