CA3140 substitute question

[dacflyer]

is there any other op amp that will work in place of a CA3140 ic , like a 741 or such??
i am wanting to use it on a circuit like the one posted in this link..
http://electroschematics.com/5397/three ... indicator/

[MrAl]

Hi there dac,


You can use an LM358, very cheap and widely available, but you can not use that circuit. The reason is because that circuit wont work anyway, even with the original ic chip. Somebody screwed up somewhere with the drawing i guess.

One problem is that the non inverting input is tied to ground. That's not right at all. It should be tied to a voltage reference of some kind, either a zener (not as good) or a true voltage reference diode ic chip (very good). The resistor values could use a little adjustment too to make adjusting the pot a lot easier.

Also strange if you noticed, the circuit says "3 state" but then the description only talks about the LED lighting up green or red. That's only 2 states.

If you like we can modify the circuit a little bit and make it work with an LM358 just fine, or we could look for a more low power ic that will draw even less current from the battery under test and go from there.

[dacflyer]

state 1 = the LED remains off in normal 12 volt
state 2 = and it lights Green when the voltage is 13.5
state 3 = and Red when the voltage is below 10 volts.


i am looking for something like this or similar,, to monitor a 12v system on my ultralight..
basically a 2 lead bi-color led red = discharge , green = charging
adjustable set points are a plus.

[MrAl]

Hi dac,


Ok, well the original circuit didnt do that, but that's no problem as we can make one that does.

Using a device like the LM339, you get four comparators in one chip, and the LM393 you get two in an 8 pin dip package. It think they make a low power version too, so that would be nice. It's also very good to include a true voltage reference diode, and these things are cheap these days anyway.

All you have to do now is specify the exact voltages (before or after adjustments) you want and what states you want the LEDs to be in. You can also decide if you want two LED's (one green, one red) or you want to use the bi color LED, either way. I like using two different LEDs for my stuff because they are easier to get and you can get high brightness LEDs too that way that take less current to light up brightly.


Ok so here is what you have to decide:

1. Turn on voltage for red (like 10 or 10.5 volts or something like that)
2. Turn off voltage for red (like 11.5v or something like that)
3. Turn on voltage for green (like 12v or something)
4. Turn off voltage for green (like 11.9v or something like that).

Or, if you want to keep it more simple then just the turn on point for red and for green, and we'll use minimal hysteresis and two pots for adjustment.

Sound good to you?

[dacflyer]

well the adjustable set points are a must since the power point you attach to is never exactly like it is at the battery.. i never really had the best of luck with comparators, but then again i guess i had cheap kits,. i had a kit with 2 LM339's and 8 leds for a bar meter. it worked, but it sucked a lot of power..

i have seen a few interesting voltage monitors onthe net, but they looked like they were PIC controled.
i am guessing i need a red turn on point of about 11volts. or which ever the voltage is on a battery thats been at rest for 10 min.
and i guess a green turn on point for 13 volts.
and i'd really like to use just 1- 2 lead bi-color led. so i only have to make 1 hole in the panel.

not sure thats possible with LM339's or ?

[MrAl]

Hi dac,


Here is a circuit to start with. The adjustment range is 9 to 15v. V1 is the battery.
For one circuit the LED connects as shown, for the other circuit the LED connects between the output and ground and the resistor becomes a pullup resistor.
Let me see if we can mod this to work with a bi color LED next.

[dacflyer]

any update yet as far as being able to use a bi-polar led ?
i been searching all week , nothing to use unless its a PIC. or already made up and for sale.

[MrAl]

Hi again dac,


Here ya go...
The output buffers make the circuit work a little better so it's a good idea to have them there.
The green is adjusted for the higher voltage, like 12.5 or whatever, and the red is adjusted for the lower voltage like 10.5 or whatever you need. Adjustment range is 9v to 15v for both sections.
Obviously not all the power supply connections are needed depending on what comparator chip you want to use. The comparators used here must have open collectors though, and most do anyway.
Take a look and let me know what you think about this circuit. Note it can also be modified to work with bi color LEDs with three leads (one common lead) and of course for two separate LEDs they can be wired inversely back to back.

[dacflyer]

in the 2 pot's there is a 60 and a 40 beside them,, whats that for ?

[MrAl]

Hi dac,


i forgot to mention that that was for the percent setting of the arm of the pot for the test. The upper pot arm was adjusted to 60 percent, so the arm is 4k away from the top and 6k away from the bottom, while the lower pot arm was adjusted to 40 percent so that arm is 6k away from the top and 4k away from the bottom. If you like you can set those two pots just like that for the initial testing and then adjust as needed.

[K3PTO]

Hi DAC,

You might consider using an LM3914. It has 10 built-in comparators with each capable of driving an LED. It can essentially be used as a low resolution DVM.

[dacflyer]

thanks , but i have my heart set on trying to use a bi-polar led , rather than use a bi-color led..

[Bygar]

Why use a sub when you can still get the real thing;
I have recently purchased Ca3140 OP AMP's at less than $1 U.S.
Try Bgmicro or SMC. They are both on the internet.
Hope this helps.
Bygar