Hello

I would like to build a circuit that can show a hit on a target, with a light. The circuit must light up for about 3 secounds or maybe if its possible, the light periode can be changed to what fits the best. But the circuit has to be very simple.

Ive found this piezo component, and I am not sure if that will do:

http://elektronik-lavpris.dk/product_info.php?products_id=104875

Does the circuit has to be build around a timer of some kind or can it be done by a capasitor to keep it simple?

/Jacob

 

Ive found this circuit, but it contains two sensors and I just want one. Can it be altered, so it will work for my purpose?

 

Any piezo should work. Know the following:

The piezo crystal is just that, a crystal. If it takes a direct hit it will crack. So, you'll have to protect it somehow.

The piezo has a very light metal coated surface. It is not easy to solder to these (though it can be done with practive and patience, I've done it hundreds of times). Conductive epoxy doesn't work well.

The crystal can (and will) generate high voltages, hundreds of volts depending on the unit. You MUST clamp the input to your "circuit". Use Schottkeys for this or a dual TVS device.

You can "trigger" just about any kind of monostable for the output pulse. A 555 circuit can have a pot to adjust the pulse. You can trigger a uC or something like these - http://parametric.linear.com/html/timerblox.

Mike T.

EDIT: The circuit you show is just two of the same circuits "diode OR'd". You can essentially remove the whole bottom circuit but there are easier ways. Look at the 555 section od this website.

 

I did this circuit awhile back for someone; it's pretty simple.

D1/D2 rectify the output of XTAL1 to DC.
D3 clamps the voltage to 15 max. A BZX79-B/C15 would be a better choice.
R1 & R2 control the amount of time to discharge C1 and Q1s' gate. R1 ensures that the LED(s) will be lit for at least a brief amount of time.
Q1 is a logic-level MOSFET that is rated to sink up to 200mA, but 100mA is a practical limit.

 

thank you very much for this circuit, it surely looks very simple.

I have a few questions to it tho.

Can I use something else, than a bzx79-b15 or the 1N4744, because the first one I can't find by my local dealer and the second one I have to buy 100 of.

The piezo, can that be what ever piezo component or does it have to be a specific one?

The variable resistor, is that a 1 mega ohm?

If I just want one LED, can I then just ignore the two others?

What component do I have to change to shorten or widen the time the LEDs are on?

How did this circuit work for you?

How did this work for you?

 

thank you very much for this circuit, it surely looks very simple.

It is.

I have a few questions to it tho.

Can I use something else, than a bzx79-b15 or the 1N4744, because the first one I can't find by my local dealer and the second one I have to buy 100 of.

Sure. Radio Shack carries 5.1v and 12v Zeners. You could use 3 of the 5.1v Zeners in series, or one 5.1v and one 12v in series. The critical thing is to not let the voltage on the gate of the MOSFET exceed 20v, as that will destroy it.

The piezo, can that be what ever piezo component or does it have to be a specific one?

Well, it needs to generate a voltage when struck. You could probably use just about any crystal that had a rugged package.

The variable resistor, is that a 1 mega ohm?

Yep. The value is approximate. The greater the resistance, the longer the MOSFET will stay on.

If I just want one LED, can I then just ignore the two others?

Yep.

What component do I have to change to shorten or widen the time the LEDs are on?

The 1 MEG pot. Just turn the knob.

How did this circuit work for you?

The person who asked for it was going to use it for a practice drum kit. They said it worked very well for them. I never built the thing myself; didn't have a use for it.

 

Okay, thank you very much. I will try to build this in the weekend and see if I can get it to work.

 

Okay, now I got it all put together, but I have one problem. The 1 meg ohm doesnt seem to do anything. Nomatter how much I turn it one way or another, the light comes on for just a bit, like a half a second or so. The circuit works well, but it can't change the time of the light to be on. WHat can I have done wrong or can I do something to change the time to a bit longer it will stay on. How long should it be able to variate from?

 

You should have one wire on the center pot terminal and another wire on one of the end terminals. The center terminal is generally the "wiper".

If you're still having problems, post a photo of your circuit.

 

I have done that and it all looks like it is connected right. I will try to reassample it tonight and see if it will work.

 

Okay, ive tried to solder it together again, but now I have another problem. When I put the batteries in, the light comes on. I can turn the light sligthly up and down using the potentiometer. When I turn it to the lowest light possible I can see the light go up, when I hit the piezo unit.

What can I have done wrong. I have looked at it now, for over an hour and I just can't figure it out.

I had the same problem earlier today and I found out that I had the 2N7000 transistor the wrong way around. I rotated it and it all worked fine, just not the time variation adjusted by the potentiometer. I have put the transistor in, the same way I did there, so it shouldnt be the problem.

Any ideas?

Just to make sure. On your schematics Sgt. the lowest leg on the 2N7000, the one that goes to the negative side is the source. The one in the middel is the gate and the top one, going to the LEDs is the drain, right?

Just for the hell of it, I tried rotating the 2N7000 around and I got, what I did earlier. What I said above, about getting the same as earlier it isnt true. This is something else. The 2N7000 seems to be positioned the same way as I had it, when it worked, but at that time it wouldnt let me adjust the time the lights was on.

I will draw a little sketch of how it is assembled, so you can see what I have done wrong.

 

Okay here is my sketch.

Can you see something wrong with it?

 

Odd. I don't see anything obviously wrong, and others have built this circuit and had it work.

 

Do you recognize the symptomes? Lights come on with the power and it can be adjusted a bit with the potentiometer. When the pieze gets hit, the light gets brighter.

 

If the LED is on when you power up, the odds are the FET is miswired. Are you sure your not interchanging the Source and Drain? The Drain-Source junction employs a protection diode that will conduct if the above condition is true.

 

If you'd connected that MOSFET backwards before, you may have damaged it.

It (the LED) should go completely dark.

 

I had it like I have it now to start with, but just to see, if I did something wrong, I tried to turn the mosfet around and it did light the LED up, but in a constant brightness and it did turn on as soon as the batteries was put in.

I have another mosfet, so I will try to change the mosfet I have in the circuit now and see if that helps.

 

Okay, now I have swapped the mosfet with a new one and it did seem to be there the problem was, but now I am back to not being able to regulate the time the light stay on. The potentiometer doesnt seem to have an effect, or the effect is so minimal that I can't see the changes. Can it be the capasitor that is to small or too big? Can it be the potentiometer, wich has to many ohms in it?

 

Try replacing the capacitor with a larger one; 0.1uf or 1uF.

 

Did you mistake C1 (10nF) for 10uf? 10nF = .01uF.