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> How Would I Add A Pulse Led To This Circuit?, Simple op-amp piezo booster.
BigJohnny
  Posted: April 03, 2012 02:42 pm
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I have this circuit which works pretty well as is, but I would like to add an LED that will flash when there is a pulse from the piezo transducer that the amp is amplifying.

Using a transistor and led as such doesn't work. It makes the waveform go half-wave negative. Also since there are 8 of these amps on one board I would like to keep the number of parts down and would like to avoid using 8 transistors if possible.


Any ideas?

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Sch3mat1c
Posted: April 03, 2012 05:05 pm
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Well, you need a series resistor on the base to limit current. 10k will do. As shown, you're shorting out the op-amp when it tries to drive more than about 0.7V, while leaving it unharmed for most negative voltages (until the base-emitter junction breaks down, -7V or so).

The LED will only begin to light up when the output amplitude exceeds +0.7V peak.

If the signal is normally stronger (>2V peak), you can drive the LED and resistor directly from the op-amp without the additional transistor and resistor.

Alternately, use a 2N7000 instead of 2N3904. This is a MOSFET which will not draw input current. It will only turn on when gate voltage exceeds 1-3V (actual value depends on manufacturing tolerance and temperature), so it isn't more sensitive than the LED, but won't load the op-amp.

Tim


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BigJohnny
Posted: April 03, 2012 05:18 pm
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Wow, that's awesome. I think that's exactly what I needed, and those are cheap.

Is there a way to bypass the op-amp with the 2n7000 such that when power is turned off to the op-amp, the input signal can pass from pin3 to pin1 as-is without amplification?




The reason for using the MOSFET(thanks) to turn on the LED is because the input signal flucuates, it comes from a piezo transducer, so I would need to be constantly changing the LED resistor value. This allows the LED to just get a single power supply.
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Sch3mat1c
Posted: April 03, 2012 05:32 pm
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You could put in an SPDT relay that runs from the same supply as the op-amp. Power off, relay switches the output connector from op-amp to input jack.

Doing that electrically will be difficult, because electronics usually need power available in the first place to conduct, and because any signal driving the op-amp's output while it's powered down will be distorted by the op-amp's output stage (which contains diodes -- if you drove it hard enough, it'll power itself!).

Tim


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BigJohnny
Posted: April 03, 2012 06:21 pm
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whats this? have I created a free energy device??? biggrin.gif

I could use a relay such as the Hamlin HE721 or similar, but someone mentioned that it could unbalance the rails causing issues with the op-amps. There would need to be 8 relays held open when the power was on. That could draw a bit more current than I'd like or am capable of putting out.

This is one small section of a much larger circuit containing 8 of those amplifiers, and an LM386/555 circuit for converting a piezo signal to an on/off signal.

Simulating it with multisim and the 7000 appears to work, the light flashes when there is a pulse.

I'm going to consult with my electronics teacher and see what he has to say about the diagrams, when it comes to electronics he can see matrix code or something in the schematics it's like he's one with the electrons.
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CWB
Posted: April 04, 2012 01:39 pm
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"...but someone mentioned that it could unbalance the rails causing issues with the op-amps."

i don't see a problem ... if you are using a dual sided regulated supply (which is a good idea) and the supply has enough current capability to run everything in the "full tilt"/worst case mode .

if one is really concerned with "unbalancing" , one could always build a simple separate supply to power the relays .

low audio/signal levels can pose a (long term) problem with relays ... the contacts can become "grunged" and not pass the signal effectively .
a couple of things help out in this area :
gold plated (or similar alloyed types) self wiping contacts
a fairly well sealed case

the hamlin type you mentioned is a good choice for design considerations ...
i did not see any mention of the contact alloy used but should be ok .
(i have seen these in broadcast equipment)
the 700-1B is the one that would work (NC SPST contacts)


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tekwiz
Posted: April 04, 2012 07:39 pm
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A relay can only unbalance power rails if it's powered by only one of them. Using a higher voltage relay across both lines will maintain the balance.
Note also that relays are available with more than one set of contacts. Two sets is a common type & they are available with 3 or even 4 sets of contacts.
The problem with doing this electronically, like Tim mentioned, is that all common semiconductor types require power to conduct. There are none that require power to stop conducting.


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BigJohnny
Posted: April 05, 2012 03:03 am
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I suppose I could run a relay at 18V, as a matter of fact I think I know of such a component that I have in my posession, it's an axicom relay.

I still don't really like the idea though as it's a bit larger than the Hamlin relay and I have to have 8 of them. that's gonna take up a lot of space.

Something else I've noticed here, about the pulse LEDs, if I turn down the first trim pot too low, the lights will fail to flash. obviously the resistor would need to be of a different value, but this raises a more interesting concern;

The voltage going into the op-amp is going to change as it's an analog signal from a piezo transducer, and ranges from about 1Vp to around or above 10Vp depending on how hard the piezo was hit.

So I'm not really sure how to go about dealing with the LED across that whole range.
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Sch3mat1c
Posted: April 05, 2012 03:22 am
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If you just want a binary hit-or-not-hit signal, you can use a comparator instead of an op-amp. Set it for perhaps 100mV input threshold (i.e., if the input voltage is greater than 100mV, the output turns on) and you'll see the LED flash for 101mV+.

Threshold is the same as gain (the pot in your circuit), which is hit sensitivity.

If you want any kind of duration, you can use a monostable (one-shot) timer to turn the series of blips into a flash of useful duration (perhaps 50ms). This will make the LED flash more uniform in intensity as well as smoothing out the strike (which will appear as a series of blips as the signal goes above and below the threshold, like a "bouncy" switch).


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BigJohnny
Posted: April 05, 2012 03:36 am
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The piezo signal must pass unchanged other than being amplified. The reason for this circuit is to amplify the signal and allow for more sensitivity/softer hits to be registered.

The circuit works, just not sure how to keep the LED flashing when the sensitivity trim is adjust below 55% at full gain or 85% at no gain.
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CWB
Posted: April 05, 2012 01:03 pm
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it sounds like (i'll take the pun) you are only looking for a minimum indication level as anything above this is "good to go" .

as was mentioned , a "pulse stretcher" and comparator circuit would help isolate the signal and so-forth ... gain is your friend as it allows resistive coupling to drop the loading effects .
taking this one step further , a buffer amplifier will provide more than sufficient isolation .
this is how the big boys do it .
the signal to drive the circuitry associated with the led indicator(s) is only "sniffed" ... there would be enough resistance involved that would "isolate" one from the other .

relay voltage ...
most relays will operate slightly below and above their rated voltage with no problems .
if the relay coil voltage you want to use is way below the available voltage , say 12 volts and the applied voltage is 18 volts (rail to rail in this case) ...
a dropping resistor can be used on each relay coil .
the current drawn by the hamlin relay mentioned is pretty low .


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BigJohnny
Posted: April 05, 2012 01:10 pm
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I'm not really sure how to build a pulse stretcher or use a comparator and amp together to achieve the end result.

I don't have the knowhow to design such a circuit, and yes I really only do need a minimum indication, the LED does not need to be hit sensitive.

I would still prefer to use transistors or FETs to do the switching instead of relays, if this is possible.


So how would I go about constructing the circuit you're referring to?
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tekwiz
Posted: April 05, 2012 09:17 pm
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The comparator/pulse stretcher circuit that Tim mentioned would be an auxiliary circuit to your amplifier. It would simply monitor either input or output(your choice) & flash an LED.
A comparator is basically an opamp circuit & CMOS opamps have such incredibly high input impedances that it would not affect your piezo signal at all. Any general purpose opamp will work for this, & the configuration as a comparator is a standard circuit.
The LM555 timer chip will then take the output pulse from the comparator & stretch it to a usable length, again your choice. The 555 monostable circuit you need is also standard. There are lots of examples of both circuits online.
All you have to do is look them up, build the circuits & chain them together. As I see it, you need 3 circuits total. Your amplifier, a comparator, & a pulse stretcher. If the pulse stretcher is 555 based, it will drive your LED directly.
Your actual output signal will come from the amplifier stage & there's no reason this cannot have variable gain.


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BigJohnny
Posted: April 05, 2012 11:40 pm
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I didn't want to add a 555 timer to each amp (there are 8) just for an LED.



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CWB
Posted: April 06, 2012 12:37 pm
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"I would still prefer to use transistors or FETs to do the switching instead of relays, if this is possible."

in theory this could be done with fets .
forget the bipolar transistors , the on resistance is much higher than with a fet .
the trick is that you would have to have a "bias battery" and/or "standby power circuit" that would constantly supply working voltage when the unit was "powered down" .

the relays are a simple and elegant solution to a need in/of the circuit requirements .


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Sch3mat1c
Posted: April 06, 2012 02:52 pm
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Actually by size, BJTs have lower "on resistance". This is more noticeable in analog switch apps, using invertible transistors (an "NPN" transistor reads the same either way, but manufacturing differences in the emitter and collector have to be carefully managed to make it also work the same both ways). Typically these devices have about ten ohms resistance, while a comparable CMOS switch or JFET has a hundred ohms or so.

BJTs still require current for operation though.

There is one passive possibility -- enhancement and depletion mode MOSFETs are normally off and on, respectively (at Vgs=0). Both require negative bias to turn off, it just happens that the enhancement mode turnoff voltage is no more negative than 0V. Depletion mode FETs are relatively uncommon though.

Tim


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tekwiz
Posted: April 06, 2012 05:57 pm
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QUOTE (BigJohnny @ April 05, 2012 02:40 pm)
I didn't want to add a 555 timer to each amp (there are 8) just for an LED.

That's 2 chips...555s are available up to 4 in one package; the LM558, IIRC. Note that you may not need the 555s. Their only purpose is to stretch very short pulses into something you can easily see. It's difficult to see a very short pulse of light in an LED. Not only do they appear dim, it's too easy to blink & miss one. But this may not matter in your app...that's up to you.


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BigJohnny
Posted: April 07, 2012 12:55 am
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Ya I just need it to blink, it's not terribly important that it be seen but I would like it to blink at full brightness even when the signal is really low, I think the minimum signal is like 1Vp
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jman 31
Posted: April 07, 2012 01:27 pm
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QUOTE (BigJohnny @ April 07, 2012 12:55 am)
Ya I just need it to blink, it's not terribly important that it be seen but I would like it to blink at full brightness even when the signal is really low, I think the minimum signal is like 1Vp

I've tried all kinds of circuits to make an LED blink from a piezo, but the most successful way that I found was with a 555 chip like everyone has mentioned. The "pulse" from a piezo is so quick that it is hard to see. I have tried amplifying it etc, but you almost have to have a pulse stretcher to be able to see it. I don't have the know-how either, but I wish I did. I would love to find a velocity sensitive trigger that was bright enough to be useful. The 555 only gives you one result no matter how hard you hit it, but it sounds like that is what you are after.

Sorry if this isn't helpful, just putting in my $.02.... laugh.gif


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BigJohnny
Posted: April 07, 2012 02:05 pm
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Alrighty, what do I need to do with a timer to get it to flash the LED?
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jman 31
Posted: April 07, 2012 03:19 pm
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Here is a circuit that I have used that works great. You can do away with the battery. Also, the pots can be swapped for a static resistance once you determine what you want the duration (P2) and sensitivity (P1) to be. You can do away with P1 altogether and make R1 about 220 ohms to increase piezo sensitivity.

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tekwiz
Posted: April 07, 2012 08:06 pm
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Good call, Jeremy. I forgot that you just went through the same process of finding a circuit to deal with piezo pulses. thumbsup.gif


BTW: In your circuit, R1 is a bit redundant...the input level pot does the same job.


@BigJohnny: The transistor type in Jman 31's circuit is not at all critical. Just about any small signal NPN silicon transistor will do. You don't need an actual 2N2222 to make things work.


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CWB
Posted: April 08, 2012 01:03 pm
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heh ... i was wondering how long it would before jman put in on this thread .
laugh.gif tongue.gif


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jman 31
Posted: April 08, 2012 01:42 pm
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QUOTE (CWB @ April 08, 2012 01:03 pm)
heh ... i was wondering how long it would before jman put in on this thread .
laugh.gif tongue.gif

Finally something I know a little bit about! haha blush.gif


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BigJohnny
Posted: April 08, 2012 02:36 pm
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QUOTE (jman 31 @ April 08, 2012 07:42 am)
QUOTE (CWB @ April 08, 2012 01:03 pm)
heh ... i was wondering how long it would before jman put in on this thread .
laugh.gif  tongue.gif

Finally something I know a little bit about! haha blush.gif

What is it that you're working on? for that matter what program did you use for the schematic and does it do simulations of the circuit?

There is no piezo in multi-sim, I'm wondering if it's possible to create one somehow.


Anyway that aside, I'll check that out.

EDIT---

Hey that works nice, but I don't see P1 really doing much of anything, P2 increases on duration, but is there a way to adjust the off duration as well?

Lastly, my drumpad would plug-in where the piezo is located, but do you have the output on the power rail as well? I can't really figure out where to put the output on my diagram because I think it works just slightly different?!

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