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I'm building a circuit using a sensor and a "Goldilocks" LED setup. The LEDs are set up to be turned on and off using the outputs from an op-amp, which is wired up in a window detector config. The first two LEDs are wired up using an NPN transistor, and the third (for simplicity) is simply wired so that when the output of the window comparator goes low the LED will go to ground and turn on: +5 volts, going through a 200 Ohm resistor, red LED, to the output pin of the MCP6002 op-amp.

The issue I'm having is that I can't get the LED to come on - I'm not sure if I'm overlooking something obvious or if it's to do with the IH0505S DC-DC that I'm using. The DC-DC is powering 3 op-amps, 5 standard LEDs (no hi-bright etc.), a temperature sensor and a humidity sensor (both of which have Iq in the micro-Amp range).

Thanks for taking the time to look over my query - I'll be able to provide you with as much additional detail as you require.

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ Too many words, not enough diagram. We have a nice schematic editor; please use it to show us the exact circuit you have created. \$\endgroup\$
    – Dave Tweed
    Commented Feb 23, 2015 at 14:19
  • \$\begingroup\$ Maybe the LED is backwards. When you think the LED is supposed to be on, confirm that Vref = 4, Vin = 4.2, check the voltage across D1, and check the voltage across R1 and make sure Vc really is 5V. \$\endgroup\$
    – user57037
    Commented Feb 23, 2015 at 17:05
  • \$\begingroup\$ By the way, 5V, 200 Ohms, red LED... It is going to be bright. \$\endgroup\$
    – user57037
    Commented Feb 23, 2015 at 17:06

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The schematics looks fine, and MCP6002 should be able to drive LED without any problem (it has I_SC of 23mA and V_OL of 25mV). Most likely the circuit is mis-assembled. You need to troubleshoot it:

  1. Check opamp output. It should be <2 volts, ideally <0.1 volts.
  2. Measure your power supply to make sure it is really giving you 5 volts.
  3. Check voltage on the connection between resistor and LED. It should be ~1.5-2 volts. More than that means LED is broken or backwards or opamp is bad. Less than that means resistor is of incorrect value, or power supply is bad.
  4. Check voltage across LED. It should be ~1.5-2 volts. If it is close to 5 volts, the LED is broken or installed backwards. If you have problems here, but steps 1-3 check out fine, you have power bus oscillations in your system.
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  • \$\begingroup\$ Thanks for the suggestions!! In response to your points: 1. The op-amp output is 0V as the comparator is giving a V0 output, due to the result of the input being negative. 2. The power supply is giving 5.23V (within tolerance so this isn't a problem). 3. The voltage seems to all be dropping across the resistor, which doesn't make sense - the LED doesn't have it's own resistance, so it's not as if there's a voltage divider... But the LED is the correct polarity. The op-amp is powering other LED branches separately (like traffic lights), so the output seems to be okay. \$\endgroup\$ Commented Feb 24, 2015 at 9:55
  • \$\begingroup\$ Are you saying you have 0 volts across LED? Then the LED is shorted. Try replacing it. \$\endgroup\$
    – theamk
    Commented Feb 24, 2015 at 16:21
  • \$\begingroup\$ I think the wire I was using was dirty so I scraped off the dull metal on the outside and it started to show a very low voltage across the LED. I've changed the circuit to run the LED off a transistor instead (the other two LEDs I'm using are working off the same type of circuit, but it seems like a waste of an LED for something so simple) - it works now. Took me a little while to get the right resistor value for Ib, but it comes on when it's supposed to. Thanks for your suggestions - much appreciated!! \$\endgroup\$ Commented Feb 24, 2015 at 16:55

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