The following doorbell circuit creates a low tone that will glide up to higher frequency. The equal total resistance linked between the base of Q1 and ground (Rbg) , and coupling capacitor C1 influences the AF oscillator’s frequency. The resistance (Rbg) is equal to (R2+R1)R3. Here is the circuit diagram :
The R2 is used to set the initial bias condition, aligned to develop a pleasant low starting frequency doorbell tone. D1 will start to conduct when Capacitor C3 charge through R6 until it reaches D1 bias voltage level. Then the value of Rbg is paralleled by R4 and D1, and R5-D2-D3, and the values of diode’s equivalent resistance is gradually decreased as the C3 voltage ramp up. This falling resistance value make the output tone slides up in frequency. Two different diode course is provided to extend the linear area of diode conduction transition pitch. With two path with different biases, after the single diode path has saturated, the second path provide further linear increase at higher voltage level.