555 one shot trigger question

[ffuzz]

Hi,

I'm trying to use a piezoelectric transducer to trigger a relay, to turn on regular house current. I am using an lm324 opamp to keep the signal positive, and was trying to use a 555 in one-shot mode to lengthen the burst from ~20ms to ~100ms.

However, it looks like the trigger for the 555 is a switch not a voltage pulse. I believe I would need to use a transistor as a switch here, but i really dont know enough to be able to choose what kind of transistor or even how to set it up.

The 555 monostable circuit:

**broken link removed**

(this image is nearly identical to example circuits in 555 manufacturer pdfs)

The output from the lm324 opamp is about 7v, and about 20ms long, but it is not a square pulse at all.

Any help would be great. I'm a software guy, but trying to learn the better side of things here.

Thanks,
ffuzz

 

[Roff]

You might be able to trigger directly from the piezo transducer. How high is the voltage burst from the transducer? Is it a pulse, or a damped sinusoid?

 

[ffuzz]

...

The piezo can get up to ~10v, or aound 15-20v peak-to-peak. It is sinusoidal, alternating current. I'm not sure that this would be any better for triggering since I still need a way to (effectively) close the switch to trigger the 555.

Is the sinusoidal AC signal better to trigger the 555?

-ffuzz

 

[Roff]

I don't know that type of transducer you're using. I simulated the circuit below based on a model I found for a 40kHz transducer. It might work for you. It might not. It won't hurt to try. Let us know your results, and, if it doesn't work, tell us the type of transducer you are using. I'm fairly sure we can come up with something.

You don't need a switch to trigger a 555. The voltage on pin 2 only has to go below VCC/3 to trigger it.

 

[ljcox]

You could use a CMOS Schmitt Trigger such as the 74C14 or 40106.

This can be used to square, invert (if necessary) and to stretch the pulse since there are 6 Schmitt Triggers in one IC package.

See attachment.