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The dual sound door bell without battery

2016-05-10 02:50  
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47.The dual sound door bell without batteryAs the popularity rate of phones is higher and higher, more and more homes have domestic phones, however, usage rate is low, therefore, it's economic to use the 48v(60v) DC current of phones as the working energy. Now we introduce a dual sound door bell without battery, the circuit principle is shown in the figure. From the figure, we can see that the circuit is changed from the common circuits of phone rings. a and b are the positive and negative poles of phone wires, respectively.