I did not mistake it with a 10uF, because I specially ordered some 10nf's and as far as I can see on them, they are 10nFs. I will try to replace them with a 0.1uF or a 1uF and see if that works. Maybe I am so lucky that I have some on hand.

 

Okay, now I have tried to put an 100nF = 0,1uF right? onto my circuit. That didnt work. I tried to take a 10uF and that one I could pump up. If I taped the piezo fast a few seconds, the capasitor build up its load and as soon as I stopped tapping the piezo it got drained very rapidly. Is this how the circuit should work? How do I make it stay on for 2 seconds or so?

 

Yes, that's pretty much the way I'd expect that circuit to respond. If you want the LEDs to stay on (with constant brilliance) for two seconds or more you'll probably want to feed the Drain to the 'Trigger Pin' of a 555 and connect the LEDs to the 555's output. Since this is the Sergeant's circuit he may have different ideas though.

 

Okay, that sounds interesting. I happen to have a 555 timer on hand. Sgt. could you maybe advice me how to connect that to the circuit, to keep the LED bright for an adjustable time, adjusted by the potentiometer?

 

Okay, I did some searching around on the 555 timer chip and figured out a monostable circuit would be the one I am looking for (I guess). I found a nice circuit, that I tried to build together with the circuit I am using now. Because I am a beginner in electronics I have some thoughts about this new circuit I have been put together.

First. Can you see some problems in the circuit?

I dont know if it is possible to trigger the 555 timer, the way I did it. I thought that now the current is down to 15v due to the zener, the 555 trigger could handle it. So I just put it right into the trigger pin.

I did some calculating with some formulas ive found and got from 0,011 seconds to 11 seconds adjusting with the capacitor and R1 I have choosen and thats a fine spectrum.

I hope that you can point any problems out, that I havnt been able to get rid of.

 

You can keep just about everything the same as Sgt.Wookie's circuit and add the 555 as shown here. VG1 is just a pulse source that I used to simulate this. U1 should hold the Led on from about 500mS (VR1 @ 0 Ohms) to over 6 seconds with VR1 at max. A 1000pF at the Gate of Q1 is all that should be needed, as the 555 will do all the stretching.


EDIT: C2 is a typo. It should read .01uF

 

Here's the circuit redrawn with Sgt.Wookie's circuit feeding the 555 trigger pin. I Lowered the value of the Gate Cap because you won't need much capacitance there any longer. The Drain resistor isn't critical so You can use anything from 1K to 22K if you like. C1 should preferably be a low leakage type like a Tantalum but for breadboarding you can use an Electrolytic. Keep in mind if you do; the pulse width from the 555 will probably be longer than shown in the graphs that I posted because of the increased leakage as compared to a Tantalum.


EDIT: C2 was a typo and has been corrected. Also, though insignificant, R2 was changed from 10K to 1K.

 

Thank you very much for the drawing. I forgot to put the drawing I made myself and it diviates a bit. But I have put the drawing here, just to show you what I got.

I have a few questions about your circuit.

R3 and C3, what are their function?

Do I need to feed the signal from the piezo though Q1, isnt the current comming from the piezo enough to trigger the 555 timer? (the way I drawed it?).

Can I change the potentiometer to 1 meg ohm (that what I bought for the other circuit, and it could be nice if that would fit).

Can I use a 220 ohm resistor infront of the LED as in the other circuit?

Do I have to put a capacitor to the control pin? I know that you do this to stop electrical noise, but would it affect the working of the circuit, if it was removed and control just went to ground?

Can you tell me what function R2 has? I havnt drawned that on my drawing, so I am currious. If I had to guess, I would say it is there to limit the current from both the piezo and the power supply to keep the current below the 15 volt the 555 timer can handle?

 

I have a few questions about your circuit.

R3 and C3, what are their function?

R3 prevents the Gate from floating. C3 is a good idea to suppress false triggering and suppress transients and noise.

Do I need to feed the signal from the piezo though Q1, isnt the current comming from the piezo enough to trigger the 555 timer? (the way I drawed it?).

Off hand I'm inclined to say no but I will have to spice it and get back to you. For certain... not the way you drew it!!

Can I change the potentiometer to 1 meg ohm (that what I bought for the other circuit, and it could be nice if that would fit).

Yes, but change C1 to ~4 to 5uF. Note!: R1 must remain. You can change its value but it should not (practically) be below 1K.

Can I use a 220 ohm resistor infront of the LED as in the other circuit?

Depends.. Is your supply still 6V as it was when you started this thread?

Do I have to put a capacitor to the control pin?

Yes

Can you tell me what function R2 has? I havnt drawned that on my drawing, so I am currious. If I had to guess, I would say it is there to limit the current from both the piezo and the power supply to keep the current below the 15 volt the 555 timer can handle?

A 555 triggers when the Trigger pin goes low, or more specifically, on its negative going edge. So... Sgt.Wookie's circuit rectifies the AC output of the Piezo, clamps the positive going signal at 15V with the Zener, and filters the signal a bit with the Gate Cap. This voltage is applied to the Gate of the FET which turns on the Drain - Source junction of the FET. This causes current to flow through R2 because current is now flowing between the Source and the Drain. Since a Common Source Amplifier inverts its output at the Drian, this will produce a negative going edge to trigger the 555.

 

Thanks for the explenation.

I have found a 1nF for C3 and but I have trouble finding a 1 meg ohm resistor for R3, so I think I will have to buy some of those. When I asked people here on the forum, in what devices I usually would find a 1 meg ohm resistor (so I could rip it from there) they thought I was crazy, but if I could find a used one, then I wouldnt have to buy them and pay the start-up fee at my local electronics store.

Ive found the 10k resistor for R2 and a 4,7uF for C1, because I will use the 1 meg potentiometer.

C2, is that a 10 microfarad or what does m stand for?

Else I have got everything. I want to have a 6volt power supply, so the 220 ohms resistor before the LED should be allright I think.

 

C2, is that a 10 microfarad or what does m stand for?

That's a typo. That should read .01uF (10nF) but it's not a critical value. A .01uF to .1uF will work fine.

Regarding your component procurement; do you live in the USA?

 

Here is what you asked for. It Spices OK but you'll have to build it to see if it works in the real world. Note that all the original components from Sgt.Wookie's circuit have been inverted for use without the FET.
EDIT: This circuit was spiced to satisfy your request for a Fet-less alternative. Keep in mind that this method is a bit unorthodox and that a FET amp/inverter, as used in my last circuit, is infinitely preferable for a number of obvious reasons.

 

Thanks very much for the new drawing. That somehow looks more confusing, than the one with the mosfet in it.

Just to be absolutly sure, that I have the right parts. The other drawing you did with the mosfet in it, is this true:

C1 = 10uF
C2 = 10nF
C3 = 1nF

Is C3 also not so critical and would a 10nF capacitor be alright to use insteed? Does R3 have to be 1 meg ohm? I will go and buy some if needed, but if I can use anything else on hand that would be prefered.

 

Thanks very much for the new drawing. That somehow looks more confusing, than the one with the mosfet in it.

Just to be absolutly sure, that I have the right parts. The other drawing you did with the mosfet in it, is this true:

C1 = 10uF
C2 = 10nF
C3 = 1nF

Is C3 also not so critical and would a 10nF capacitor be alright to use insteed? Does R3 have to be 1 meg ohm? I will go and buy some if needed, but if I can use anything else on hand that would be prefered.

Since you have a 1Meg pot already refer to the schematic in post 27. This is the circuit I'm basing the following on and the one you should build.

Replace VR1 with 1Meg
C1 can be anything between 5uF and 20uF (Low leakage type like tantalum). If C1= 5uF max time (LED On) will be ~ 6 Sec. If C1=20uF max time (LED On) will be ~ 24 Sec. Both will be true when VR1 is at max. With VR1 at min the LED would just flash briefly.

C2 can be anything from .01uF to .1uF.
C3= .001uF also stated as 1000pF.

 

I havnt had time to go to the store and get some components, but ive found a little circuit simulator, that is very interesting. It can be found here: http://www.falstad.com/circuit/

The simulator has some preset circuits in it and it does have a monostable 555 timer circuit to show. It looks very similar and I tried to play a bit around. One thing I did notice was, that the circuit also contained a resistor, like the R2 on the circuit you posted at post 27. But when I tried to mess with the numbers of ohm on that resistor it didnt change anything. I tried to remove the resistor and nothing seemed to happen. Can you tell me what the R2 is needed for and what its function is? I know I have a lot of stupid questions, but I am trying to learn what each component does for the circuit.

 

Free Spice usually lock out many functions to entice the user to purchase it.

R2 is the FET's Drain (load) resistor. In that circuit it also serves as the 555's Trigger pull-up resistor. When the Trigger pin goes low the 555's output pin goes high for ~ the RC time constant of T= 1.1 x ((VR1 + R1) x C1)).

 

I have now put the circuit together and it works, but is there anyway to make it more sensitive? Or can I somehow amplify the output from the piezo or does it not produce any electricity at all if the circuit doesnt make the LED light up?

 

If I put an amplifier before the 2N7000 mosfet, it would kick the low voltage comming from the piezo up, so it would be enough to trigger the 2N7000 and with it, the whole circuit, right? If I put a potentiometer in the amplifier circuit bit, then wouldnt I be able to control the sensitivity of the circuit to some extend?

 

If I put an amplifier before the 2N7000 mosfet, it would kick the low voltage comming from the piezo up, so it would be enough to trigger the 2N7000 and with it, the whole circuit, right? If I put a potentiometer in the amplifier circuit bit, then wouldnt I be able to control the sensitivity of the circuit to some extend?

That's what was done in this circuit.
http://forum.allaboutcircuits.com/showpost.php?p=314331&postcount=27

By the way....Did you know?>>> If you're looking to simply light a LED with a light touch of your finger, it's easier than using to do than wacking a Piezo!

 

So if I understand you right, I will have to change C3 to a 100pF capasitor or so, to make the piezo more sensitive? Atm. I dont have any equiptment to make PCBs, so what do you think would be best, a predrilled copper board with dots, or a predrilled copper board with lanes.

I have read that 2N7000 can be swapped out with a BS170 (wich are easier for me to get) is that true?

I was wondering what to do if I want two piezo's, so if one gets shot I can still shoot, using the other one for triggering. I guess it is easily done, by putting another piezo in parallel with the other one, but I may be overlooking something?