Astable Op Amp circuit

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Kim

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Aug 18, 2021, 7:06:23 AMAug 18
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Hello,

I'm a bit new and currently studying from home. I've been trying to build a twin LED alternately flashing circuit with a LM741 Op Amp but having problems. Not helped by the fact I think there might be an error on the schematic I was given. 

As far as I can tell, LED2 and R5 will be shorted out in the original schematic, so I simulated in LTSpice removing the extra connection (from output to R2/R5) and adding a ground after R5. Now I am getting a nice pulse in LED2. But the voltage across LED1 is not a pulse, it's the capacitor sawtooth shaped voltage. 

On my breadboard of the circuit, LED2 is constantly lit and then flashing brighter. If I connect the LEDs in series, they both flash on and off at the same time. But I'm not 100% sure I have breadboarded correctly. I can add a picture of the breadboard, if helpful or any of the readouts from LTspice. Any help/pointers/tips would be much appreciated please, I know I'm making an error somewhere, just narrowing down where I'm going wrong.flasher.jpg

Richard Ibbotson

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Aug 18, 2021, 8:53:34 AMAug 18
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Connecting R5 to ground looks right but maybe also the LED1 should go the the +ve supply rail and not the feedback resistor?
Richard


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On 18 Aug 2021, at 12:06, 'Kim' via rLab / Reading's Hackspace <reading-...@googlegroups.com> wrote:

Hello,

I'm a bit new and currently studying from home. I've been trying to build a twin LED alternately flashing circuit with a LM741 Op Amp but having problems. Not helped by the fact I think there might be an error on the schematic I was given. 

As far as I can tell, LED2 and R5 will be shorted out in the original schematic, so I simulated in LTSpice removing the extra connection (from output to R2/R5) and adding a ground after R5. Now I am getting a nice pulse in LED2. But the voltage across LED1 is not a pulse, it's the capacitor sawtooth shaped voltage. 

On my breadboard of the circuit, LED2 is constantly lit and then flashing brighter. If I connect the LEDs in series, they both flash on and off at the same time. But I'm not 100% sure I have breadboarded correctly. I can add a picture of the breadboard, if helpful or any of the readouts from LTspice. Any help/pointers/tips would be much appreciated please, I know I'm making an error somewhere, just narrowing down where I'm going wrong.
<flasher.jpg>

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<flasher.jpg>

Tim Masson

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Aug 18, 2021, 9:52:25 AMAug 18
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Yes, Richard is right here, there is no path for current through LED1 but I think there is a bit more.  The link between the output of the 741 to the R2/R5 node is clearly wrong and this point should be connected to ground.  R2 should connect directly between the output of the amp on pin 6 and the + input on pin 3 of the Amp.  This should be biased to ~2.5 volts by R1 & R4 which are unchanged.  LED 2 sinks the output of the 741 through the diode drop (say 1.7 V and then 330 ohms.  The anode of LED1 to +5 volts, but through a 330 ohm current limiting resistor and this will sink current into the amplifier when the output is low.  Now, when when one diode is On the other is Off as the amplifier output swings from just above 0 to just below 5 volts.

Kim

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Aug 18, 2021, 10:10:41 AMAug 18
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Interesting, I tried connecting LED1 to the +Ve and both LED's remained constantly lit. I'm guessing because it's a path that bypasses the capacitor and OpAmp? (I'm not 100% there with my OpAmp studies yet.)

I spoke to my tutor who wasn't sure if the circuit was proven or not (?!) But I'm not the only one who's had trouble. He said as long as I test, make changes and show the differences between simulating and practical, I will fulfil the requirements. 

Still, it's sort of bugging me now. I'd like to be able to make them flash alternately. For now, I can connect them in series and change the flashing speed with a smaller capacitor and R3. 

Thank you guys.

Richard Ibbotson

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Aug 18, 2021, 10:44:19 AMAug 18
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The LEDs with 330ohm series resistors is really too heavy a load for a 741 op amp and likely the output voltage swing is not sufficient with that load. Can you measure the output voltage swing at the op amp output with the LED1 connected to the +ve supply?
You might try increasing the 330ohm resistors to say 1k. Or if the LED is red type you might change to green or blue with a higher voltage drop. Though you might then struggle to turn them on!

In your simulation are you using a 741 op amp type or an ideal op amp? What voltage swing do you see in the simulation compared to the prototype?
Richard


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On 18 Aug 2021, at 15:10, 'Kim' via rLab / Reading's Hackspace <reading-...@googlegroups.com> wrote:

Interesting, I tried connecting LED1 to the +Ve and both LED's remained constantly lit. I'm guessing because it's a path that bypasses the capacitor and OpAmp? (I'm not 100% there with my OpAmp studies yet.)

Nigel Worsley

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Aug 18, 2021, 10:50:51 AMAug 18
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I hope you aren't using a 741 on your breadboard. It is a VERY old design and the performance is terrible by modern standards, the output swing is rubbish and the datasheet says it requires a minimum of +10V and -10V for the power supply.

Nigle

Kim

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Aug 18, 2021, 11:16:34 AMAug 18
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Ok, the output voltage with the LED connected to +VE is about 4.86V (that's with my multimeter) On the simulator it's a pulse from 1 up to 4V, that's with the 741 type parameters. 

Perhaps I can trying increasing the V and see what happens?

Kim

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Aug 18, 2021, 11:18:06 AMAug 18
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Nigle, unfortunately I am. It's not my choice, it's the exercise question schematic. I believe it's an old question, I should probably give up and move on... I just wanted to try and understand it.

Bob Dunlop

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Aug 18, 2021, 12:19:32 PMAug 18
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Hi,

Hope we are not doing your assignment for you.

Didn't realise they still made the good old 741, horrible device by
modern standards but pretty bomb proof. Still lot's a features and
limitations to train the student rather than giving them an "ideal"
device.

I agree with others in that I think LEDs and their current limiting
resistors should go between the op-amp output and the two supply
rails. Limitation one, the 741 can drive a max of about 20mA so
you're probably hitting the short circuit protection. I increased
the resistors to 1K (what I had easy in the box), modern LEDs don't
need 20mA drives.

Next limitation the 741 can't drive it's output rail-to-rail unlike
some modern devices, especially when you are running it at 5V, the
bottom of the spec sheet. In the real world (see attached) I measured
the output at 1.8V low, 4.3V high, and a period of approximately 6
seconds. Slow enough you can measure it with a DVM rather than a scope.

That's another of my pet hates, this is a high impedance circuit and
timing will be a bit of a race between the 47K charging resistor and
the leakage in the capacitor. Every build of the circuit will be
different.

On to the LEDS, the one connected to the supply rail will see voltages
of 3.2V and 0.7V which should be enough swing in the right range to
turn it on and off. The LED connected to ground on the other hand
sees a low of 1.8V and won't turn off. A realy dirty trick is to add
a regular diode in series with the LED, the additional voltage drop
may be enough to extinguish the LED. On my bread board two additonal
diodes were required. Never use a trick like this in a real design
as diode forward voltages are only specified with a maximum and
possibly a typical value, never a minimum. Every build will need a
different number of diodes.

No much of a tutor not to have checked the circuit first, or is the
assignment a test of your diagnostic skills?


On Wed, Aug 18 at 01:51, 'Kim' via rLab / Reading's Hackspace wrote:
> Hello,
>
> I'm a bit new and currently studying from home. I've been trying to build a
> twin LED alternately flashing circuit with a LM741 Op Amp but having
> problems. Not helped by the fact I think there might be an error on the
> schematic I was given.
...

--
Bob Dunlop
IMG_20210818_161429322.jpg

Kim

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Aug 18, 2021, 12:58:12 PMAug 18
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Hi Bob, 

No my assignment has already been submitted, it was only necessary to show the simulation, take various readings and explain why a practical circuit could vary. 

I wanted to have a go at getting it working on a breadboard and I just get stuck working on something until I can understand what is happening (or give up in a huff of course). The last circuit assignment used a LM301A which was a huge £3.99 for 1! II wanted to see how it would behave practically versus sim, helps me understand more thoroughly I think.

Thank you for the pic and explaining more, I have ordered a few different values of capacitors and resistors so I can have a play changing the timing. I have some diodes somewhere so will dig them out and give them a try too.

No, not much of a tutor and not much of a college. I know that it's registered with the examining body at least, but I think it's more aimed at professionals who know their stuff and are just looking for the bit of paper. It is a challenge.

Thanks again

Kim

Kim

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Aug 18, 2021, 4:55:41 PMAug 18
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I'm thrilled, I found some 1k resistors, connected LED 1 to the positive rail and added a couple of diodes, one at a time to see the voltage drop. And it worked.

Next I'm going to try a few different values for C1/R3 for different time periods and experiment with a 555.

Thank you all kindly for your help. 

DSC_3651.JPG
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