Traditional Chinese Practical Convenient phone light: Circuit Design, Parts List & Workshop Prototype Guide

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Convenient phone lightThe convenient telephone light can provide convenient lighting for answering calls at night and picking up the handset to dial (press) phone numbers. When the phone rings or the handset is picked up, the convenience light automatically lights up; when the phone stops ringing or the handset is put down, the convenience light stays on for a few seconds and then turns off.Circuit working principle:The circuit is shown in Figure 206. Among them, S1 is a tact switch linked with the microphone button. It is usually in the off position and is on when picking up the handset. BC is a piezoelectric ceramic piece pasted on the telephone ringer and speaker BL. L1 and L2 are telephone lines. The DC voltage of about 50V (off-hook) is used to "forward" through the bridge VD (1N4001×4) (the a/b terminals of VD do not need to consider the polarity and can be connected between the L1 and L2 lines at will). R1 reduces the voltage, VD1 stabilizes the voltage, and C6 filters to obtain the 12V power supply VDD that supplies the light circuit. Normally (the microphone is placed on the machine), S1 is in the off position, VT1 (9013) is cut off due to no bias, and the collector voltage Vc4, which is close to the VDD value, is added to the trigger terminal TR (pin ②) of the monostable IC (NE555); the other control signal of the IC pin ② comes from the ringing sound pickup, amplifier BC and VT1/VT2. At this time, there is no ringing signal, so there is no AC voltage at the collector of VT2, the rear stage of the cascade amplifier. Therefore, the potential of IC② pin is mainly determined by VC4, that is, IC② pin is high level. Therefore, the IC output pin ③ is low level. The KL relay is in the release state, its normally open contact KL1 is broken, and the H light does not light up. When someone calls and BL sounds, BC is vibrated to generate alternating voltage VBC, which is coupled by C1 and amplified by VT1/VT2. The ringing amplified signal VBC generated at the collector of VT2 (9013) is coupled to IC pin ② through C3. During the negative half cycle of VBC, the monostable IC will be triggered and enter the "transient state". The ③ pin changes from low level to high level, KL pulls in, KL1 is turned on, and the mains lights the H light. As long as the intermittent period of the ringing signal TD < Ta (=1.1R4·C4: IC's "transient" period), the H light will stay on. If someone picks up the handset to answer the call, S1 is connected. VDD injects base current into VT4 through R10 and S1, so VT4 is saturated and turned on. When VC4 on IC pin ② drops below VDD/3 (4V), pin ③ maintains a high-level "self-locking" state. As long as the handset is held in place, the H light will remain on forever (that is, the call time is not limited). After making the call, when the handset is put down, S1 returns to the off state, Vc4 rises to the VDD value, and the IC pin ② is at high level again. At this time, VDD charges C4 through R4. When the voltage on C4 rises to 2VDD/3 (8V), the IC will return to the "steady state", pin ③ becomes low level, KL is released, and the H light turns off. According to the given value in the figure, the delayed self-lighting time of the H lamp is Td=1.2R4·C4=11S. To extend Td, increase R4 or C4 as needed. When you need to make a call and pick up the handset, S1 will be connected and the H light will light up immediately (the principle is the same as above). VT3 and photoresistor R9 in the picture form a "brightness identification" circuit to ensure that the H light does not light up when using the phone during the day. Due to the strong light during the day, the resistance of R9 decreases, and the result of the voltage division between R9 and RP makes VT3 tend to be saturated, so the voltage of the strong reset terminal MR (④ pin) of the IC is less than 1V. At this time, no matter how the potential of other pins of the IC changes, pin ③ will be "always" low, so the light will not light up during the day (the situation is opposite at night).Figure 206Component selection:BC chooses HTDZO type buzzer. KL is HZ00F12-0-A closed small DC electromagnetic relay (2.05×16.5mm, coil: 400Ω± …

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