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Photodiode / LED Lightbar circuit - almost complete


Uman
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My class project is to build a battery operated light detector circuit using a photodiode and a LED light bar to register the light intensity. The photo diode output is connected to a single-supply 5 volt Op Amp (battery operated). The Op Amp feeds a national semi LM3914 IC light bar driver, which powers the lightbars.
When the photodiode detects light, the light bars illuminate according to the light intensity; and works fine. Unfortunetly, I need the reverse; I need the light bars to illuminate when the photodiode is in the dark and extingush when the photodiode detects light. What can I put between the Op Amp out put and the lightbar driver to inverse the driver signal? The OP Amp is battery operated and is powered by +5v, so inverting the Op amp connection will not work.
The LM3914 LED lightbar driver input single is 0 to 2.5vdc for full off or full on respectfully.

Any help or discussion would be a big help.
Thanks all.

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Hi Uman,
You didn't post your circuit so I am just guessing at how it is connected:
1) Your photodiode is probably reversed-biased with a resistor to 5V. Therefore in the dark, its voltage output is 5V.
2) With light shining on the photodiode, it leaks current and its voltage reduces.
3) Therefore your opamp is an inverting circuit with a 0V output in darkness and 2.5V when bright.

Of course you can invert the opamp connection to make its operation the inverse. If you make it a non-inverting circuit, its output will be 2.5V in darkness and 0V when bright.

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