Here is a simple and low-cost cir-cuit of heart beat monitor using readily available components. It uses the piezo electric plate of audible piezobuzzers as the sensing device, which can be purchased for around Rs 2 only from component vendors. The sensor is pressed against human body near the heart region. It should make a solid contact with your palm to convert heart beat sound into low-frequency electrical variations. These electrical variations are amplified by transistor T1 that is configured as a common-emitter amplifier. Amplified signals are coupled to tran sistor T2 for driving the audio power amplifier stage. The speaker reproduces heart beat notes as audible sound.
Circuit diagram :
The two BEL188 silicon transistors used in the power output stage are freely available. In case you use AC188/128 germanium transistors in place of BEL188 silicon transistors, replace 220-ohm resistors with 47-ohm resistors and 680-ohm resistors with 1-kilo-ohm resistors.
Author : Pradeep G. - Copyright : EFY