Timer not working properly??

[RobH]

I have found another noise circuit in the same book as the previous circuit, and is an extension of it.(The previous circuit did work after getting help from here thanks)

This circuit uses x2 555 timers along with a LM741
The 1st timer / 555 is called 'a noise duration timer', and it is supposed to work like this:
When a noise is picked up by the electret mic the beeper sounds, and in theory when the noise stops , so should the beeper. This is done by the voltage going low, when the noise stops, using the reset pin on the 1sr timer / 555
The voltage on pin 4 without noise is 9V and when it goes low, it goes down to practitly 0V, but doen't stop the noise of a beeper.
The x2leds which are in circuit with each timer do come on and go off as soon as the noise stops, but as soon as the beeper is connected and activated it goes on forever.

Qoute:
'When the Noise Duration Timer reaches the end of its' period, its' output will go low which will shut down the external noise-making device. It will pass through a coupling capacitor to the Pause Duration Timer , 2nd 555 /timer, and the sudden drop in voltage will trigger it'
End Quote

On pin 2 of the 2nd timer I get 9v with no noise, but when I tap the mic the voltage goes up to 10v and then back down to 8V, but with the beeper connected to pin3 of the 1st timer, after going up to 10v , it goes down to just 9v

What do I need to do to put this right
Thanks

[bidrohini]

Hi, did you check the beeper that you're using? Are you sure that it is good?

[Ian.M]

Quote from the book in question ("Make: More Electronics" by Charles Platt), at the end of the chapter containing this circuit:

Just One More Little Thing
While I was tinkering with the circuit, something happened that I haven’t even mentioned yet. I happened to leave the loudspeaker close to the microphone. Can you guess what happened next? Audio feedback, of course! When the loudspeaker made its Protest Output sound, the microphone picked this up. The microphone isn’t smart enough to tell the difference between the Protest Output and someone shouting, so the circuit continued to be active. The noise went around and around and would never stop.

This is a conceptual problem, not a hardware problem. The concept of a device that responds to a shouting person by making a noise that is even louder was flawed from the start. The circuit ended up shouting at itself!

Did you foresee this, when I wrote out the specification? I didn’t see it, because I had tunnel vision, which is a very common problem when a new device is being designed. I was focusing on the goal (in this case, making a noise in response to someone shouting) and forgot the larger picture.

Often you don’t discover an obvious problem until a prototype is up and running. Then you feel embarrassed, because everyone will say, “That should have been obvious!”