Basic circuit help please

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I have put together a circuit from here:
https://circuitdigest.com/electronic-circuits/light-fence-circuit-diagram-with-alarm

and this is the schematic:
https://circuitdigest.com/fullimage?i=circuitdiagram/Automatic-Light-Fence-Circuit-Diagram-with-Alarm.png

I have the led, ldr and the 100k pot part of the circuit working fine,  
but the buzzer from the 555ic does not work or sound as it is supposed  
to do. (the buzzer is new and and has worked on another project)

I am using a BC327 pnp transistor as I don't have a BC557 transistor

The author clearly has it working fine, but I cannot get it fully working.

Hopefully as per the schematic, I have a wire from pin 2 on 555ic to the  
emitter of the BC327, and also a wire from pin 3 of the 555ic to the  
positive side of the buzzer.

Now, the strange thing is after removing the said wire from the BC327  
emitter, the buzzer does work, albeit when the ldr has not been  
covered.This seems to be due to the capacitor across pins 1 and 2 of the  
555ic.

Thanks

Re: Basic circuit help please
On 30/01/2020 4:48 am, RobH wrote:
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What type of buzzer?  ie. Is it one that just requires  a DC voltage  
supply or one that needs an an oscillating supply to drive it. If the  
former then put it across the LED, perhaps with a "back" connected diode  
just in case it produces som back emf. If the former then check the  
output of the 555.

Re: Basic circuit help please
On 30/01/2020 00:44, RheillyPhoull wrote:
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The output of the 555 pin 2 across the 10uf capacitor is 6.3v with no  
ldr detection, and 4.6v with detection. The output from pin 3 is 0v.

The buzzer requires between 3v and 5v to work, so needs a DC voltage supply.
Would a piezo electric buzzer do what I want it to do, beep when  
detection is present.
Thanks

Re: Basic circuit help please
On 30/01/2020 7:50 pm, RobH wrote:
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Just leave the 555 out and put the buzzer across the LED........"KISS"  
principle

Re: Basic circuit help please
On Fri, 31 Jan 2020 07:44:20 +0800, RheillyPhoull

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Leave the op-amp, and transistor out and trigger the 555 as a
monostable directly from the photo detector.  The input impedance to
the internal comparator is high, the data sheet gives the current as
point five microamps.  The sensor would have to connect to VCC and the
pot be in the ground leg to sense darkness.

Re: Basic circuit help please
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With 10uF and 1M the timing the 555, it is running at less than 1Hz,
so a buzzer is the right choice there, not a loudspeaker.


--  
  Jasen.

Re: Basic circuit help please
On 29/01/2020 20:48, RobH wrote:
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I don't see how this ever worked. The op amp + input is effectively  
connected to the + rail. The op amp output will never change state. Does  
the LED change?

Re: Basic circuit help please
On 30/01/2020 12:16, Andy Bennet wrote:
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Yes, the led does light up when the ldr is shaded. Now I have connected  
the 5v buzzer across the led , and it beeps when the ldr is shaded .

The author shows it working fine, but he did not state what type of  
buzzer he used. I just bought an Adafruit 3v to 5v buzzer, and expected  
it to work, but it didn't in the way it was connected.
Had I realised or even checked that there was no output from pin 3 on  
555 initially, I would have eventually found a way round it.



Re: Basic circuit help please

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There are several design errors in the schematic IMO.  If he managed
to get it working, it is doing so in spite of the design, not because
of it.

In addition to my earlier comment, another WTF idea hit me, why in
hell does he find it necessary to use a op amp at all?  A small signal
transistor has more than enough gain to operate directly from a CDS
cell, a mosfet would have more than enough to operate directly from a
photo transistor.  I'll bet that explains how the reversed logic works
too.  (the op amp isn't doing a damn thing necessary to the circuit)

Piezo buzzers use very little current and can operate directly from a
555 output without a driver transistor.  If you are new to electronics
get a piezo buzzer that works at the voltage you have there, and don't
get a piezo transducer - they require driver circuitry to emit sound.

Re: Basic circuit help please
On 30/01/2020 15:08, default wrote:
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Thanks, and yes I might as well be new to electronics as the last time I  
did anything in this work was over 35 years ago, and have forgotten most  
of what I knew then.

i should say that I am doing this as a hobby and not as anything serious  
or  professional.

Re: Basic circuit help please

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I'd suggest you scrap that schematic and start from scratch with a new
one.

An excellent source for just what you are attempting here would be one
of the Forest Mims "Engineer's Mini-Notebook," Series, published by
Radio Shack.

There's a separate book(let) for the 555, op amps, and one entitled
"opto-electronic circuits"  That last has a break beam alarm circuit
in it (using a pulsed LED transmitter - and filtered receiver, to make
it more immune to false operation)

Simple to understand and just what you need to know.

I downloaded the books some time ago when they appeared on Bit  
Torrent, I still have the torrent file, but I checked and there's no
one "seeding" it at present.

Low and Behold:

https://archive.org/search.php?query=Forrest%20Mims%20engineer%27s%20mini-notebook

Archive org has the books

Re: Basic circuit help please

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everything you need in one book

    TimerOpAmpOptoelectronicCircuitsProjects

https://archive.org/download/TimerOpAmpOptoelectronicCircuitsProjects/Timer%2C%20Op%20Amp%20%26%20Optoelectronic%20Circuits%20%26%20Projects.pdf

or  

https://tinyurl.com/rad7ate

I'd still go with the individual books - less daunting that way...

Re: Basic circuit help please
wrote:

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Ditto that.  An op amp running from a single supply should be
referenced to a voltage divider setting the input at 1/2VCC or some
voltage less than VCC to work properly.

Another glaring mistake is the 100K pot wired to go to zero ohms. Zero
ohms or close-to with full light on the photocell may well burn out
the pot.

AND isn't the logic all backwards?  Looks to me like the photocell
seeing light will drop the voltage at the inverting input which should
send the output of the op amp high, turning on the transistor/LED, and
sending the input to the 555 low, triggering it on.  (output at pin 3
high, piezo buzzer sounding)

Typically you'd want to detect the absence of light as the beam is
broken.  Or did I miss something here?

AND I'd use a photo transistor those CDS cells are often
moisture/humidity sensitive and degrade over time in wet environments
unless hermetically sealed.

Re: Basic circuit help please
On 29/01/2020 20:48, RobH wrote:
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I don't see how this ever worked. The op amp + input is effectively  
connected to the + rail. The op amp output will never change state. Does  
the LED change?

Re: Basic circuit help please
On Wednesday, 29 January 2020 20:48:16 UTC, RobH  wrote:
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LED output is dc. Your 5v buzzer can go across LED & R2, but do add a diode across it. The 555 is not needed.


NT

Re: Basic circuit help please
On Thu, 30 Jan 2020 06:56:10 -0800 (PST), snipped-for-privacy@gmail.com wrote:

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The 555 is desirable IMO.  You'd want the thing to sound for a time to
raise an alarm.  A person might run through the beam and break it for
a ~100 milliseconds or so.

That could  be accomplished with a capacitor and RC network too, but a
555 is easily controlled and more predictable IMO.

Re: Basic circuit help please
On Thursday, 30 January 2020 15:14:56 UTC, default  wrote:
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Very true, I was not braining.


NT

Re: Basic circuit help please
On 30/01/2020 14:56, snipped-for-privacy@gmail.com wrote:
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Ok thanks.
I have some diodes which I could use.

Pardon my ignorance, but as I am only doing this as a hobby and have not  
touched electronics in over 35 years. I mistakenly thought that the  
circuit and design I saw on that said website was how it should be, but  
have now found that it is not.

Re: Basic circuit help please
On 1/30/2020 9:28 AM, RobH wrote:
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  Hey guys, he said that pin 2 is moving up and down by changing light  
input to the ldr. The problem as I see it, he said the voltage went down
to 4.6V on pin 2, that's not low enough, it must go below 3V to switch  
the 555.
                    Mikek

Re: Basic circuit help please
On 30/01/2020 15:39, amdx wrote:
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Using a 5v supply the voltage across pin 2 on the 555 was 4.01v and went  
down to 2.57v after changing the light input to the ldr. On pin 3 it was  
0v as before.

Thanks


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