Hi, I'm trying to figure out how to trigger a 555 timer using sound, sort of.
Basically I have a small speaker and want it to charge a cap by lightly tapping it. When the cap discharges, it triggers the timer. By changing the value of the cap I should be able to change the time it takes to charge the cap. So I can set the how long you have to tap the speaker before the timer fires.
Sure could use some ideas....please
Thanks
George

 

Hi G Man,

basically you can trigger an NE555 with a sound signal. However you can't trigger an NE555 with a positive going pulse or voltage. The trigger signal has to be negative against the supply voltage.

The second problem you'll be faced with is the fact, that the input is re-triggerable, meaning that the output will change its state only once, on initial triggering.

Therefore you must interrupt the AF-signal delivering the trigger signal as soon as the timer output changes its state.

So you need: an amplifier for the mike, a rectifier circuit, an inverting amplifier to trigger the timer and a flip-flop which will disonnect follow-up AF signals to the circuit, to be reset after the timer changes its output to low again.

Regards

Hans

 

What do you want the timer to do when it fires? Should it reset your cap? How long does the timer stay on?

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Ron

 

Basically flash some led's and trigger a playback IC, I have all that set. Its ok if the timer gets re-triggered I want the 555 to stay on for now. Basically when the object brushes against the speaker, say for 1 minute, the 555 fires.

 

Hi Gman,
You get the same answers on this website.
It isn't easy but if you make the circuit in stages then it will work and we will help you with any problems with it.

Before you had a microphone or a speaker used as a microphone.
Now you are brushing against a speaker which will have a much higher output than if it was a microphone. So maybe a preamp is not needed.

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Uncle $crooge

 

Maybe I didn't explain what I was trying to do well enough? Sorry, I'm not real good at that. Yes, brushing against the speaker, only way I can do what I need to do. I don't think a preamp is necessary, basically charge a cap with the noise created by the moving speaker "really small one from a pr of headphones" when the cap discharges, it triggers the 555.

 

Brushing against a speaker makes AC. A capacitor is charged with DC so a rectifier is needed to convert the AC to DC.
What will discharge the capacitor?

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Uncle $crooge

 

The capacitor will discharge when it charges to its listed voltage.
Might need a diode, yes.

 

A capacitor might take weeks to discharge itself. It needs to have a transistor to discharge it.
The trigger pin 2 of a 555 timer needs to be pulled down to a voltage that is less than 1/3rd of the power supply voltage. A capacitor can't pull it down.

You are talking about charging a capacitor? Why? a 555 charges its timing capacitor then discharges it. You need two 555 timers.

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Uncle $crooge

 

I want to use a capacitor so I can adjust the discharge time by changing the value of the cap.

 

You adjust the 555 time by changing the value of the capacitor or by changing the value of a trimpot.

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Uncle $crooge

 

G_man, I think you know what you want, but you have not defined it well enough here for me to feel like I could design something for you. I also think you need to drop your love affair with capacitors. It may well be that that's the way to go, but there's more than one way to skin a cat. A broad but concise statement of your goal is what is needed. I feel like we're having to drag it out of you.

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Ron

 

Yeah I do have a problem with that. OK as best as I can describe what I'm trying to achieve here. I have a small poly speaker. I want to fire the timer by gently rubbing-touching the poly speaker. I need to be able to adjust the time it takes to fire the 555 ie cap or trimpot as audioguru said, by increasing the value of the cap you have to tap the speaker longer. One idea is the speaker connected to a darlington array charging the cap and maybe a comparator to discharge it??..I'm a bit further along than yesterday. But don't have it working yet.
George

 

What happens when the threshold is reached? I take it you want your capacitor, or whatever, discharged and ready for the next fondling of the speaker. When the threshold is reached, a timer is also apparently triggered. How long should the timer be on? Is the speaker stroking circuit (charging capacitor, etc.) disabled until the timer times out? Think about everything you want, and then describe it here. Do you have to drive a load, or an LED, or what?

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Ron

 

Exactly, when the threshold is reached, the cap should be discharged and ready for another fondling...LOL...Is there a way to adjust the time the 555 runs with a trimpot? this would be ideal as would disabling the speaker circuit either altogether or for a variable duration, again trimpot maybe? Honestly though, just getting a semi working prototype is all I'm after here, if not disabling the stroking circuit after it fires the timer is easier, thats ok too for now.
Basically it will be driving two led's. I have an astable setup right now running two led's on a breadboard.
Thank you for being so helpful.
George