Phone hang reminder

The telephone hang-up reminder introduced in this example can send a "feeding" reminder when the phone hangs up, telling the user whether the phone has been hung up.
Circuit Operation Principle The telephone hang-up reminder circuit consists of a trigger circuit, an inverter, a delay circuit, an audio oscillator, and an audio output circuit, as shown in Figure 3-149.

The trigger circuit is composed of a capacitor C1, a resistor R1, R2, a Zener diode VS and diodes VD1, VD2.
The inverter consists of Dl-D4 inside the six Schmitt non-integrated circuit lC (Dl-D6).
The audio output circuit is composed of a resistor R6, a transistor V, an inductor L, a diode VD4, and a piezoelectric buzzer HA.
When the telephone is off-hook, the voltage at both ends of Cl is about 1 OV, VS and VDl, VD2 are all in the off state, the input of D1 is pulled to the low level by R2, and both D2 and D4 output the low level, the audio oscillator Does not oscillate, HA is silent.
When the phone hangs up, the voltage at both ends of Cl rises to 48V (the specific value varies depending on the switch), VS breakdown is turned on, and the input terminal of D1 is clamped to a high level of about 1.4V by VDl and VD2, D2 "Output high level, and charge C2 through R3. If the on-hook time exceeds ls, C2 is fully charged, D3 outputs low level, D4 outputs high level, so that the audio oscillator oscillates, its output oscillating signal is After the V is amplified, the HA is prompted to sound a reminder. After the phone hangs up, the working power of the IC is supplied by the stored power, so the HA only emits a "beep" and the circuit stops working.
Component selection
Rl-R6 uses 1/4W carbon film resistor or metal film resistor.
Cl selects a polyester capacitor with a withstand voltage of 160V; C2 selects an aluminum electrolytic capacitor with a withstand voltage of 16V; C3 selects a monolithic capacitor; C4 selects an aluminum electrolytic capacitor with a withstand voltage of 1OOV.
L selects TDK color code inductor.
VDl-VD3 selects 1N4148 type silicon switch diode for use; VD4 selects 1N4001 type silicon rectifier diode for use.
VS selects 1N4753 (1W, 36V) silicon Zener diode.
V selects S9013 type silicon NPN transistor.
IC selects CD40106 type six Schmidt non-gate integrated circuit.
HA uses HTD27 piezoelectric ceramic buzzer.