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# Stupid newbie needs help

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#### lordv2500

##### New Member
Like the subject says I'm a complete novice when it comes to electronics and I found this forum while searching for somewhere to ask for help. If somebody doesn't mind taking some time to help me, could you please help me whith this circuit that I'm supposed to build(for highshool) and explain. I can build it no prob but I haven't been studying for very long and don't really know much.
Ok, heres the schematic for the thing I'm supposed to build:
http://www.geocities.com/lordv2500/sch.txt
Please, could someone explain how this works and what it does.
I know I'm asking a lot, but please even if you just explain the function of a little part, like even the 741 opamp(showing the formulas I'd need to to find Vout).

i think it is a dorbell or some kind of ringer. with many tones. and maybe the switch you press will produce a tone. cant figure it out exactly.
use some simulation program to see what it does.

If i think right, this is a simple door alarm,with go-in and go-out delay.The 741 circuit a comparator with hystersis. Its make prell-free for contact (key) and generates negative trigger for the following 555.(the key and condenser is parallel)
The key"C" function is armed/disarmed. The last two 555 (before the loudspeaker) is a sirene circuit as bogdanfirst wrote.

With calculations? Cause I can't seem to understand it. It seems that when the switch is closed vout=+vcc(15v) and when it's open it's -vcc(0v), why? Please help me understand!
Vth=3.7v
Vtl=0v
Also could someone recommend a simulation program to use?

Unfortunately i never used simulation program.
Operational amp. work as comparator if no negative feedback. When You connect a +voltage to -in (e.g.+5V while Vcc15V) as a reference, the output is low if +inV<-inV and suddenly go to high when +inV increase over -inV (open loop gain very high)In this case because for 555 need negative trigger - and for stability a hysteresis - the +refV(3.7V) coming from output divider to +in.If the switch is closed, -in=0V, out=15V,when the switch opened, the 100nF "slowly" charged via 10k, and when increase to +3.7V, the out go to about 0.5V (Uc-e sat from output transistor), +in near to 0V.

why isn't the 0.1uF charged when the swith is closed?
Is it as if the 0.1uF doesn't exist and the resistor get the full voltage and nothing is left for -in

As first i wrote the condenser and switch is parallel,Its a draw failure: no crosspoint - 100nF, switch, 10k and -input connected.

Okay thanks!!!
Now for the next part!
When outage is -vcc on the comparoror, the 555 creates a delay determaned by (Ra+Rb) and C right? t=1.1*C*Rab, afterwards what happens with the second 555 timer?
Sorry for so many questions

Or maybe someone could recommend some sites on the net explaining this type of stuff with examples or recommend another forum which is more active than this one, because there don't seem to be many people using this one with an average of 3 registered users online at a time.

When by first 555 determined time go to end started the second 555 and the syrene. The syrene runtime determined by second 555. If this circuit a door alarm, impossible go out without alarm.

I think I might need some help here too.

For the 741 circuit, it looks to me like it is a feedback circuit. I can understand the circuit used to eliminate contact bouncing (with the switch) for the IN- input, but aren't the 3.3k and 10k resistors part of a feedback network?.

I don't understand what the 741 circuit is doing. If it were a comparator then why is there any feedback at all?

Okay, I was just thinking a bit more about it.

Initally, you hook everything up and the "space" key is open. IN- = 15V after cap has charged. Due to negative feedback IN+ becomes 15V (virtual short circuit), while the output also is at 15V (the gain of the system is 4, but VS+ is 15V so this is the max output)

When you then close the "space" switch, IN- becomes 0, output becomes 0 and there you have your negative triggering for the 555 timer.

Is this right or have I wandered away from the track completely?

What happens with the condenser and tthe parallel resistor and diode. I don't get when the second 555 is triggered. Can out be more than 15v on the first 555(impossible right?). So when output is 15v and vcc is 15v what happens to the diode? and also I never studied the 4001 what do I consider logic 1 and what do I consider logic 0?

Zainny, acctually first when the switch is closed -in becomes 0 therefore smaller than +in(max 3.7v) and out is 15v and when switch is open the cap is charged, once it reaches over 3.7v out becomes 0v and when the switch is closed again the cap discharged and once it's lower than 3.7v again the out is 15v. Well that's how I understand it now , hope that's right.

I understand how a comparator works lordv2500, because that is basically what you have described. If we simply want to compare if the input is above 3.7V then I don't understand why the circuit is as it is...

From what I can see. We want:
15V out for input < 3.7V
0V out for input > 3.7V

Why not have the Op-Amp and simply have IN+ @ 3.7V and IN- as the voltage over the cap?
Why is there a resistor branch from the output to IN+?

Half of the parts are there to make the whole thing more confusing. The same reasult could be achieved with fewer parts. It's just that my teacher wanted to include everything we were studying. Frankly, I see no reason for the 741 at all, it seems to be an overly large switch. It serves no needed purpose.

Here take a look at this, does this seem right.
http://www.geocities.com/lordv2500/graphs.txt

Zainny,the voltage divider make hysteresis for the comparator.
Lordv, the D1 diode need the proper discharge of output capacitor after triggering. I agree, you can build simplyer circuit with fewer components and i mean the 4001 first part bad choice, here better the NAND gate.

so do the graphs I dis seem right to you?
Just a few more things and I'll understand this thing completely!
On the 3 NOR what happens to the paralell capasitor and resistor. I now when the second switch is closed it should be 1 but how does that happen.

After that, between the third and forth NOR is this what happens. If out(10) from third NOR is 0 then the capasitor is charged and in(12) is 1.
When out(10) is 1 then in(12) is 0. right?

And last, what is the purpose of the capasitor just behind the speaker?

I think the 3. and 4. NOR gates wanna be a flip-flop, but in this case cant work. When after power on the pin11=0 You can change to 1 by key.(pin8=0 by 4.7k,pin9=0 by pin11. If the key closed,pin8=1,pin10=0,until condenser charging pin12=0,pin11 = pin9 go>0. no more work.)
The serial capacitor make DC decoupling.
4.7k is a pull-down resistor,101nF for key debouncing.

I think the 3. and 4. NOR gates wanna be a flip-flop, but in this case cant work. When after power on the pin11=0 You can change to 1 by key.(pin8=0 by 4.7k,pin9=0 by pin11. If the key closed,pin8=1,pin10=0,until condenser charging pin12=0,pin11 = pin9 go>0. no more work.)
The serial capacitor make DC decoupling.
4.7k is a pull-down resistor,100nF for key debouncing.

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