I need to increase the gain in this circuit by running the signal thru the second stage. Was wondering if I wired it correctly. First attachment is regular schematic, second is red-marked mods I made.

 

hi john,
You have the + and - Inputs incorrectly marked.

Pin2 is -vin and pin 3 is +vin, also the 2nd OPA is crossed over.

If it was configured as in your second image and the wires were correct, it still would not work as you expect.

E

 

I agree it does not match the data sheet BUT, this device seems to work. Check out the attached plan.

 

just increase the feedback resistance, how much dc voltage output do you need?

 

I really dont want to change the resistors but double of what it is now would be nice.

 

I really dont want to change the resistors but double of what it is now would be nice.

hi,
What are the Light and Dark Vout values of the OPA, as in the original circuit.?

E

 

have not measured it but I would like double the output hence I was thinking of running it thru another opa stage.

 

have not measured it but I would like double the output hence I was thinking of running it thru another opa stage.

hi john,
I figured that you wanted to double the output, but what I want to know if there is a DC level on the output when the sensor is Dark and what the Vout is when the sensor is lit.

You could do a quick check in less than a minute.

Eric

 

I dont have the device with me but dark = ZERO. When lit its .2 or .4 just from memory on a digital meter and I didnt look at the units so I dont know if its mV or V.

 

Also, your second schematic has the 2nd amplifier connected open loop, for an additional gain of approx. 100,000, not "double". I suggest connecting the gain-set feedback resistors to the output of the first amp as in the original schematic, then configuring te second amp for a gain of 2 or more.]

And ericgibs is correct, the original schematic in your pdf attachment is incorrect.

ak

 

The circuit would probably work much better without the second opamp. No kidding.

 

Analogkid, is this what you are thinking. Maybe with a resistor into pin 2? I think the only thing wrong with the schematic is the + and - are reversed, a typo on the creators page.

 

I dont have the device with me but dark = ZERO. When lit its .2 or .4 just from memory on a digital meter and I didnt look at the units so I dont know if its mV or V.

hi John,
IF your voltages are correct.?
You could consider a modification as per this image.

BUT I think you need to verify with a DVM the actual Vout voltages, they do not sound correct to me.

E

 

I will double check those voltages tonight.

 

as i said earlier increase the 1 meg resistor to 2 Meg ohms, this will make the output double on the number 6 setting,

or just double all the resistor values for all ranges, From 100R, 1K, 10K, 100K, 1M Ohm resistors
TO 200R, 2K, 20K, 200K, 2M

 

jonfin, your sch was close but not there yet. See ericgibbs' sch for the correct second stage. To his sch I would add a 2.4K resistor from pin 1 to pin 5 instead of the direct connection. You are dealing with some pretty small voltages, and the LM324 is an old part with pretty bad offsets by today's standards. The extra resistor helps to equalize the input offset currents of the two inputs for a more accurate output.

ak

 

I'm dubious about this circuit. You're trying to operate that amplifier with inputs that are at or very near 0 volts, which is also the amplifier's negative power supply voltage, and I'm not sure that it can perform in a linear manner in that mode. If this were my project, I'd expect to generate a pseudo-ground (say 1V) which you could easily do with one of the other amps in the package, then connect pin 3 of the amp and the photodiode anode to it. That would make sure that the inputs of the amplifier were in a region where it could operate properly.

But the LM324 is a cheap component with fairly poor specs. There are jobs that it's adequate for, but if accuracy is important, you might do better to spend a little more money on something with higher quality.

 

hi John,
The LM324 specification is very good down to 0V, in fact it will work about 0.1V below 0V.

This image is a LTS simulation of the basic circuit, using a LM324.
The behavioral current source, diode and 1meg represent the photo-diode.

The PD current is set from 0uA thru 50uA.

E.

 

I was skeptical enough about the ability of a humble LM324 to work correctly all the way own to 0V that I just had to build the circuit and try it out. So one way had a diode (not having the right part, I used an IR LED, knowing it would function just fine as a photodiode) connected to Gnd with a 1M resistor as feedback, and the other way had a simple pseudo-ground at about 1V and made the connections to that instead. Then I checked the output with very dim light, and it was down around zero, and with an incandescent desk lamp a couple of feet overhead, it was up around 0.6V. Both circuits functioned pretty much the same, so I suppose I have to accept that whatever limitations the LM324 may have, functioning around 0V is one thing it can do well. I didn't know that.

 

The 324 has a PNP input stage, so the common mode rating is Vee - 0.3V and a typical range is more like -0.5V. Plus the little puppies are almost indestructible. The 324 and 339 were just plain magic when they came out over 40 (!!!) years ago, and the world still consumes about a billion a year even today.

ak