Photo-transistor - Op-Amp Design Build Issue

Thread Starter

RTTD1977

Joined Mar 25, 2019
6
Hello
I'm new here and have come across a very strange problem that I can't get to the bottom of that maybe someone out there can suggest a resolution to. I've designed a light level detector with photo-transistor, MCP601 op-amp, MCP42100 dual digital pot and LM311 comparator (circuit attached). I also have a handful of PCBs manufactured by EuroCircuits to test out the design. The problem I'm having is that about 75% of the op-amps in the circuit don't seem to work, almost as though they are faulty, which I'm sure they're not. The problem with the non-working op-amps is that the negative input and output are set at almost supply voltage and altering the light level on the photo-transistor doesn't affect the input voltage at all.
I can remove the op-amp from the circuit and replace with another which will then work just fine (or maybe after replacing a few times!). So I'm presuming my design is OK and that there's something amiss that causes the circuit not to work. It can't be the PCB as that works, once I find a working MCP601. It's almost as though there are some slight manufacturing differences in the MCP601 that cause my circuit to work or not as I'm pretty sure they're not faulty.

Can anyone shed any light on why I might be experiencing the above please? I'm at my tethers end :-(

I'm sure it's not a faulty op-amp as I use them elsewhere and I'm very carefully avoiding static. I've even bought different batches from different suppliers to rule out faulty supplies. These are all SMD components BTW

Thank you
 

Attachments

AnalogKid

Joined Aug 1, 2013
8,675
As a courtesy, always post a link to the datasheets of the devices being questioned.

Your input voltages are out of range, and some devices tolerate this better than others. As you can see on the datasheet, the input common mode range does *not* include the positive rail. For you circuit to work with the non-inverting input connected to +5 V, the device positive power rail must be at least +6.2 V.

OBTW, your schematic does not show the power rails. What are they?

ak
 

Thread Starter

RTTD1977

Joined Mar 25, 2019
6
As a courtesy, always post a link to the datasheets of the devices being questioned.

Your input voltages are out of range, and some devices tolerate this better than others. As you can see on the datasheet, the input common mode range does *not* include the positive rail. For you circuit to work with the non-inverting input connected to +5 V, the device positive power rail must be at least +6.2 V.

OBTW, your schematic does not show the power rails. What are they?

ak
Thank you AK.. The power rails are +5V (USB powered) and 0V.. Cheers
 

mvas

Joined Jun 19, 2017
537
Should the 10K Resistor for the (+) Input Pin of the MCP601 Op Amp, connect to ground, instead of +5v ?
 
Last edited:

Thread Starter

RTTD1977

Joined Mar 25, 2019
6
Should the 10K Resistor for the (+) MCP601 Op Amp Input Pin, connect to ground, instead of +5v ?
Thanks mvas. In my circuit the 10K resistor is connected to 5V, and the circuit worked. It's just when I built a few more, most of them didn't work with the same circuit, so something obviously amiss. How would I bias the input voltages to be 1.2V lower than the supply voltage, if you think this to be my issue? Cheers
 

mvas

Joined Jun 19, 2017
537
See figures 2 - 5 here ...
http://www.bristolwatch.com/ele2/pdiode_op_amp.htm

Your "Inverting" Op Amp, with (+) Input Pin @ 5V, and a "SIngle Supply" has me confused.
Figure #4 best describes your configuration.
NOTE: Negative Voltage Output.
But your schematic is ???

Does a Non-Inverting Configuration seem better for your 0 - 5 volt supply ?

A very simplified, Non-Inverting, Single Supply ( +5v & Gnd ) , Photo-Transistor Ampliifer ...
https://i.stack.imgur.com/Nhnpg.png
 
Last edited:

Thread Starter

RTTD1977

Joined Mar 25, 2019
6
Thank you. My photo-transistor circuit is inverting, and gives 0V (or thereabouts) with light levels > 200 lux, rising to 4.47V in darkness. I've built a few of these circuits and it's the fact that some work and some don't has me flummoxed. AK above suggested my input voltages are out of range and may be the root of my problem. Can anyone please suggest how to reconfigure the op-amp inputs with a photo-transistor so as not to exceed the input voltage (Vdd - 1.2V with a 5V/0V supply), or is it just a case of employing a potential divider to the inputs?
Thanks again, and apologies for my ignorance.
 

AnalogKid

Joined Aug 1, 2013
8,675
The opamp you have chosen *does* include the negative rail within its common mode range. If you turn the circuit "upside down" so it is a non-inverting amplifier with the inputs referenced to GND rather than Vcc, the output voltage will increase with increasing flux. Reversing the inputs to the comparator will restore the logic polarity.

ak
 

Thread Starter

RTTD1977

Joined Mar 25, 2019
6
See figures 2 - 5 here ...
http://www.bristolwatch.com/ele2/pdiode_op_amp.htm

Your "Inverting" Op Amp, with (+) Input Pin @ 5V, and a "SIngle Supply" has me confused.
Figure #4 best describes your configuration.
NOTE: Negative Voltage Output.
But your schematic is ???

Does a Non-Inverting Configuration seem better for your 0 - 5 volt supply ?

A very simplified, Non-Inverting, Single Supply ( +5v & Gnd ) , Photo-Transistor Ampliifer ...
https://i.stack.imgur.com/Nhnpg.png
OK, I might just give that a try. Thanks again
 

Thread Starter

RTTD1977

Joined Mar 25, 2019
6
The opamp you have chosen *does* include the negative rail within its common mode range. If you turn the circuit "upside down" so it is a non-inverting amplifier with the inputs referenced to GND rather than Vcc, the output voltage will increase with increasing flux. Reversing the inputs to the comparator will restore the logic polarity.

ak
Right, that sounds like a good suggestion. I'll give it a go. Thanks again
 
Top