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.

 

If you were to add a power supply inverter chip like ICL7662 from Maxim, then you could use the inverting function of your op-amp. Basically, the ICL7662 chip gives a negative output for a positive input (+5vdc in = -5vdc out), use it's output as the negative supply for your op-amp. The chips are real easy to use; you just need the chip and 2 capacitors. Here's a pdf datasheet link:http://pdfserv.maxim-ic.com/en/ds/ICL7662-Si7661.pdf
JB

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Nothing is impossible for the man who doesn't have to do it himself - Weiler's Law

 

Post a schematic, or at least a detailed description, of your photodiode and op amp connections.

 

Without adding anything, you can reverse the opamp connection.
Probably your photodiode is reverse-biased with its anode at ground and its cathode connected to a pull-up resistor to +5V.
Your opamp is probably an inverting circuit so that without light, the photodiode doesn't conduct and its output voltage is high. Therefore the opamp inverter gives a low to the LM3914. Light makes the photodiode leak current, so its output voltage falls, and the inverting opamp gives an increasing voltage to the LM3914.

With the opamp wired as a non-inverting circuit, its output voltage follows the photodiode's voltage.

Depending on the light intensity on the photodiode, it may have enough current to drive the LM3914 directly, without an opamp follower.

 

Just reverse the inverting and non-inverting inputs it's comparing. Or swap the positions of the photodiode and pullup as audioguru suggests.

 

Here is the photodiode lightbar circuit.

Attached Images

photodiode_circuit.jpg (127.5 KB, 227 views)

 

This should start at 2.5V in total darkness, and go low as the light intensity increases.

Attached Images

photodiode_circuit.gif (10.9 KB, 217 views)