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Build this 2-tone alarin Inodule You've just finished designing an alarm project and you need an ear-catching alarm to go with it. If that's the case, then this alarm sounder is the go. It produces a melodic 2-tone sound, packs quite a punch for its size and uses only a few parts. By DARREN YATES This circuit was basically designed for all those times we needed a simple circuit that made an "easy on the ears" sound. It's very similar to the alarm siren in the Egg Timer project published in the November 1990 issue of SILICON CHIP. It's perfect for any alarm type application such as a water level monitor, automotive gauge monitor, or in a kid's game to indicate a winner. The module uses "easy to get" parts, can be built in a couple of hours and shouldn't cost any more than about $10 to make. It has only one input point and is simple to operate. Just connect the control input to the positive supply rail to make the alarm sound, and ground it to turn the sound off again. Circuit diagram Let's take a look at the circuit diagram in Fig.1 and see how the circuit works. The module uses a single 4093 Schmitt trigger NAND gate IC, a few transistors, a couple of diodes and little else. Gate IC1a is connected up as the control oscillator. This determines how fast the circuit toggles between the two tones. If pin 1 is connected to the positive supply rail, the gate is enabled and the circuit begins to oscillate. If you look at the truth table in Table 1, this will show you why. If one of the input pins of the gate is low, then it doesn't matter what you do with the other, the output will always remain high. This corresponds to lines 1 and 2 of the truth table as well as lines 1 and 3 since it doesn 't matter which input is taken low. However, once you take one of the input pins high , the output then becomes the opposite of the value on the other input pin, matching lines 3 and 4 as well as lines 2 and 4 in the truth table. The NAND gate here is a special type known as a Schmitt trigger gate. Normal gates have a very small threshold area or "hysteresis" level where an input voltage causes a change in the output level. For a normal 4011 NAND gate, the threshold level is about half the supply voltage plus or minus a few millivolts. The Schmitt trigger gate, however, The parts for the 2-tone chime are all installed on a small PC hoard. The project can drive a small loudspeaker to good volume & is ideal for a wide range of applications requiring an audible alarm (eg, door chimes, games & automotive monitors). 48 SILICON CHIP ----------------------+3V-15V 16VWI 10 + 01 .,, 1 PCB,codeSC08111901, 73x 46mm. 5 PC pins 1 8Q 57mm loudspeaker 47k 1M 120k Semiconductors 1 4093 quad 2-input NANO Schmitt trigger (IC1) 1 BC558 PNP transistor (01) 1 BC338 NPN transistor (02) 1 BC328 PNP transistor (03) 3 1 N914 signal diodes (D 1-D3) -:- 150k B ELJc VIEWED FROM BELOW TWO-TONE CHIME MODULE Fig.1: the circuit is.based on three Schmitt trigger NAND gates: ICla, IClc & ICld. ICla is the control oscillator & sets the rate at which the circuit toggles between the two tones generated by IClc & ICld. The outputs from the tone oscillators are then mixed & fed to an audio amplifier (Ql, Q2 & Q3). has a fairly large area, usually about 1/3rd of the supply voltage. This means that the input voltage must go below 113rd of the supply voltage before the gate recognises it as a low and above 2/3rds of the supply rail for it to be seen as a high. So, if we have a 9 volt supply rail then the upper threshold is about 6 volts and the lower threshold is about 3 volts, giving us a hysteresis level of about 3 volts. By adding just two components, we can turn this gate into a simple squarewave oscillator. Looking back at the circuit diagram , notice that there is a lMQ resistor connected between the output and one of the input pins (pin 2), and a 0.lµF capacitor from that input pin to ground. Ifwe assume the other input is held high, then this is how the oscillator works. Initially, the capacitor has no voltage across it and the gate recognises this as a low. If you look back at the truth table in Table 1, you can see that one high input and one low input gives us a high output. The capacitor now begins to charge up via the current flowing from the gate output and through the lMQ resistor. The output remains high until the voltage across the capacitor reaches 2/3rds of the supply voltage. When this happens, the gate realises it now has a high on both inputs, and so pulls the output low. (Check this again PARTS LIST on the truth table shown below). The capacitor now begins to discharge through the lMQ resistor, out through the low output. Once the voltage falls below 113rd of the supply voltage, the gate sees a low on one of its inputs and so sends the output high again. The capacitor again begins to charge up through the lMQ resistor until it reaches the upper threshold level and so the cycle c.ontinues. There are three of these oscillators in the circuit, formed by gates ICla, c & d. ICla forms the control oscillator while IClc and ICld are the two tone oscillators. The frequency of ICla is much lower than the other two so that the two tones can be easily distinguished. The frequency of this type of oscillator can be easily varied by changing either the resistor or capacitor value. Reducing the value of either component increases the frequency and increasing the value decreases the frequency. TABLE 1 INPUT A INPUT B OUTPUT (pin 1) (pin 2) (pin 3) Low Low High High Low High Low High High High High Low Capacitors 1 100µF 16VW electrolytic 1 10µF 16VW electrolytic 1 0.1 µF metallised polyester (greencap) 2 .018µF metallised polyester 1 .0039µF metallised polyester Resistors (0.25W, 5%) 1 1MQ 2 10kQ 2 150kQ 1 1kQ 1 120kQ 1 68Q 1 47kQ Miscellaneous Hookup wire, solder, etc The outputs from these two oscillators are then coupled together by two lOkQ resistors to a common .0039µF capacitor. These components form a very simple audio mixer. The signal is then fed to the base of transistor Ql, which is biased on by the 4 7kQ resistor. Transistors Ql-Q3 together form a low-power audio amplifier. Ql provides the voltage gain and biasing for the push-pull output stage based around transistors Q2 and Q3. The 68Q resistor and diode D3 provide a voltage drop of about 1 volt between the bases of transistors QZ and Q3 to reduce crossover distortion, to provide a clean output signal. They also set the quiescent current to about 5mA. (You could even use this amplifier circuit for your own projects!) The amplified audio signal appears at the junction of the emitters of Q2 and Q3 and is coupled to the loudspeaker via a lO0µF capacitor. Construction If you make or buy the PC board, check that there are no shorts or breaks in any of the tracks. If there are JANUA RY 1991 49 Fig.2: be sure to orient all polarised parts correctly when installing them on the PC board. These parts include the IC, transistors, diodes & electrolytic capacitors. any, correct them now before you do any soldering. Once you're happy that everything is OK, solder in the PC pins and the wire links. Next install the resistors and the greencaps. Use the wiring overlay diagram to make sure you have them in the right place on the board. Now install the electrolytic capacitors. Check that you have them correctly installed before you solder them in. Once you've done that, solder in the diodes, transistor and the IC. Testing If you're happy that everything is OK, you can now test the circuit operation. First, connnect a wire link between the control pin and the posi- CAPACITOR CODES 0 0 0 0 Value IEC Code 100n 0.1µF .018µF 18n .0039µF 3.9n EIA Code 104 183 392 Fig.3: use this full-size artwork to etch your own PC board & to check the board for etching defects. tive supply pin. If you now connect up a voltage supply of between 3 and 15 volts, you should get an instant chime from the speaker. Disconnect the power and remove the link between the positive supply and control pins and replace it with a link from the control pin to the ground pin. If you again switch on the power, you should hear nothing. If this doesn't work, disconnect the power and check the board for any accidental solder splashes between the tracks. You should also check that you have all the components correctly installed. Experimenting Since we are using only dirt-cheap and common components, you may like to experiment a bit with the circuit in the following way: To increase the frequency at which the circuit cycles between the two tones, decrease the lMQ resistor connected to ICla. You will notice some strange effects, the lower in resistor value you go, but don't go below about lOkQ otherwise the gate begins to dissipate too much power. You can decrease the frequency also by increasing the resistor value. If you want to change the frequency of the actual tones themselves, you can increase or decrease the value of the 150kQ and lZ0kQ resistors connected to IClc and ICld respectively. As before, increasing the resistor value decreases the frequency and decreasing the resistor value increases the frequency. Again, don't go below l0kQ. The volume of the chime can be increased by replacing the .0039µF capacitor with a larger value - say .0082µF. Be careful though, as this increases the current consumption of the amplifier from about 30mA to 90mA. Don't go above .0lµF or you may end up damaging the output transistors. Most importantly, do not take the control input higher than the circuit supply voltage otherwise you will kill the IC. What you can do is to run the chime module from an existing power supply and generate the control signal from that supply voltage. SC RESISTOR CODES 0 0 0 0 0 0 0 0 50 No Value 4-Band Code (5%) 5-Band Code (1%) 1 2 1MQ 150kQ 120kQ 47kQ 10kQ 1kQ 68Q brown black green gold brown green yellow gold brown red yellow gold yellow violet orange gold brown black orange gold brown black red gold blue grey black gold brown black black yellow brown brown green black orange brown brown red black orange brown yellow violet black red brown brown black black red brown brc,wn black black brown brown blue grey black gold brown 1 2 SILICON CHIP