#### meowth08

##### Member

We were asked to design an alternating tone using a 555 timer. One tone is 450Hz that lasts for 90 seconds and the other tone is 15kHz that lasts for 60 seconds. Actually, I've already started with the design patterning it to the "burglar alarm system" schematic Sir Eric provided last time. In this design, I used 2 monostable to produce the 90 seconds and 60 seconds time requirements. Each monostable is connected to an astable circuit. Each astable circuit was made to oscillate at the required frequencies with these following assumptions:

first astable: 450Hz

frequency=1.44/[(R1+2R2)C]

duty cycle=R2/(R1+R2)

I'm not sure if the equation for duty cycle is correct. I've also read some articles that computes duty cycle as R2/(R1+2R2). which is which???

second astable: 15kHz

frequency=1.44[(R1+2R2)C]

duty cycle=R2/(R1+R2)

Since what is desired is that the tones alternately sound, I used a total of (90sec+60sec)=150 sec to finish 1 sequence (i.e. 450Hz then 15kHz). Dividing 90 by 150, I got a duty cycle for the 450 Hz tone as 0.6 while 60/150 yields 0.4 duty cycle for the 15kHz tone.

Giving a value for C for the astable, and substituting values, R1 and R2 were readily computed.

My problem is that I do not know exactly what to do next. I am thinking of delaying the first tone for 90 seconds. But how would I do that? After 90 seconds, the 15kHz will prevail. Then after 60 seconds, the 450 Hz again which will continuously alternate.

To achieve the alternation I was also thinking to connect the outputs of the the two astables to a 2to1 demultiplexer, if there is such, then to an amplifier.

If there are mistakes in my design or if the design is a total mistake, please educate me my learned friends. Many thanks in advance.

first astable: 450Hz

frequency=1.44/[(R1+2R2)C]

duty cycle=R2/(R1+R2)

I'm not sure if the equation for duty cycle is correct. I've also read some articles that computes duty cycle as R2/(R1+2R2). which is which???

second astable: 15kHz

frequency=1.44[(R1+2R2)C]

duty cycle=R2/(R1+R2)

Since what is desired is that the tones alternately sound, I used a total of (90sec+60sec)=150 sec to finish 1 sequence (i.e. 450Hz then 15kHz). Dividing 90 by 150, I got a duty cycle for the 450 Hz tone as 0.6 while 60/150 yields 0.4 duty cycle for the 15kHz tone.

Giving a value for C for the astable, and substituting values, R1 and R2 were readily computed.

My problem is that I do not know exactly what to do next. I am thinking of delaying the first tone for 90 seconds. But how would I do that? After 90 seconds, the 15kHz will prevail. Then after 60 seconds, the 450 Hz again which will continuously alternate.

To achieve the alternation I was also thinking to connect the outputs of the the two astables to a 2to1 demultiplexer, if there is such, then to an amplifier.

If there are mistakes in my design or if the design is a total mistake, please educate me my learned friends. Many thanks in advance.

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