I am trying to make a small, simple, inexpensive circuit to only power an LED from a piezo transducer after a minimum impact/voltage has been reached. I would like this to be tunable and prefer not to use op amps. Can this be done with a zener diode or switching diode and a small pot? Any ideas would be appreciated. Thanks!

 

What will be vibrating/hitting the piezo transducer?
Tuned to what?
Which piezo transducer? Can you post a link to its datasheet?

 

Thanks for replying.I want to use this on something like a hammer to indicate a minimum impact was reached. I will tune it with a calibrated impact transducer. I am using a piezo buzzer from Radio shack right now.

• This 85dB Piezo Buzzer operates on 4-28VDC, carrying 5mA current at 12V, with a loud buzzer tone rating of 3,600Hz.

 

So you're hitting the piezo element of the buzzer with a hammer ?

 

I will mold the piezo into the polymer hammer or embed it in epoxy into a slot.

 

If the natural resonant frequency of the piezo is 3.6kHz then the couple of damped oscillations you'll get when it receives an impact will be too brief to give any noticeable light from a LED, even assuming the generated voltage exceeds the LED Vf (which it probably will). You will need a circuit such as a '555-based monostable to 'stretch' the oscillations into a well-defined pulse. What power supply do you have for that?
I still don't know what you mean by 'tuning' in this context.

 

Thanks for responding. I don't have a power supply. If you connect a piezo to a LED and tap on the piezo it will briefly illuminate. The harder you tap, the brighter it illuminates and higher voltage on a meter. I don't want it to illuminate until you tap on it hard enough. So the tuning is to only allow current after a minimum voltage is reached, tunable with a calibrated impact sensor.

 

A zener diode is a place to start. It would need to be low voltage, and you would place it in series with the LED, and of course reverse biased.

Another thing you could use for experimentation is to just keep adding regular diodes in series with the LED and forward biased. Each one you add will shave off 0.7V, or 0.4V if you use Schottky diodes. Just find the number that does the job, and then replace them all with a single zener.

Just be aware that every diode will suck off power and reduce the brightness of the LED, even once the voltage threshold of the LED is met.

I'm wildly speculating here. We were discussing current-shunt circuits in another thread. You might be able to use that concept here, and place your LED in series with the shunt transistor. Hmmm... Here's the relevant circuit from that other thread. R3 would be replaced by your LED, and R4 would be replaced by maybe a 100K resistor. R1 would be eliminated (shorted) and who knows what R2 should be, maybe 10K to start? V1 and R6 are your piezo. You'd use some small general purpose NPN for Q1.

 

Absence of a power supply prevents use of an accurate comparator.
You could try something like this :-

Any low-power LED, and any silicon diode, will do.

 

You guys are great! I know more mechanical and chemical and just a little electrical, so I was on the right track with the diodes but had no idea how to build the circuit.
I will try these ideas and any others that I get.
Alec_t- not sure what you meant by "Absence of a power supply prevents use of an accurate comparator."
My thinking was that the piezo should put out the same voltage with the same impact force each time, and increase with increasing force.
What I would do once this is fixed inside my device, is tune or calibrate it with a powered, calibrated impact sensor, for the LED to illuminate above the minimal impact force that I want. Also this does not need to be really accurate, just within a range.

 

Oh,that's completely different. In that case you just need a comparator circuit. Check out the LM3914. It contains 10 comparators and can give you a bar graph or dot style display using 10 LEDs. There's a log version and a linear version, 3915 and 3914 I think.

 

Alec_t- not sure what you meant by "Absence of a power supply prevents use of an accurate comparator."

A comparator is a circuit which senses the difference between an input voltage and an accurate band-gap reference voltage. Such a circuit requires a power supply (which you don't have). The circuit I suggested uses a 3.3V zener diode to provide a threshold voltage, but that is not as acccurate as using a band-gap reference (such as is part of the LM3914/5 suggested by wayneh).