Traditional Chinese Password electronic doorbell: Circuit Design, Parts List & Workshop Prototype Guide

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Password electronic doorbellThe password-type electronic doorbell can only ring if the corresponding buttons are pressed in accordance with the password sequence within the specified time. Otherwise, the doorbell will not ring if the buttons are pressed. This password-type electronic doorbell is only suitable for use by family members, relatives and friends. It can keep out uninvited guests and naughty children and save unnecessary trouble.1． How the circuit worksThe circuit of the password-type electronic doorbell is shown in Figure 65-1. It is composed of a monostable circuit, an AND gate circuit, a switch circuit, and an audio and display circuit.Figure 65-1In the figure, the time base integrated circuit IC1 forms a monostable circuit. In normal times, the ②, ⑥ and ⑦ pins of IC1 are all above 2/3 of the power supply voltage, that is, high level. Then IC1 is in a reset state, that is, the ③ pin of IC1 outputs a low level. Relay J does not operate due to no power, and its normally open contact J1 disconnects the power supply of the rear circuit, and the rear circuit does not work. When the push-button reset switch AN1 is pressed, AN1 short-circuit the capacitor C1, and the charge on it is quickly discharged, causing the potential of IC1's ②, ⑥ and ⑦ pins to become zero, IC1 is set, ③ pin outputs high level, relay J is electrically closed, and the normally open contact J1 is closed, turning on the power supply of the rear circuit to make the rear circuit work.At this time, if AN2 is pressed, a forward current is applied to the base of transistor BG1 to conduct. After BG1 is turned on, its collector potential becomes low, causing transistor BG2 to cut off, and the high level output of the collector of BG2 is sent to pin ① of IC2. After that, although AN2 is released and disconnected, since the capacitor C2 is full of voltage, this voltage will slowly discharge to the base of BG1 through the resistor R2, so that BG1 continues to be on and BG2 is in a cut-off state, that is, the input pin ① of IC2 remains high for a certain period of time until the voltage on C2 is discharged to lower than the base working voltage of BG1, then BG1 can be cut off, BG2 is turned on, and pin ① of IC2 can become low level.Since IC2 is a four-input AND gate circuit, the input pins ①②④⑤ of IC2 must input high level at the same time, so that the output pin ⑥ can output high level. That is, after pressing AN1, you should press AN2, AN3, AN4 and AN5 accordingly. If you press less than one IC2, it will not work. The working principle of pressing AN3, AN4 and AN5 is the same as AN2.It can be seen from the above that: the time for AN2 ~ AN5 to be pressed one after another should be less than the discharge time of capacitor C1. If the time for AN2 ~ AN5 to be pressed one after another exceeds the discharge time of C1, relay J will be released and the power supply to the rear circuit will be disconnected. Even if the password is pressed correctly, there will be no sound. This limits the time for selecting and pressing the password switch, and improves the confidentiality of the password switch.When the ⑥ pin of IC2 outputs a high level, the one-way thyristor SCR is triggered to conduct, and provides working voltage to the music integrated circuit IC3 and the light-emitting diode through the voltage-reducing resistors R14 and R15 respectively, causing the speaker to emit an electronic doorbell sound and the light-emitting diode LED to light up.In order to confuse outsiders, 5 fake button switches were also installed. Since these 10 button switches are arranged in different forms, they form different passwords. According to the working principle of the circuit, AN1 must be the first position of the password switch. Only by pressing it first can the power of the rear circuit be turned on, and then pressing AN2 ~ AN5 (the order is not limited) can the sound and light circuit function. Therefore, the password is 5 digits and can be arranged arbitrarily, such as 36947, 58213..., among which 3 and 5 (the first one) must be AN1.2. Component selection and installationIC1 is a time base integrated circuit, which can be selected from 5G1555, NE555, SL555, SF555, μA555 and CA555.IC2 is a four-input AND gate circuit, and you can choose CMOS integrated circuit C036 or C031. Its pin arrangement is shown in Figure 65?. In this circuit, only 1/2 of IC2 is used.Figure 65-2 Figure 65-3IC3 is a high-quality “ding-dong” music integrated circuit, model number is KD-253, and its pin arrangement is shown in Figure 65-3.J is an ultra-small relay, the working voltage is 3V, optional HG4099-003, JZC-22F or JZC-6F (4098), etc. SCR is a one-way silicon controlled thyristor, …

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