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This circuit is a doorbell with counter , that means it counts how many times the bell is triggered . It uses a chip synthesizer sound, the HT -2811 ms nothing prevents you from using another device sound generator . The HT -2811 reproduces the sound of a " ding-dong" doorbell . Besides this ic , the circuit includes a CMOS 4026 and Display Driver IC which counts the total number of visitors .  Electronic Doorbell with Counter Circuit Diagram

This is a simple project of Electronic Voltmeter Circuit Diagram.This circuit provides a simple means to determine the voltage of a low-impedance voltage source. It works as follows. P1, which is a 1-W potentiometer, forms a voltage divider in combination with R1. The voltage at their junction is buffered by T1, and then passed to reference diode D1 via R3. D1 limits the voltage following the resistor to 2.5 V. An indicator stage consisting of T2, R4 and LED D2 is connected in parallel with D1. As long as the voltage is not limited by D1, the LED will not be fully illuminated. This is the basic operating principle of this measurement circuit.  Electronic Voltmeter Circuit Diagram   

Here is electronic project of Simple Electronic Door Buzzer has a very simple function and is very easy to build.  This door buzzer circuit use a LF351 operational amplifier and other few common electronic components.  When the S1 push-button is depressed an initial positive voltage is placed on the C2 and the non-inverting terminal of the LF351 operational amplifier and the circuit will oscillate at a low frequency. Simple Electronic Door Buzzer Circuit Diagram: The used speaker for this project must have a impedance between 40 -80 ohms.  The door buzzer electronic circuit needs to be powered using a 9 volts DC power supply.