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A low-cost electronic doorbell Has your doorbell broken down? This unit uses readily available parts to produce a realistic & pleasant "dingggdonggg" sound. By DARREN YATES These days, like most consumer goods, doorbells are treated as a "throw-away" item - the cost of repairing one invariably exceeds the cost of buying a new one. If your doorbell has broken down or if you don't already have one, then here's an ideal opportunity to build a deluxe unit for yourself. As you can see from the photographs, we built two versions, one housed in a commercial doorbell case (from Dick Smith Electronics) and the other in a plastic zippy box. Apart from that, the two units are identical. Go for the Dick Smith Electronics version if the unit is to be seen as well as heard. In other situations, the zippy box version may be the more convenient, particularly if the unit is to be hidden from view on top of a cupboard or a bookcase. The last time that we used a 2-tone chime circuit was in the Door Minder project, published in February 1988. That circuit used an SAB0602 2-tone chime IC which has subsequently become hard to get and expensive. This project overcomes that problem by using common CMOS ICs. In fact, parts of the circuitry are similar to the Executive Thingie project in last month's issue. When the front doorbell switch is pressed, this project generates the familiar "dinggg-donggg" sound by sequentially feeding the outputs from two tone generators to a small power amplifier. These tone generator outputs are fed to the amplifier via separate CMOS FETs which act as variable resistors to give the required decay characteristics. Power is derived from a 6V battery pack consisting of four 1.5V AA cells. If you use alkaline types, they should last for about one year or so. Alternatively, you could replace the batteries with a 6V 300mA plugpack and forget about them. Block diagram Refer now to Fig.1 which shows the block diagram of the Doorbell. We'll Left: this view shows the completed electronic doorbell in the commercial case that's available from Dick Smith Electronics. This case has a dark brown front panel with metallic highlights and this attaches by four screws to a cream-coloured midsection. The rear-panel, which is moulded in black plastic, is secured to the wall and the front assembly then clips over it. 30 SILICON CHIP Ii BELL PUSH DEBOUNCE HALF MONOSTABLE HALF MONOSTABLE IC1a IC1b IC1c D1 D2 VOLTAGE CONTROLLED RESISTOR IC2a AMPLIFIER BIAS CONTROLLER VOLTAGE CONTROLLED RESISTOR IC2b [(J SPEAKER HIGH TONE OSCILLATOR LOW TONE OSCILLATOR IC1d IC1e AMPLIFIER 03-06 Fig.1: when the doorbell switch is pressed, the two half monostable stages (IC1b & IC1c) are activated in sequence & apply bias to voltage controlled resistor stages IC2a & IC2b. These stages then modulate the outputs from the tone generators (IC1d & IC1e) & the resulting signals are then fed to an audio amplifier stage (Q3-Q6). due to switch contact bounce or due to spikes generated in the switch leads by nearby lightning strikes or mains transients. From there, the output is then fed into two half-monostables based on IClb and IClc. These provide the correct time delay between the two tones of the doorbell. Each half-monostable is connected to a voltage-controlled resistor network (IC2a & IC2b) and these modulate the outputs from tone oscillator stages ICld & ICle to give the characteristic ringing sound. The outputs from the voltage controlled resistor stages are then fed to the audio ampli- just go through it briefly at this stage, so that you will understand the basics of the circuit. The front doorbell switch is connected to a debounce circuit based on ICla. This prevents false triggering Fig.2 (below): the final circuit uses the FETs from a 4007 IC as the voltage controlled resistors. When S1 is pressed, Ql turns on & provides bias to IC2a which allows through signals from oscillator stage IC1d. Ql then turns off & Q2 turns on to provide bias for IC2b which then passes signals from tone oscillator IC1e. 100 + 16VWJ T 6V 1 + Circuit diagram Let's look now at Fig.2 which shows the complete circuit details. The project is based on two common CMOS ICs and six transistors. Let's start again with the doorbell * 150k I ...I... fier stage (Q3-Q6) and to the loudspeaker. We've pulled rather a neat trick with the audio amplifier stage, however. Because the circuit is powered continuously, we have to make the quiescent current (ie, the current flow when the circuit is in its idle state) as low as possible, to conserve the batteries. By using CMOS ICs in the front end, we have no problems here but we have to control the quiescent current of the audio amplifier. The easiest way to achieve this is to use the half-monostable outputs to control the DC bias of the amplifier via an OR gate (Dl & DZ). When the circuit is in its idle state, the monostable outputs are low and no forward bias is applied to the amplifier. Thus, the quiescent current drawn by the audio amplifier is reduced to zero. Conversely, when either half monostable output is high, the output of the OR gate is also high and bias is applied to the amplifier which then operates as normal. This allows us to keep the quiescent current down to only 160µA. Not bad,huh? 2.2M +6V 01I t14 IC2b 1 OOOR I BELL S1 10 3.3M .,. 8.2k 2.2 + 25VW+ t9 -¥- 12 -:- +6V * 120k +6V 01 + 10k 02 1N914 IC2a 4007 03 BC54& 8. eLJc 2.2 + 25VW+ VIEWED FROM BELOW 10k .,. *SEE TEXT DOORCHIME MAY 1992 31 Fig.3: you can install the parts on the PC board in any order but take care with the orientation & placement of the transistors & ICs. The two resistors marked with asterisks may need adjusting to get the correct tones. switch. The 2.2MO resistor, the 0.lµF capacitor and Schmitt trigger stage ICla form the de bounce circuit. These components clean up the waveform produced by the bouncing switch contacts to give a positive-going pulse on pin 2 of ICla each time the switch is pressed. This positive-going pulse is then AC-coupled to the first of the halfmonostable stages. This stage consists of IClb and its associated 0. lµF capacitor and 4.7MO resistor. Normally, pin 5 ofIClb is held low by the 4.7MO resistor and so its output at pin 6 is high and transistor Ql is off. When pin 2 of ICla switches high (ie, when the doorbell is pressed), IClb is immediately triggered and switches its pin 6 output low. Pin 6 of IClb then switches high again a short time later on the trailing edge of the input pulse, as set by the RC time constant on pin 5. When pin 6 ofIClb goes high again, the second monostable based on IClc is triggered into action. This halfmonostable works in exactly the same manner as the first and switches its pin 8 output low for a brief period. Thus, the two half-monostables work in a sequential fashion, with IClb first briefly switching its output low and then IClc doing the same. Voltage-controlled FETs Transistors Ql and Q2 buffer these outputs and in turn drive the voltage controlled resistor stages IC2a and IC2b. These stages are actually the ~channel FETs from two complementary pairs inside a 4007 CMOS IC. When pin 6 of IClb switches low, Ql turns on and quickly charges the 2.2µF capacitor on its collector. This turns on N-channel FET IC2a by pulling its gate (pin 3) high, so that it now passes signal from tone oscillator stage ICld. When pin 6 of IClb switches high again, Ql turns off and the 2.2µF capacitor discharges via its parallel 680kO resistor. This sets the decay time for the tone oscillator signal. As the voltage across the 2.2µF capacitor decreases, the source-drain resistance of the FET increases so that the signal level on pin 5 decays to zero. Q2, IC2b & ICle operate in exactly the same fashion. When Ql turns off, Q2 immediately turns on and this turns on IC2b which now passes signal from tone oscillator ICle. The two tone oscillators (ICld & ICle) are standard Schmitt trigger configurations with RC feedback components. ICld is the high tone oscillator and ICle is the low tone oscillator. Or, to put it another way, Ql, ICld and IC2a produce the "ding", while QZ, ICle and IC2b produce the "dong". Output amplifier The tone outputs from the FETs are mixed via two 10kO resistors and coupled to the base of Q3 in the audio amplifier via a 0.22µF capacitor. A .033µF capacitor then filters the waveform on Q3's base to give it a more "mellow" sound. The audio amplifier is a fairly standard 4-transistor class B arrangement but note that no fixed bias is applied CAPACITOR CODES (10%) 0 0 0 0 0 Value IEC Code EIA Code 0.22µF 0.1µF .033µF .01µF 220n 100n 33n 10n 224K 104K 333K 103K RESISTOR COLOUR CODES 0 0 0 0 0 0 0 0 0 0 0 0 0 32 No. Value 4-Band Code (1%) 5-Band Code (1%) 1 4.7MO 3.3MO 2.2MO 680k0 330k0 220k0 150k0 120k0 10kO 8.2k0 6.8k0 1.5kO yellow violet green gold orange orange green gold red red green gold blue grey yellow brown orange orange yellow brown red red yellow brown brown green yellow brown brown red yellow brown brown black orange brown grey red red brown blue grey red brown brown green red brown not applicable not applicable not applicable blue grey black orange brown orange orange black orange brown red red black orange brown brown green black orange brown brown red black orange brown brown black black red brown grey red black brown brown blue grey black brown brown brown green black brown brown 2 1 1 1 4 2 1 2 SILICON CHIP The PC board is secured to the rear panel of the DSE case on 25mm tapped standoffs. Note that the lOOµF capacitor at top right is mounted with its body flat against the PC hoard so that it clears the loudspeaker. to Q3 or to the complementary output pair (Q5 & Q6). Q3 is wired as a common emitter amplifier and provides most of the voltage gain. Its collector output drives Q4 which in turn functions as a driver stage for Q5 and Q6. Note that the bottom end of Q6's 1.5kQ base bias resistor has been connected to the output rather than to ground. Because Q5 and Q6 together function as an emitter follower with a voltage gain of almost unity, there is almost no AC voltage across the 1.5kQ resistor. This means that very little signal current flows in the resistor and thus its impedance does not load the output of the preceding stage, Q3. This technique is called "bootstrapping" and it results in greater signal output. The gain of the amplifier is set to approximately 4.5 by the ratio of the 6.8kQ and 1.5kQ feedback resistors. The amplified audio signal appears at the emitters of Q5 and Q6 and is coupled to the loudspeaker via a 100µF capacitor. Bias control The DC bias control circuit for the amplifier is quite simple but very effective in reducing the quiescent current to zero under no-signal conditions. As mentioned earlier, it is based This view shows how the batteries are hidden in one channel of the DSE case. The front & mid-sections of the case are held together by four self-tapping screws. on diode OR gate Dl and DZ. This OR gate drives a voltage divider consisting of ZZ0kQ and 330kQ resistors. The voltage developed at the junction of this voltage divider then provides the DC bias for the audio amplifier. Let's see how it works. When Sl is pressed, Ql immediately turns on as described previously and pulls Dl 's anode high. Thus, the output of the diode OR gate also goes high and this applies approximately 3.ZV of DC bias to the base of Q3 via the voltage divider. Similarly, when Ql turns off and QZ turns on, the output of the OR gate is pulled high via DZ and base bias is applied to Q3 as before. When the output of the OR gate subsequently goes low (ie, at the end of the two tones), the lOµF capacitor immediately begins discharging via the ZZ0kQ and 330kQ resistors. This progressively removes the DC bias from Q3 over a 5-second period, thus effectively shutting the amplifier down until the button is pressed again. By the way, if the doorbell switch is pressed and held down, the Doorchime will only operate once and then stop. That's because IClb can only be triggered when pin Z ofICla switches from low to high (ie, the change of state is necessary to trigger IClb). Thus, to get the Doorchime to sound repeatedly, it is necessary to repeatedly press the doorbell switch. Board assembly Fig.3 shows the parts layout on the PC board. This board is coded SC031069Z 1 and measures 108 x 74mm. Before starting the assembly, check The four plastic ribs on the back of the front section must be snapped off before the final assembly, to provide clearance for the PC board. MAY 1992 33 Now for the big test. Connect the loudspeaker, bell push (S1) and battery connector to the PC board, then snap the battery into position. If everything is OK, the circuit should trigger immediately power is applied. After that, you will have to press S1 to trigger the unit - disconnecting and reconnecting the battery will not do the job unless you discharge the lO0µF filter capacitor across the supply. Adjusting the tones A plastic zippy case can be used to house the PC board if the unit is to be hidden out of sight (eg, on top of a cupboard or bookshelf). the board carefully by comparing it with the published pattern. If you find any defects, correct them immediately, then install the four wire links. Make sure that the links are straight, so that they don't short against other parts. Once the links are in, you can install the resistors, diodes and capacitors. The two resistors marked with asterisks should be soldered to PC stakes and not directly to the board itself. This makes it easy to make adjustments to the tone oscillators, as we'll explain later. The accompanying table shows the resistor colour codes but it's also a good idea to check them on your multimeter before installing them on the PC board. Make sure that the diodes and electrolytic capacitors are correctly oriented. Finally, you can complete the board assembly by soldering in the ICs and the transistors. Be sure to use the correct transistor type at each location, as both NPN and PNP types are used in the circuit. Fig.2 shows the pin connection details for the transistors. Install a plug and socket in series with the battery leads if you are using the DSE case, so that you can easily separate the front section from the rear panel assembly. This will make it much easier to replace the batteries when they eventually go flat. 34 SILICON CHIP At this stage, the tones might sound a bit ''wonky" but that's easily fixed by changing the values of the feedback resistors in the tone oscillators (ie, those marked with asterisks). You simply increase the resistor values for lower tone frequencies and decrease them for higher frequencies. For example, to lower the frequency of the "dinggg", increase the value of the 12okn feedback resistor in the high tone oscillator. If you're the cautious type, then connect your multimeter (set to the mA range) in series with one of the battery leads before initially applying power. It should indicate a peak current of about 40mA as the doorbell sounds, but this should then rapidly fall away to about 160-170µA after 10 seconds or so. If you get any readings other than these, (eg, if you get a current of 40mA or more continuously), switch off immediately and check for wiring errors. In particular, check for incorrectly oriented parts and for shorts on the copper side of the board (eg, shorts between adjacent IC pads). Final assembly Once the circuit is working correctly, you can install it inside the case. If you're using a plastic zippy case, first attach the adhesive label to the front panel and drill out a pattern of 5mm-diameter holes to let the sound escape from the loudspeaker (note: drill small pilot holes first). The loudspeaker can then either be glued•in position or secured to the lid using two small aluminium brackets, machine screws and nuts (see photo). The PC board is mounted on the bottom of the case at one end, so that it sits underneath the loudspeaker. Check that there is enough room at the other end of the case for the batteries before drilling the mounting holes. 91 n (/) C) w ~ C) (J'I "°nJ Fig.4: this is the full-size etching pattern for the PC board (code SC03106921). The board can then be mounted in position on 5mm spacers and secured using screws, nuts and star washers. The assembly can now be completed by drilling a small exit hole in the side of the case for the leads to the doorbell switch . .These leads can be made as long as is necessary to connect the unit and doorbell switch together. The alternative Dick Smith Electronics case is a 3-piece assembly of moulded plastic. The front piece is dark brown with m etallic highlights and this attaches by fo ur screws to a cream-coloured mid-section. The rearpanel, which is moulded in black plastic , is secured to the wall and the front assembly then clips over it. If you're using the Dick Smith Electronics case, it's simply a matter of gluing the loudspeaker into the mould on the rear panel and mounting the PC board over it on 25mm-long plastic standoffs. These standoffs are mounted on either side of the loudspeaker and the m ou lded battery holder (see photo). Don't forget to connect the loudspeaker leads before mounting the PC board with its co de number adjacent to the loudspeaker. Battery arrangement The zippy box version has a series of 5mm-diameter holes drilled in the front panel to allow sound to escape from the loudspeaker. The battery holder moulded into the rear panel is not used here. Instead, the batteries are clipped into a separate long holder which is cleverly hidden in one of the channels that run down both sides of the case. You gain access to these channels by undoing the four screws that hold the front and middle sections together. The battery holder is installed in the righthand channel, as viewed from the back, and the leads passed out through the cutout in the centre of the channel. By the way, it is a good idea to install a plug and socket in series with the battery leads so that you can easily separate the section containing the batteries from the rear panel as- PARTS LIST 1 PC board, code SC03106921, 108 x 74mm 1 plastic zippy case, 150 x 90 x 50mm; or 1 commercial doorbell case from Dick Smith Electronics (see note 1) 4 1.5V AA cells (alkaline) 1 4 x AA battery holder (square for zippy case, long for DSE case) · 1 75mm 8-ohm loudspeaker 1 front doorbell switch (DSE Cat. P7554; Jaycar Cat. SP-0780) 4 5mm spacers (25mm for DSE case) Semiconductors 1 74C14 hex Schmitt trigger inverter (IC1) 1 4007 complementary pair plus inverter (IC2) 3 BC558 transistors (01 ,02,04) 1 BC548 transistor (03) 1 BC337 transistor (05) 1 BC327 transistor (06) 2 1N914 signal diodes (D1 ,D2) Capacitors 2 100µF 16VW RB electrolytics 1 10µF 16VW RB electrolytic 2 2.2µF 25VW RB electrolytics 1 0.22µF 63VW MKT 4 0.1µF 63VW MKT 1 .033µF 63VW MKT 2 .01 µF 63VW MKT Resistors (0.5W, 1 4.?MQ 5% 1 3.3MQ 5% 1 2.2MQ 5% 2 680kQ 1 330kQ 1 220kQ 1%) 1 150kQ 1 120kQ 4 10kn 2 8.2kQ 1 6.8kQ 2 1.5kQ Miscellaneous Tinned copper wire; insulated hook-up wire; figure-8 bell cable; machine screws, nuts & washers. Note 1 : the Dick Smith Electronics doorbell case is available only as part of a complete kit. sembly. This will make it much easier to replace the batteries when they eventually go flat. Finally, it is necessary to remove four plastic ribs from inside the case before clipping it over the rear panel assembly. These ribs can be easily snapped off by hand. SC MAY 1992 35