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Problem with my OPT101 circuit

Other Parts Discussed in Thread: OPT101

I made exactly the same circuit shown(source of the circuit is part of figure 6 took from  www.ti.com/lit/ds/symlink/opt101.pdf ) here. The only change was that I used +5 volt power supply(across pin 1 and 3) instead of +15V as drawn in the data sheet). 

After making the circuit, I made direct sunlight to fall on the IC, output voltage was almost zero. I am able to see a positive spike(around 1.5 V) only at the moment when I block the light path and make no light fall on the IC. After the spike, the voltage is again zero. When I unblock the light, again a spike(this time negative spike(around -1.5 V)) appeared at that moment

I mean, the circuit gives a non zero output ONLY WHEN THERE IS SUDDEN CHANGE IN INTENSITY. But instead, there must be a output voltage depending on intensity of light and not as a function of rate of change of intensity.

What mistake am I doing? 

  • Deepak,

    I'll set this up in the lab and let you know what I find but what you are seeing is certainly peculiar.  V+ = 5V should be no problem.

    One thing the data sheet mentions but never shows is a load resistor on the output, and it is almost always 10kOhm.  Try putting 10kOhm between pin 5 and 3 and see if this changes anything.

    I will check this out and let you know what I find.

    Jason Bridgmon

  • Deepak,

    Is it possible that you are looking at the output waveform on an oscilloscope?  If so, might the probe be AC coupled instead of DC?  Often that creates the transient pulses exactly as you described.

    regards,

    Jason Bridgmon

  • Also I know the diagram shows pin 8 grounded, but I see in the text earlier in this thread the emphasis was on grounding pin 3...please ensure both pins 3 and 8 are grounded and pins 4 and 5 are connected together.

  • Pins 3 and 8 were grounded and pins 4 and 5 were short circuited.

    Yeah I will try with 10Kohm load and also I will check whether it was AC coupled or DC coupled and then report to you.

  • Hi ,

     You were absolutely correct about AC coupling in my oscilloscope. But now the problem is that, at my output terminal, I am getting combination of 50 Hz and 100 Hz noise. I mean, some times my Digital oscilloscope reads the signal as 50 Hz and some times 100 Hz noise. Any guesses about the source of 100 Hz noise(mostly 100 Hz noise)? And 100 Hz noise occurs even after I turned Off all tube lights in the room in which I was working.

    Actually I wanted to develop pulse oximeter setup.

  • Hi Deepak,

    The 100Hz noise is almost certainly due to light from an incandescant bulb reaching the OPT101.  The explanation is that  an incandescant bulb when operated from a 50Hz AC source will actuallly have it's light output peak at both positive and negaitive peaks of the 50Hz AC thus doubling the frequency (no such thing as negative light :))...the OPT101 is simply detecting this.  Not sure why you might see 50Hz signal....maybe you have some test equipment ground loop or maybe the scope triggered to the AC line.  While you may think the OPT101 was in the dark when you turned the room lights off, it almost certainly had plenty of light still it was detectng, it is a very sensitive device.  Think of it this way...if you can see it, there is enough light for a significant output...in fact perhaps the 50Hz is related to the screen refresh rate of your oscilloscope or a nearby computer monitor.  The OPT101 sounds like it is working fine in your lab... to quickly create a dark setup, place a thick dark cloth cover over the OPT101 (like a jacket or blanket) then you will see that the output is right around 7.5mV with no AC component.