I'm trying to use a PVDF Piezoelectric Sensor to trigger a Dual D Flip Flop. Once the sensor undergoes strain and produces a voltage, I want it to switch the flip flop on and power 2 LEDs. Once the sensor is hit again, it will switch the flip flop off and the LEDs will go off. Currently I'm running the piezo through a 741 Op Amp through a 5.1V Zener into the CLK of the Flip Flop, but I'm not getting any results. I'm not very familiar with these devices, but from what I've read, this process should work.

Any help/tips for using these devices is much appreciated.

Also, I drew up a basic schematic of my circuit in Paint, if that helps any one. I'll upload that in a little bit.


Nick Shuman

 

Here is the basic circuit diagram I drew in Paint. If any one sees some possible errors, please let me know.

Uploaded with ImageShack.us

 

There's so many things wrong in your circuit that I don't know where to begin.

The 741 is a split supply device but your indicating a single positive supply.

Your feeding input of a 5V logic block with an 18V rail voltage from the 741.

The 741 is not directly TTL or CMOS logic compatible.

You have no limiting resistor for your LEDs.

 

Thanks for the reply. In my diagram, I neglected the 5.1V Zener between the Op Amp output and the CLK. I also left out the limiting resistor (47 Ohms) for my LEDs.

How do I go about "splitting" my supply to get a positive and negative supply? Can I use 2 separate 9V supplies for the positive and negative terminals?

What does the 741 not being CMOS or TTL logic compatible mean? I'm not familiar with those terms, and after looking them over on the site, I'm still not sure what your statement means.

Should I be using another method to get the Piezo to produce enough voltage to trigger the CLK?

 

I would abandon your approach completely. I see no good reason or purpose for the D Flip Flop or the Opamp. We had a topic some months back similar to this that triggered a 555, which makes much more sense. Let me scan my files to find it and I'll get back to you.

 

Another item you're missing is that the Q\ output of the 7474 needs to be connected to the data input, and the data input should not be connected to Vcc.

As it is, if the 741 could ever clock the 7474, the Q output would stay high all the time instead of toggling on and off.

 

What does the 741 not being CMOS or TTL logic compatible mean? I'm not familiar with those terms, and after looking them over on the site, I'm still not sure what your statement means.

While you did not tell us what flip flop you are using these generally come in one of many families. TTL parts are made to work with other TTL parts, same with CMOS. Since these are digital devices they use pretty specific voltage levels to represent a digital 1 or 0.

Op amps fall into the class of analog devices, where the output can be almost any voltage.

So to use an op amp to drive a flip flop is not the usual way things are done, except for the time I did exactly that when I drove a microprocessor off the output of an op amp driven by a piezo element.

To do it right you need a single supply op amp, one example is the ALD2702PAL, $3.30 each and it has 2 amps per package. That one comes in a DIP package, most are surface mount devices (harder to work with).

With enough gain from the op amp the piezo will make a signal to clock the flip flop. However, tapping the piezo is not going to get you one clean pulse, you will probably get more then 1 pulse; get an even number and the light changes on to off nice, get an even number of pulses and nothing happens.

But if you get the circuit sometimes working when you bang on it then we can fix it for only one pulse.

 

Another item you're missing .......

Ah, deja vu! Sergeant, you were involved in this thread too as I recall.

http://forum.allaboutcircuits.com/showthread.php?p=314270#post314270

 

So, in general, the flip flop requires a very narrow range of voltage to CLK, whereas the op amp outputs a wide range of values. Also, the vibration of the piezo would produce multiple positive edges making it difficult to predict.

Luckily, I have a pair of 555 timers, so I will try out that design tomorrow morning and let you know how it goes. I'm trying to design a basic setup for a pressure-sensitive light-up crosswalk system for one of my classes, so a timed LED response would actually be much better for the overall scheme. And from what I read on the other thread, the time could be set for around 6 seconds, which would be right around what I need.

Thanks for the all the help and input.