OBJECTIVE

Here is a simple and very easy to build alarm siren producing gadget capable of producing five different types of siren tones. The gadget is built around a single IC 556 which is nothing but a dual IC timer 555.That is, it has two independent 555s   within the same package. The type of siren can be selected from an appropriate combination of switches to be closed given in the accompanying table. The gadget could also be constructed using two 555s in place of one 556.

CIRCUIT DESCRIPTION

For a clear understanding of this circuit functioning, it is very important to identify those pins in IC556 that represent two independent 555’s. IC555 has eight terminals namely the COMMON (Pin-1), TRIGGER (Pin-2), OUTPUT (Pin-3), RESET (Pin-4), CONTROL (Pin-5), THRESHOLD (Pin-6), DISCHARGE (Pin-7) and +Vcc (pin-8). IC556 which is a 14-pin IC has two fully independent 555s with only a common supply and a common ground point. lts pin connection diagram as shown in Fig. 22.2 tells about the pin numbers for the different terminals of two 555s inside this IC. As is clear from this diagram, one of the 555s is represented by pins 1 to 7 and 14 with pin-7 being the ground pin and pin-14 the supply pin The second 555 is available on pin numbers 7 to 14.

SIMPLE SINGLE TONE ALARM

With switches S10, S13 and S14 closed and all other switches open, the given circuit is reduced to a simple astable multivibrator configuration feeding the speaker. The astable multivibrator produces a waveform at the output that has a high time depending upon R1, R4 and C6 and a low time depending upon R4 and C6. The values for these components are so chosen that the frequencies the output waveform is about 4.5kHz. You would notice that the second 555 remains disconnected from the output side.

FACTORY SIREN

With switches S6, S7, S10, S12 and S14 closed and other switches open, the given circuit is once again an astable multivibrator built around oneof the 555s The high time this time depends upon the resistance R1 , the resistance offered by potentiometer P3 and capacitance: C6. The low time depends upon potentiometer P3 setting and capacitor C6.You would notice that there is an RC network connected from supply to ground with the capacitor connected across control pin (Pin- 3) of the 555 being used. The RC time constant is variable upto a maximum of 10 seconds. Thus as the circuit switched on, voltage at the control pin rises towards Vcc with the time constant depending upon P1 setting and C7. This rising voltage changes the output frequency.Thus what we hear is a frequency modulated audio tone resembling that of the one from a factor siren.The second 555 is again not coming into figure.

AMBULANCE SIREN

With switches Sl, S4, S5, S9 and S10 closed, the

given circuit is reduced to a configuration where the first 555 wired as an astable multivibrator drives the control pin of the second 555 again wired as the astable multivibrator. The frequency of the first astable multivibrator is decided by R1, P4 setting and C5 while that of the second depends upon R2, P2 setting and C3 in addition to voltage. being fed externally at its control pin (Pin-1 1). The second astable multivibrator operates on two different frequencies corresponding to two different voltage amplitudes present at its control pin due to the low and high potions of the waveform appearing at the output of first 555. Both frequencies of the second astable are much higher than the frequency of the first astable. The final result is therefore a siren with two different tones repeating alternately.

The second astable sets the frequencies of these different tones arid the first astable decides the durations of these tones.

POLICE SIREN

With the switches S2, S4, S5, S9, S10 and S1 1 closed and other switches open, the circuit that we get is again similarly what we have just seen in case of an ambulance siren with the only difference that the control pin of the second astable instead of being driven from the output of the first astable is fed from the trigger-threshold junction point of the first astable multivibrator. The more or less triangular waveform appearing at this junction continuously frequency modulates the output frequency of second astable output which feeds the speaker.The sound like a police siren is the result.

BEEPING SIREN

The circuit configuration is again similar to what we have seen in the two immediately preceding cases. In the present case, the output of the first astable multivibrator feeds the reset terminal of the second astable. The second astable output finally feeds the speaker. Wheh the output of the first astable is high, the second astable functions normally and produces a tone. As the output of the first astable goes low, the second astable is reset and its output goes to zero. Thus result is a siren comprising of a repetitive seqence of presence and absence of tone.Figs. 22.3 and 22.4 respectively show the PCB layout and the components layout.

PARTS LIST

Resistors and Capacitors

R1, R2 : IK, 1/4W

R3 : 68K, 1/4W

R4 : 15K, 1/4W

P1 : 4.7K ,preset

P2 : 100K, preset

P3   : 100K , preset

P4 : 4.7K, Preset

C1 : 100µF, 25V (Electrolytic)

C2 : 10µF, 25V (Electrolytic)

C3, C4, C9 : 0.01µF (Ceramic Disc)

C5 : 1000µF, 25V (Electrolytic)

C6 : 0.01µF, 25V (Polyester)

C7 : 1000µF, 25V (Electrolytic)

C8 : 1000µF, (Electrolytic)

Semiconductors and ICs

IC-1 : IC 556C

Miscellaneous

S1 to S9, S11 to S14: DIP switches, S10: Toggle: switch, 8 ohm speaker

+9V battery Solder wire, multistrand wires etc.

TESTING GUIDELINES

1. The alarm gadget should be tested for its functioning in all the five different alarm settings separately by closing the appropriate set of switches and leaving the rest open. In all individual settings, check that you get an overall sound effect from the alarm that it is supposed to generate.

2. You would notice that S10 has got to be closed in all the five alarm settings. It is rightly so as it is the switch in series with the 9V battery it is suggested that this switch be closed last of all after you have closed all other relevant switches when trying to test a particular alarm setting.

3. In case of those alarm settings which have potentiometers in the frequency determining portions, the effect of varying the potentiometer resistance on the output should be seen and the resistance should be set for the best overall effect.

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