This design is suppose to light up the LED When the sensor is under light, and turn off the LED when the sensor is covered.

I noticed two problems:

  1. My LED won't light up.

  2. When I measure the voltage input difference (expecting +15- V) without the wire connecting the second prong of my 50k potentiometer (25th node) to the second prong of the op-amp, I measure 15 volts. But when I insert that wire how it is in the picture, my same input voltage measurement starts fluctuating. I assume this is a problem

Of course, I tried changing the values of the potent. and sensor but had no luck. I also tested my op-amp and it seems to be working correctly. The LED is also working correctly because it lit up in a different scenario. & Yes, I am also aware there is no voltage input in the photo. These were taken after I unplugged The input.

Will you please look over my design and help me locate the problem?

My breadboard / setup

Setup at a different angle


EDIT: New circuit based off new understanding

New design

EDIT2: Just talked to my instructor. I indeed originally wired the purple wire wrong, but it should be fixed in the last two photos. I also confirmed that my resistor is not shorted and the photo is just confusing everyone, so I will upload a new one once I get home.

EDIT3: Less confusing photo (made sure you could tell the led is not shorted) Less confusing photo

SOLUTION: Thank you @jonk and @TonyStewart!

My circuit was wrong in that I did not realize my potentiometer should not connect to the op-amp's 3pin (basically I didn't understand the schematic).

Later on, @jonk helped me come to the realization that there was something wrong with my op-amp, so I used a new one and it now works.

  • \$\begingroup\$ Probably not the cause, but where is your decoupling capacitor? \$\endgroup\$
    – winny
    Mar 1, 2017 at 6:11
  • \$\begingroup\$ @winny I haven't learned about / used decoupling capacitors. This lab should work without one. \$\endgroup\$
    – Smeboo
    Mar 1, 2017 at 6:14
  • \$\begingroup\$ Pot is pointing at 15V try <1V \$\endgroup\$ Mar 1, 2017 at 6:34
  • 1
    \$\begingroup\$ I can see the arrow is pointing to the pin connected to V+. Get a voltmeter and report every pin with pot set to 0V. \$\endgroup\$ Mar 1, 2017 at 6:48
  • 1
    \$\begingroup\$ Look carefully at your LED connection. You have both LED pins on the same horizontal row, which means they are connected together, and nothing connecting to them. You need to rotate the LED 90 degrees, then connect the resistor and comparator properly. \$\endgroup\$ Mar 1, 2017 at 12:42

2 Answers 2


I think you may have wired it up wrong. It looks to me like you did this:


simulate this circuit – Schematic created using CircuitLab

I think you want this:


simulate this circuit

Pull the end of the purple jumper off of pin 3 and move it to your \$+15\:\textrm{V}\$ rail.

Below is a piece of your schematic and some information I've added. The arrow points to a "cross-over" and NOT to a connection. Just in case that helps.

enter image description here

Your newly added picture does appear to show a shorted LED. However, your first picture does not appear to show a shorted LED. I wouldn't have been able to tell from when I was reading your post, because your last picture wasn't added by then. Now, looking at it it does appear that it could be shorted out. Did you move it between pictures???

Anyway, that is another detail to look at and double-check. I think you already know (knew) that all five holes along a row are all connected together. That's a detail that everyone needs to know just to use a protoboard. So I assume you are aware of that. But anyone can accidentally get a lead in the wrong hole, too. So it's worth checking out just to make sure.

Best wishes here.

  • \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$
    – Dave Tweed
    Mar 2, 2017 at 2:05

@jonk was correct in identifying the purple wire was wrong.

@WhatRoughBeast was also correct , your LED is shorted out and R is connected wrong to LED. FIX IT enter image description here You must use a a voltmeter to debug your connections and learn to say "pin one" and write as p1 or U1-1 or pin1.

Compare my expectations below and record your results.

Pin    Function  Voltage
---    -------  --------
1     (out)   15V  V+ with LED off     0V with LED on     
2      (-)     0 to 15V pot V . Set to middle pin3 range 
3      (+)     0V in dark  3V in bright light
4       V+      Supply voltage

11      V-      Supply return or ground ( 0V reference )

The LED must never see reversed voltages or inserted backwards as -15V will damage LED. (-5V max) The Series R limits the current from the output switched from 0 to V+ and the voltage across RED LEDs rises to 2V when lit.

The pin 2 & 3 connections are reversed on schematic to match your description. Ambient light +V in makes output + and LED goes off when connected to LED -ve (cathode)

As is Pin 2 (-) is your reference level input from pot and if room is lit, Pin 3 (+) rises and if more than (-) then output goes high. ( We think by saying if + is greater than - then output is +true (high))

  • \$\begingroup\$ It's starting to make sense now but I won't be home for a little while to do some more work. I'll check on it again, but I'm pretty sure the LED isn't shorted, it's just bad picture quality (Should be the same as it is in picture 1). \$\endgroup\$
    – Smeboo
    Mar 1, 2017 at 15:22
  • \$\begingroup\$ By shorted , I mean in the same row of pins meaning connected together. and the resistor goes between them without shorting but to the wrong pins. Try to layout the leads more logically like the schematic. I think this caused your confusion. They were also shorted in picture 1 but @WhatRoughbeast had better eyes on this than I did. \$\endgroup\$ Mar 1, 2017 at 15:30
  • \$\begingroup\$ I will look at it. Are you sure they are shorted in the very first photo? Because I cannot, for the life of me, see how they aren't in series. \$\endgroup\$
    – Smeboo
    Mar 1, 2017 at 18:05
  • \$\begingroup\$ Sorry 1st photo is now correct \$\endgroup\$ Mar 1, 2017 at 18:15
  • \$\begingroup\$ Haha! Alright. Was worried! I just started learning about op-amps today so once I get home I will double check the input voltages. \$\endgroup\$
    – Smeboo
    Mar 1, 2017 at 18:35

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