Basic circuit help please

so, connect trigger to where?

--
  Jasen.
Reply to
Jasen Betts
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am-with-alarm

I have a very vague memory of an opamp being treated like that and yet work ing. Looking at opamp input circuits reveals there are voltage swing limits and so-called phase inversion that complicate the picture. At this late ho ur I can't work out if it could actually work, but can't dismiss it out of hand.

NT

Reply to
tabbypurr

General opamp theory says it does not work and never could.

If it does work then that is due to parameters way off the datasheet, therefore not guaranteed to work for any or all devices.

The circuit "designer" does not have a clue, and seems to be playing with all combinations of all pins of random devices until something appears to work - and then publishing it.

Reply to
Andy Bennet

gram-with-alarm

,

working. Looking at opamp input circuits reveals there are voltage swing li mits and so-called phase inversion that complicate the picture. At this lat e hour I can't work out if it could actually work, but can't dismiss it out of hand.

General opamp theory doesn't come into it. General opamp theory only applie s when 2 Rs are used on the input to set a known voltage.

if it works, and the op at least says it does, it'll be due to the input ci rcuit issues of the opamp, which should be sufficiently consistent for any one type of opamp.

I really ought to lash one together & test it, but am way too occupied with other stuff atm.

NT

Reply to
tabbypurr

I think we lost our OP. Mikek

Reply to
amdx

ignore the trigger pin, no connection.

If you did wire a monostable (not the astable as shown) and use the trigger, and hold it low longer than the time constant, it will glitch periodically as it times out then resume timing. (assuming it is DC coupled as this schematic shows)

Shorting the timing cap to ground insures the output changes state and stays that way (no glitch) until the short is removed then times normally until it times out.

The glitch is very fast, but can be noticeable in the load.

Reply to
default

No, I'm still here reading the posts.

As I am a hobbyist with electronics and use youtube for most of my answers, I am very interested in why some posters says it doesn't or shouldn't work. I know that you can get a flashing led with just a 555, but I'm completely ignorant whether an op amp is also needed or not as the case may be. I'd do some reading up on the 741 and watch some videos about it.

I see the point of what some people say about the author not having a clue etc, as I am very aware that that is also the case with other topics on youtube.

I only posted my question because the buzzer didn't work originally in the way it was postioned on the schematic. I know that it does work now, but I am also looking at or for other similarish circuits. I have found something on the hobby-circuits site which I'll put together when I have all the parts.

Reply to
RobH

The way it is suppose to work is the 741 is set up as a voltage comparater. When the voltage on one of the input pins is larger or smaller than the other, the 741 output changes from low to high. The

741 is not a good device to use and a few more resistors should be used to get it to work consistnatly. The transistor seems to invert the signal and drive the led. Then the 555 gets triggered to send an output voltage to the buzzer for a period of time.

The 741 is usually an amplifier and there are dedicated comparators that work much better.

Reply to
Ralph Mowery

I have not put any thought into the 741, I concentrated on getting the

555 to work properly first. The OP pointed out the the 741 and transistor did not pull pin 2 low enough to trigger the 555. So I don't doubt some 741 problem. I wonder if it's possible to trigger the 555 directly from the resistor/ldr connection and skip the 741/transistor circuit all together. Hey ROBH, When you light the ldr, what does the voltage drop to? (at 100k at ldr connection) You might need to keep adding 100k resistors in series with you 100k pot until you get it to drop to just under 3 volts. This might not work after you connect it to pin 2, but it is worth a try. If it doesn't work at least you will know why, even if we have to tell you. IF you do this give us light and no light voltages and then the same when connected to pin 2 of the 555. Mikek
Reply to
amdx

Measuring the voltage between the 1ook pot and the ldr was stationary at just under 8v, even when the ldr was shaded. The voltage at the R2 210 resistor was 6.75v with no shading and 5.98v with shading. I have also found that the 10uf capacitor between pins 1 and 2 on the

555 does nothing. Although it might change the frequency of the buzzer??
Reply to
RobH

it can overload the one side of the differential pair and drive the next stage directly, invoke the fault condition known as phase inversion etc... the op-amp may still be amplifying, but it's not running in any mode that's blessed by the data-sheet.

--
  Jasen.
Reply to
Jasen Betts

They're faster amd more efficient, but the 741 is plenty fast enough to watch the output of a light dependant resistor. sure a LM339 (or LM393) would out-perform it, but in this application you wouldn't notice, other than the price.

--
  Jasen.
Reply to
Jasen Betts

Something wrong there! Unhook the 742 pin 2 from the connection and put your meter at the connection. It should change voltage when it is dark. What meter are you using? You could put two more 100k in series with the 100k pot. that will lower the 8v, but also allow a larger swing between light and dark.

The voltage at the R2 210

If you will recall, there should not be any connection of the 10uf to pin 2. At this point I need a drawing of the 555 circuit you are using. Post it where ever you can and send me the link. Once I see it, I will post any corrections needed. Mikek

Reply to
amdx

ROBH, this is exactly the circuit you want.

C1 is your 10uF, R1 is 100k resistor that will give you a 1 second buzz,

200k will be a 2 second buzz. R2 can be prety much anything but use 10k. Pin 3 is your buzzer. For the switch if you don't have one, just use a wire to momentarily ground pin two. If after you set this up it doesn't work, take a picture and post it somewhere or send it to my email address. Let me know, I want to get something working. Mikek
Reply to
amdx

There is a photo I took here:

formatting link

Reply to
RobH

I don't know why, but all I can pull up from the link are my own files. Anyone know how I can view his picture? Mikek

Reply to
amdx

I know why, you have to log in, but I have shared it now with the email address I see here. I don't know how to make it public yet.

Reply to
RobH

Ok, I put this circuit together, and what happens is that when I either hold or put the 10k resistor from pin 2 into to the gnd rail, the buzzer beeps continually. When I either let go or disconnect from GND it stops beeping.. Where does the LDR go, and or the led.

Thanks

Reply to
RobH

files.

post it on a sensible image hosting site Would be interesting to many people see a video of this thing actually work ing, including clear view of the input side of the 741.

NT

Reply to
tabbypurr

I'm working with him offline. He now has a working one shot 555, so that is solved.

For a start I'm doing just ldr-resistor with the center connection to Pin 2. I realize there is a level transition problem with a single neg pulse to Pin 2. However we are working towards that.

We are just now adjusting the resistor-ldr to drop below 1/3Vc to trigger the 555. As built is only dropped to 4.3 in the dark, so he is now adjusting the resistor value.

Next we will need to differentiate the negative going signal from the ldr. I do see a problem with a slowly decreasing light situation and we may end up with some fast switch between the ldr and the 555 circuit. I think he has limited parts, so we may have to do something with the

741 or two PNPs. I think I saw he only has PNP transistors. If you have an Idea to make cause a fast switch from a slow transition, I'm open to ideas. He has the 741, so can you make that work, or two NPNs. If this is daylight to sunset sensor, we will need a fast transition.

Mikek

Reply to
amdx

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