Description In all the houses exist the bells in the door. All want, they have the possibility of being possible to change the intensity, the tone of sound. With this circuit we have this possibility. With the materials round the gates, we can change the sound. For the materials that exist in the circuit the frequencies are roughly 5 KHZ and 2 KHZ. The circuit functions only for small time interval afterwards the pressure of switch. Simultaneously with the sound we have also optical clue from LED D2. Changing the capacitors C2-C3, with different price of capacity, we create new sounds. Circuit diagram Part List R1=470Kohm R2=22Kohm R3=3.9Kohm R4-5=330 ohm R6=2.2Kohm R7=1.2Kohm C1=22uF 25V C2=470uF 25V C3=1 uF 25V D1=1N4001 D2=LED RED 3mm IC1=7413 Q1-3= BC 550 Q2= BC 560 BATT=4X1,5V or external pack 6Vdc SP= Speaker 8R /0,5W All Resistors is 1/4W 5% circuit from http://users.otenet.gr/~athsam/melody_door_ringer.htm