Urgent help ! ! !
I have made a voltmeter and the LED DC voltage indicator circuit was put in Multisim 12 and breadboard.
I bought LM741 operational amplifiers served as comparators that drive LED indicators.Voltage thresholds are 3, 6, 9 and 12V. Above each incremental threshold an additional LED turns on. This is the datasheet of LM741 and the schamatic diagram is given below.
Why can’t it totally turn? Oh, I’m really confusing.
Thank you in advance.
My dear Mene , R3,4,5,6 (in your schematic with the value of 10Kohms ) are way too big ! Change its with 1Kohms , and everything should be O.K. !
Keep us posted !
Oh, my aching head.
It’s been a long time since I analyzed a circuit, and op amps were never my strong suit.
I agree that your voltage divider network needs some changes, but I don’t think cera@1967 went far enough. As I understand the circuit, you want D6 to fix U1P2 to 12 volts. Which means the voltage divider circuit wants to put a little more than 12 volts on the Zener. Keeping the 1K’s that cera@1967 suggested, I would change R2 to 1.8K. If you remove the Zener, that would make the calculated voltage at this point 12.4 volts. Once the Zener enters the equation it only has to deal with 0.4 volts of reverse bias.
Thanks for the challenge, and if anyone sees an error in my analysis, PLEASE point it out.
The thing with Zener diodes is that they are not perfect devices. You need to put some current through them to get the rated voltage. They also start to conduct way before their rated voltage is reached.
D6 needs to be loaded at >10% of its rating (4 mA for a typical 1/2 W unit) to get it in the good part of its voltage curve. So R2 needs to be about 3 k, as it is.
R3+R4+R5+R6 needs to be >> than R2 to not affect that 4 mA loading on D6. They are OK as is.
As D5 start to conduct (around 9 V) , we will see a progressive voltage drop across R1 and we will never get 12 V at D5. Either make R1 much smaller, or use a higher voltage Zener for D5.
The LM741 is a terrible opamp, not designed for rail-rail output either. The old, cheap, quad LM324 would be great for this application.
First of all ,D5 is there not to let the input voltage to go over 12V , in case the measured voltage is more than 12 V ! It is working like a over voltage protection !
Second , if you look closer , D6 is connected to the ground , therefore in parallel with the 4 resistors ,and the current thru it is about 2 mA ( 6V/3300=1.8 mA ). Because R3,4,5,6 ( at 1Kohms each ) ,are in series , the current thru its will be 3mA (12V/4000ohms=3mA ). Accordingly with Kirchhoff rules , the current will go in both paths with a value dictated by the resistance in each circuit. This side of the circuit will have a consumption of 5 mA , divided in 2mA for D6 and 3mA for 4XR(3,4,5,6).
You are right about : the values of the resistors BUT ,only for LM324 ! As you said 741 are terrible opamp , and that's why the resistors has to be much lower than 10K...
the value of R1 seems to be too high ...
Ummm, why use 4 separate ICs instead of a quad, and why use an op-amp (“amp” as in amplifier) as a comparator? That’s like using pliers as a wrench. It might work, but…
An ancient LM339 quad comp would be perfect for something like this. 1 chip, 1 set of +V/gnd connections, easy to route all the commons (all those pin-3s above) together.
They’re open-collector, active low, so you’d flip the direction of the LEDs.
Oh yeah, there’s absolutely no need to feed it from a pair of 9V batteries, either. You can make life so much easier by using a nice cheap voltage reference (eg, 1.2V) and just scale down the input. Amazingly, it’s an exact 1:10 ratio from your 12V comparison (zener).
So your Vref is 1.2V, and with 4 equal-value resistors (okay to make them a higher value now) would be 0.3V, 0.6V. 0.9V, and the straight-through 1.2V. You can now power the critter from USB, a pair of AAs, whatever. And the ’339 is more or less current-limited on the output so you may not even need ballast resistors if the voltage is low enough. It might sink a maximum of 10mA or so (forgot the spex).
So 7V in gets divided down to 0.7V, and gets compared, which’d light up LEDs D4 (>0.3V) and D3 (>0.6V). Done.
D5 is probably more for reverse polarity protection. Point is it will already conduct before getting to 12 V. And R1 being 10 k will give a huge voltage drop when D5 starts to conduct. Which I suspect is the problem in the OP, if there is not a language issue. 15 V Zener and R1 1 k should be OK.
Don’t know where I got the 3 k for R2, it should be about 1.5 k (keeping the others at 10 k). 3.3 k should still work though, the 12 V reference may just be too low.
Comparators would be better, but probably the OP is playing around with opamps?
That D5 is hooked up in reverse. It’ll conduct fast when reverse-biased (to keep from popping the inputs of the op-amps) and be current-limited by the 10k resistor on regular overvoltage. Otherwise, it’ll just clamp the input voltage to 12V maximum, meaning D1 may or may not ever get turned on(!).
I’m not a fan of that circuit, as you might tell.
I like simplicity. How about this solution:
Just a random search result.
Cheers
Kel
Ah, old school, i see
Now for this project i think i will quickly grab atmega328 and some resistor, xtal, upload some code, and work like a charm
I know it’s overkill. But hey, it works
Actually Atiny is a better choice than atmega328
I have not analyzed the circuit fully, but a few details:
Supply voltage for the 741 is a bit low, data sheet says minimum 20V (+/-10V)
The 10K resistor looks fine
The 741 do not work near supply voltages, i.e. it will not work near 0V (Within 2-3 volt of 0V)
A much better OpAmp is a LM324
ALL of you are right ! The possibilities are almost endless and MUCH better too , but , we have here a guy that made this particular setup , and he want help for THIS one !...
More than 30(!) years ago I made this: LED Voltmeter by using a quad device such as LM324 or four 741 opamp ICs | The road less box Blog .
At that time was a schematic from a German electronic magazine and it worked as it is.., you are right again , is the old school , but , if you guys remember , all of us started from the bottom , in the early age of electronics !...It is a good exercise that I recommend , instead of buying some kits already tested , you need just a solder station and some skills , and you can call you, an " electronic engineer " ...!!! .The gentlemen about my age (60) will understand what I'm talking about.., is the pleasure of doing something with your head and hands , fails and successes (more fails , if I remember good !!!). Is all about fun and proud , a feeling that is hard to explain these days when " JUST BUY , DON'T THINK , WE ALREADY DID IT FOR YOU..." became a dangerous practice...sorry, is just an opinion , and who am I to judge ?...
What is curious is that Mene didn't step back in here ,to tell us about the progress ...! Help for someone that do not need it anymore ? Hellooooooo!...