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Does your doorbell just go ding dong or worse, don’t you have any doorbell at all? Either way, you can improve your whole lifestyle by building and installing this musical doorbell. It plays a sequence of nine notes each time someone presses the button. Design by BOB FLYNN Musical come clean. Maybe A building this musical doorbell might not make a huge difference to LRIGHT, WE’LL your lifestyle but then again maybe it might. One of the visitors to your home might be so impressed by your unique doorbell that they might offer you a partnership in a huge new electronics venture. You never know . . . The new doorbell uses just three cheap ICs and three tran­sistors in the circuit. There are two good ol’ reliable 62  Silicon Chip oorbell D CMOS 555 timers, a 4017 counter and not much else. We are presenting this project just as a PC board, knowing that you will want to make your own arrangements as far as the case and loudspeaker are concerned. The unit will play virtually any tune of up to nine notes although there is a proviso which we will come to later. Let’s have a look at the circuit of Fig.1. There are two separate 555 oscillators and a 4017 decade counter. What happens is that the first 555 oscillator (IC1) produces the clock pulses for counter IC2. IC2 then counts from 0 to 8 and then stops on the tenth clock pulse. The whole circuit waits until the next time the door bell button is pressed but we’re getting a little ahead of ourselves. Each output of the 4017 counter is used to produce a sepa­rate frequency from the second 555 timer, IC3. This then drives an amplifier stage consist- Fig.1: IC2 is the heart of the circuit and its nine outputs cause IC3 to produce nine different notes as it counts through. ing of two transistors, Q2 & Q3, which drive the loudspeaker. How it works Now let’s have a more detailed look at how the circuit works. IC1 is a conventional 555 timer circuit with its output frequency variable between about 1.3Hz and 5.5Hz, depending on the setting of the 2MΩ trimpot VR1. The only unusual feature of the circuit of IC1 is the con­nection to pin 4. In normal free-running 555 oscillator circuits, pin 4 is tied to the positive supply rail but in this case we use pin 4 to start and stop oscillation. The output pulses from pin 3 of IC1 are fed to the clock input of IC2 and each of its outputs from Q0-Q8 goes high in turn for the duration of a clock pulse. Each 4017 output is fed via a diode and two resistors (R1 & R2) to pin 7 of IC3. Depend­ing on the values of R1 & R2 connected to each 4017 output, IC3 can then generate a different note in a nine-note sequence. On the tenth clock pulse, the Q9 output of IC2 goes high and this pulls the clock enable pin 13 high and also pulls the base of transistor Q1 high. The combination of these two events effectively stops IC2 on the tenth count and ensures that when pushbutton S1 is pressed, the pin 3 output of IC1 immediately goes high to give a full first count in the nine count sequence from IC2. IC3 generates the nine notes. The frequency of each note is determined by the sum of the resistance of resistors R1 & R2, the 51kΩ resistor between pins 6 & 7 and the .01µF capacitor at pins 2 & 6. Table 1 shows the values to provide one octave of notes including sharps and flats. The overall pitch of all the notes can be shifted up or down by the tune control, trimpot VR2. This takes advantage of the fact that you can shift the upper and lower thresholds of the 555 timer with an adjustable voltage divider connected to pin 5. The pin 3 output of IC3 drives a rud­ i­ mentary amplifier stage consisting of complementary emitter followers Q2 & Q3. These in turn drive the 8Ω loudspeaker via a 220µF coupling capacitor and a 27Ω current limiting resistor. Since the output waveform is essentially a pulse train, the complementary amplifier stage can operate in class-B and do without such niceties as quiescent current. By the way, the output from pin 3 of IC3 has a varying duty cycle, depending on the frequency, since the resistor between pins 6 & 7 is constant at 51kΩ while resistors R1 & R2 are varied. Construction As noted above, we are presenting this project just as a PC board, knowing that you will want to make your own arrangements as far as the case and loudspeaker are concerned. The PC board measures 129 x 79mm and is coded 11211971. Fig.2 shows the com­ponent overlay. After checking the PC board for any etching defects or undrilled holes, November 1997  63 Fig.2: the component overlay for the PC board. You will need to select the values for R1 & R2 from Table 1. Fig.3: here is the full-size etching pattern for the PC board. install the PC stakes for the supply, loudspeak­er and pushbutton connections. This done, install the wire links, the diode and the resistors. It is a good idea to check each resistor value with your multimeter before you install it. Ah, now what values should you use for R1 & R2? Table 1 shows the values for various notes so if you have a favourite few bars of music you can determine the notes you want and pick the resistors accordingly. But there is 64  Silicon Chip one little drawback to be noted. Since the 4017 counts from one to nine in a continuous sequence, the notes are produced in the same sequence, with no gaps in between. This does not present a problem if all adjacent notes in the sequence are different but if you have two adjacent notes which are the same you do have a problem. Instead of having two separate notes you will just get one long note. The only way of overcoming this, short of adding extra gates to provide a short break between each note, is to leave a one-note gap between two identical notes. This means that your tune will be shortened to eight notes and you will then need to omit the diode and resistors R1 & R2 for that note position. Going back the circuit of Fig.1 for a moment, that is why the diode and the resistors associated with the Q4 Parts List 1 PC board, code 11211971, 129 x 79mm 6 PC stakes 1 momentary contact pushbutton switch (S1) 1 6V or 9V battery or DC plugpack (see text) 1 8Ω loudspeaker 1 2MΩ trimpot (VR1) 1 2kΩ trimpot (VR2) This musical doorbell can be arranged to play a nine-note sequence each time you press the pushbutton. Make sure that all parts are correctly oriented. output of IC2 have the note “Omit”. The circuit actually shows the note se­quence for “Westminster Chimes” and since the fourth and fifth notes are both C, we’ve had to omit the resistors and diode for the Q4 output of IC2. So this long-winded explanation makes the point: if you have a tune with two adjacent notes the same, you will need to leave gap (in time) otherwise you will get one note the same. The obvious alternative to this Table 1 Semiconductors 2 7555 timers (IC1, IC3) 1 4017 decade counter (IC2) 1 BC547 NPN transistor (Q1) 1 BC337 NPN transistor (Q2) 1 BC327 PNP transistor (Q3) 9 1N914, 1N4148 small signal diodes (D1-D9) 1 1N4004 silicon diode (D10) dilemma is to choose a tune which does not present this problem. Having inserted all the resistors, you can now finish the board assembly, taking care to ensure that all the semiconductors and electrolytic capacitors are inserted the correct way around. Capacitors 1 220µF 16VW PC electrolytic 2 100µF 16VW PC electrolytic 1 0.33µF MKT polyester 1 0.15µF MKT polyester 3 0.1µF MKT polyester 2 .01µF MKT polyester 1 .0033 MKT polyester Testing Resistors (0.25W, 1%) 1 560kΩ 1 15kΩ 2 100kΩ 2 10kΩ 1 56kΩ 1 27Ω 1 22kΩ Values for R1 & R2 depend on desired notes – see Table 1. To test the finished board, you will need a 9V battery or DC power supply, an 8Ω loudspeaker and a pushbutton. If all your work is correct, the board should run through its sequence of notes as soon as the supply is connected and then fall silent. After that, nothing should happen until you push the button and then the note sequence should be produced. If the circuit does not work as it should you can check the operation of each stage with your multimeter. For example, the output at pin 3 of IC1 should pulse up and down at around two or three times per second if trimpot VR1 is at its mid setting. You can check this with your multimeter set to read DC. Similarly, you can check that each of the outputs of IC2 go high in turn and so on. Options You have several options for powering the circuit. First, you can use a 9V battery but ideally this should employ six C or D cells to obtain long battery life. If you use a 9V (Eveready 216 size), the battery will not last long. You could also use a 6V lantern battery but then the 27Ω resistor in series with the loudspeaker should be reduced to 18Ω. Alternatively you could power the circuit with a 6V or 9V DC plugpack, bearing in mind that their operating voltage will typically be around 50% more; ie, 9V and 13V, respectively. Do not use a 12V DC plugpack because the output voltage will be too lightly loaded for this circuit. You would run the risk of blowing the chips as the plugpack voltage is likely to be as high as 17V. Finally, if you do use a DC plugpack instead of a battery, you can save a little money by using ordinary 555s instead of 7555s. Their current drain will be a lot higher but that does not SC matter with a plugpack. November 1997  65