Pulse beat monitoring sound and light alarm circuit

Working principle of the circuit: Usually we touch the pulse on the wrist with our fingers and can feel it beating. This device picks up this signal
with a sensor and converts it into an electrical
signal, and then displays it in the form of sound and light
. Figure 134． This is the circuit schematic diagram of the pulse beating sound and light
alarm. The circuit is mainly composed
of 74LSOO four-two input NAND gates
IC1-IC4, VT, piezoelectric ceramic sensors
HTD1-HTD2 and other components. It can be divided into 4 parts:
sensing amplifier circuit , shaping
amplifier circuit, sound alarm, and light-emitting alarm circuit.     (1) Sensor amplifier circuit The sensor amplifier circuit is composed of an inverter 1C1. To pick up the pulse signal, the sensor HTD1 made of piezoelectric ceramics is used . Since the electrical signal sent by the sensor is extremely weak, an amplification circuit with high input impedance must be used. Applying the IC1 inverter as an analog device, as long as appropriate linear bias is added, a high-impedance amplifier is formed. The feedback resistor Ri connects the input terminal and the output terminal of IC1 to form an amplifier circuit with negative feedback. The capacitor C. Its function is to short-circuit the high frequency to prevent the amplifier from self-oscillation.     (2) The inverter of the shaping amplifier circuit IC2 acts as a shaper and works in a switching state. If the output level of IC1 is higher than the turn-on level of IC2, IC2 will output a low level "o"} If it is lower than the turn-on level of IC2 level, IC2 will output a high level "1". If the value of the feedback resistor Rz of the second-stage amplifier circuit is properly selected, the output of Icl will just exceed the turn-on level of IC2. Once the sensor has a weak signal output, it will output a high level "1" after amplification by IC1 and shaping by IC2. When IC2 is in static state, it outputs low level "0".     (3) Sound alarm circuit The sound and light-emitting tube drive circuit is composed of 1C3, IC4 and transistor VT. IC3. and IC4 form a controlled multivibrator. When IC2 outputs a high level , the oscillator starts to oscillate. By adjusting the resistors Y, Y and capacitor G, the oscillation frequency can be changed. At the same time, IC3 drives any electric ceramic chip HTD2 and sends out an audible alarm signal.     IC4. Connect the transistor VT to drive the light-emitting diode to emit light. At the same time, connect a speaker to the collector of VT for double alarm.