Design and production of dual-tone doorbell circuit

       1. Working Principle

The circuit schematic is shown below. When the doorbell button AN is not pressed, the voltage across UC3 is zero, the R reset terminal of 555 is valid, the output pin ③ is 0, and the doorbell does not ring. When the doorbell AN is pressed, the D2 diode conducts forward and charges the capacitor C3, making the voltage across UC3 close to +5V, R=1, and the 555 chip works.

When the doorbell AN is pressed, D1 is turned on, and +5v charges capacitor C through D1, R1, and R2. When charged to UC≥2/3VCC, the 555 timer is set to O, and the output jumps to a low level; at the same time, the discharge switch is turned on, and the capacitor C→resistor R2→pin 7→ground GND begins to discharge. When the capacitor is discharged to UC≤2/3Vcc, the 555 timer is set to 1, and the output potential jumps to a high level again. At the same time, the discharge switch (VT) is turned off, and capacitor C starts to charge again, repeating the above process. This cycle repeats itself, and the circuit oscillates. Oscillation frequency f=1/(T1+T2)

T1=0.693*(R1+R2)*C

T2=0.693*R2*C

Release the doorbell AN, diodes D1 and D2 are turned off, and C3 forms a discharge circuit through R4. When the discharge reaches 1/3 of the power supply voltage (i.e. 1.4V), 555 resets and stops oscillating. The oscillation frequency during this period is f=1/(T1+T2)

T1=0.693*(R1+R2+R3)*C

T2=0.693*R2*C

When the doorbell AN is released, the doorbell keeps ringing for about 1.3×R4×C3. In summary, the doorbell does not ring when it is static, but it rings at the frequency of fl when it is pressed, and the speaker makes a "ding" sound. When it is released, it rings at the frequency of f2, and the speaker makes a "dong" sound. The length of the "dong" sound can be changed by changing the value of the variable resistor R4.
　　

       
2. Component Selection

555 time base chip, 2 3kQ carbon film resistors, 1 3.9kΩ resistor, 1 22kΩ sliding variable resistor, 1 0.1μF and 0.01μF ceramic capacitors, 2 47μF/16V electrolytic capacitors, 2 diodes, 1 8Ω/0.25W speaker. SV power supply or 4 No. 5 dry batteries in series.

3. Printed Circuit Board Design

Using ProtELDXP software, a single-layer printed circuit board is designed as shown in the right figure (omitted), which is 70mm long and 48mm wide. The copper film conductor width is 50mil; the power line and ground line need to be widened to 70mil; to meet the needs of single-sided board manufacturing, the outer diameter of the pad is 100mil and the inner diameter is 30mil.

4. Make printed circuit boards

① Printing: Print the designed printed circuit board with a laser printer (prepare thermal transfer paper as required), and print the PCB diagram on the thermal transfer paper at a 1:1 ratio.

② Transfer: Place the thermal transfer paper on the copper-clad plate (the copper-clad plate should be polished with fine gauze before transfer, which is conducive to transfer), and put it into the transfer machine for graphic transfer　　

③Corrosion: Use a special correction pen to repair the incomplete transfer or small defects, and put the repaired copper-clad board into the corrosion tank for corrosion (about 15 minutes).

④Inspect and apply flux: Take out the corroded printed board and clean it with clean water. Check the entire printed board, pay special attention to short circuit and open circuit defects, and correct the errors (the short circuit can be engraved with a knife, and the open circuit can be scraped off the protective layer in preparation for solder connection). After checking, apply flux (rosin water).

⑤ Drilling: Use a bench drill to drill holes at the location of the pad to complete the production of the printed circuit board.

⑥Finally, the tested components are shaped, installed, and soldered to the printed circuit board to make a dual-tone doorbell circuit.