two-tone chime

by P Sicherman two-tone chime EE 92 July/August 1986 171 This electronic chime is easily built from commonly available, inexpen- sive parts. Depression of the door bell button, S2, causes inverter Ti to pass a logic low level to NAND gate NI, which responds with a logic high level at its output, enabling the oscillator composed of N2 and N3 to toggle at about 1 Hz. Since buffer capacitor CI remains charged for some time after S2 has been released, the oscillator N1....N4 = IC1 = 4093 IC2 = LM386 will continue to provide 1 Hz pulses to C4 and Cs, as well as to a second oscillator section, composed of N4 and associated parts via Rs. A logic high level at pin 10 of inverter N3 enables T2 to connect preset P2 in parallel with frequency determining parts 127-Pt, which arrange the fre- quency of N4 to toggle at a few hertz. The two superimposed frequencies may be adjusted to individual taste with Pi and P2. HP LS 4.8% 5w I 86431 In addition to controlling the tone fre- quencies of the c...