Basic circuit help please

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.

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

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?

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?

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.

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.

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

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.

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.

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.

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.

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

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:

Archive org has the books

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

everything you need in one book

TimerOpAmpOptoelectronicCircuitsProjects

or

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

Pin 2 needs to go below 1/3 VCC, so with a 9 volt supply, that would be 3V, with a 5V supply that would be 1.6V. If you put the 9v supply back in and put a 47 ohm resistor from pin 2 to ground, that should bring pin 2 down to about 1.3V and trigger pin 3 to sound your buzzer. OR you could just ground pin 2 to make the buzzer buzz. All of this is not the fix for your circuit, but this is to help you understand how the 555 behaves. If this works then we will figure out how to make the transistor pull it lower. Let me know. Mikek

Using the 9v supply and either shorting or using a resistor on pin 2 to ground caused the led to stay on and the buzzer to beep all the time. The voltage on pin 2 was 6.58v when shorted and 1.9v when using a 100 ohm resistor. (Smallest I had)

OK, so now you know that the 555 will trigger when pin gets low enough. How long did you want the buzzer to buzz? Was it any time the ldr signaled it to or just one beep when the LDR made the change. I don't know the purpose of the circuit. We either have one problem to fix or two problems to fix. But I need to know what the buzzed is supposed to do. JUst to give you a head start, I suspect your transistor is not being turned off hard enough, meaning you can't bring pin 2 low enough. Might increase the 210 ohm to 500 ohms or 1000 ohms to start. But that's getting ahead. Mikek

The idea of the buzzer , from the circuit I followed, was to beep when the ldr sensed detection?, or was shaded by anything or anyone, I guess. So yes , it was for when the ldr signaled it to.