random tune doorbell

The circuit diagram of the `random tune` doorbell is shown in figure 1. As can be seen, it basically consists of two squarewave generators, a counter, and a current controlled oscillator. The fre-quency of the first squarewave genera-tor (Nl/N2) can be varied between ap-prox. 12 and 900 Hz, whilst that of the second generator (N3/N4) is roughly 1 kHz. The counter, IC3, is enabled when the clock enable input is taken low. However due to the integrating effect of C3/R4, the negative going edge of the first squarewave enables the counter for only a brief period. The count is only incremented when a positive going edge from N3/N4 coincides with a negative pulse from Nl/N2.