I thought the 324 is a single supply opamp which only requires a single positive supply? Isn't this how single supply opamps work?

Yes but yours inverts so it tries to make a negative output voltage that is impossible without it also having a negative supply.

 

Thank you. I read about single supply opamps and I realized that I have to level shift the noninverting input. I raised the non-inverting input to about 1.15V and I don't think I am getting the correct results. With a 1K feedback resistor, the output is pulled high to about 3.25V and I don't see any oscillation of the signal when the light hits the photodiode (output is only pulled high to about 3.4V when 1M resistor is added in feedback. Could it be that I just can't see the oscillations because they are on the order or 20mV? Any help would be appreciated. Thank you very much.

 

The LM324 is a low frequency opamp. Its max gain at 10kHz is only 50, at 100kHz is only 5 but its output swing is a max of only 0.5V p-p.

The signal from the photo-diode is very low so the feedback resistor should be a high value, not 1k.

With a 1k feedback resistor the small signal from the photo-diode does nothing so the output of the opamp should be +1.15V, not +3.25V.

 

Thanks Audioguru. With any feedback resistor value (1-1M) I noticed that the output is being pulled to the non-inverting level of 1.15. However, when I add the photodiode output to the inverting input (with or without incident light on the diode) the output gets pulled high to +3.25V. I cannot figure out why the output is being pulled to this value.

I noticed that when I remove the anode of the diode from ground, the output returns to 1.15V. Also, I am only supplying 1Hz LED frequency to the diode for testing.

 

The LM324 has PNP input transistors that have a bias current that pulls high without a resistance to ground. Then the output goes low.
When there is a feedback resistor and the photo-diode has no light so it does not conduct then the opamp is a simple follower making its output voltage the same as its non-inverting input voltage.

Maybe you connected the photo-diode upside-down so that it is a forward-biased 0.6V diode to ground. Then the output will be high at about +3.5V.

 

I beleive the diode is biased correctly (reverse). The end of the PIN is connected to a BCN connector that has a red and a black alligator clips on the other end. I connect the red lead to ground and the black to the noninverting input. I checked the output of the diode with incident red light alone before being inputted into the lm324 and the output was pulled to about 300mV (from constant white light shining into the diode) and there were pulses of about 40mV when the light hit the diode.

 

Yes, your photo-diode is connected upside-down.
By itself it is a mini solar cell and when it is upside-down with its cathode connected to ground then its anode produces a positive voltage.

In your circuit it is supposed to be with a reverse bias voltage from the feedback resistor so with no light it does not conduct then the output from the opamp is the same as the inverting input pin. When there is light then the reverse-biased diode leaks a current through itself which causes the output of the opamp to go high.

 

Thanks Audioguru, however I think I am a little unclear on what you are trying to say. From what I understand from reversed biased diodes is that the anode (positive red alligator lead) is tied to ground while the cathode (black negative alligator lead) is the input of the inverting LM324 (with a feedback resistor to the output), which is what I did. No light there should be no conduction, when light is incident a current is delivered from the diode to the circuit. This is what I have and the output of the opamp (pin 1) is a constant 3.25V. What am I missing? I am very confused.

 

What am I missing? I am very confused.

The output of your inverting opamp is high because the photo-diode is conducting.

 

My anode is at ground. I thought this was reverse biasing. Should the diode be biased at a negative voltage? Most of the charge amp circuits I saw had their anode at ground for reverse biased configuration.

 

Yes the anode of the photo-diode should be at ground. But i think you have the cathode at ground. Look at the photo of the photo-diode in its datasheet.

 

You were right, the anode is the outer shell of the photodiode casing. I reversed my configuration and now I am getting the attached output from the 324. When LED light is indicent there is a smooth peak at 3.25V, and when there is no LED light there is what looks to be high frequency oscillations or noice from 0V-3.25V. I have a 100k resistor in the feedback loop.

 

The "oscillation" is probably the wire to the photo-diode is an antenna that picks up mains hum. Use very short wire or use a shielded audio cable.

 

I am using the attached cable. This is not shielded?

 

Hello,

The show cable is partly shielded.
The wires to the crocodile clamps are not shielded (10 - 15 cm I estimate).

Greetings,
Bertus

 

Thanks everyone for the help. I got the circuit working after adding a capacitor (bypass) from the the voltage divider output to ground.

 

I suspect the capacitance of the diode plus the capacitance of the cable is making the op amp oscillate. Try adding 10k between the center conductor of the cable and the inverting input of the op amp. Connect the cable shield to GND, as before.

 

Now my circuit is working, I want to replace the LM324 with a single source (5V) high frequency opamp. Any suggestions?

 

Now my circuit is working, I want to replace the LM324 with a single source (5V) high frequency opamp. Any suggestions?

This might be a good time for you to start learning to use various mfr's op amp tables. Try Analog Devices, TI, Linear Technology, Maxim, Fairchild, STMicroelectronics, Intersil, ON Semiconductor, etc. If you don't understand the tables, post your questions here.
If I recommend an op amp, I will first have to do the same thing.

 

Hello,

If you want to understand the opamp parameters take a look at this PDF from TI.

http://forum.allaboutcircuits.com/attachment.php?attachmentid=16011&stc=1&d=1265470331

Greetings,
Bertus