Recently, I began work on a adjustable regulated power circuit in search of a short circuit protection / indicator add on. A fellow forum user issued a solution that involved an LM321 as a comparator in the image below:

Power circuit with LM321 Comparator as short-circuit indicator

As a possible solution as a short circuit protector, the output of the LM321 is to be connected to the gate of an SCR. The SCR's anode will be connected to the regulator's adjust pin and short it to ground thus rendering the output of the regulator to as little output as possible. (I'm not sure if the SCR part is relevant to my question yet so I included it's purpose to the comparator for now).

The current design has two LED's. The Green LED is active when the load is within specified parameters. The Red LED will activate when LM321's output is high indicating that Voltage (load) drops below Voltage (ref) or that too much load is present. With the SCR in place, the regulator's output drops off this deactivating the Greed LED. The Red LED will be active. My question is, is it possible to design a bi-color LED (two-lead or three lead) version of this? Any help is greatly appreciated. Many thanks in advance.

Edit: Not sure why this got down voted. Question was pretty straight forward. What was wrong with the question? :/

  • \$\begingroup\$ It wasn't me - but I'm guessing because the way it was phrased made it a question very specific to your own problem, among other things - you will find that stack exchange sites' scoring system makes people answering inclined to want well defined questions which fit well into the criteria set out in that site's help centre, such as expecting problems which will clearly be of use to others - not that I don't think yours would be. \$\endgroup\$
    – Jodes
    Apr 9, 2015 at 6:34
  • \$\begingroup\$ Perhaps the question was too wordy or something. I attempted to be focused on the question as best possible. If I hosed the question somehow, my apologies. Your comments have greatly helped and have pushed me in the direction I needed to finding my answer. Once again, many thanks! \$\endgroup\$ Apr 9, 2015 at 6:40

2 Answers 2


Change the circuit above, replacing the circuit on the left with the circuit on the right. The RED/GREEN LED combined can be found as a standard bi-colour LED with two pins.

I think this is what is intended:


simulate this circuit – Schematic created using CircuitLab

  • 1
    \$\begingroup\$ I have added a variation of your circuit which I think is what you intended to put - please delete yours or mine above as desired. Your cct works with a bipolar supply and R1 is not needed - but green is never drive on. A cct typo I suspect. \$\endgroup\$
    – Russell McMahon
    Apr 6, 2015 at 12:37
  • \$\begingroup\$ Ah yes, thank you! Kind of you to offer the excuse of a CCT typo, but it was in fact just a brain fart! \$\endgroup\$
    – Jodes
    Apr 6, 2015 at 13:19
  • \$\begingroup\$ I will be using the output from the comparator to drive the gate of an SCR. Namely a 2N5060 to short the adjust pin of the LM350 to ground. In this particular setup with a bi-color LED, I think that I'm pulling directly from the comparator output before the LED's. Am I correct to believe that V+ from the upper rail in the diagram on right will will mean that a high signal is always present between the LED's and the comparator's output? I'm not sure I am following correctly the diagram on the right. \$\endgroup\$ Apr 7, 2015 at 0:14
  • \$\begingroup\$ I understand that the Red LED will active on high signal from the comparator's output but with it's output low, the Red is inactive and the Green is active is getting V+ from the rail but it's return is being fed inward though the output of the comparator(?) Please advise. As an aside, I tried this with a 741 op amp just for rough testing on a breadboard but couldn't get it to work. Then I realized that the 741 circuit setup may require a dual power source (V+, GND, V-) :P \$\endgroup\$ Apr 7, 2015 at 0:17
  • \$\begingroup\$ For the circuit on the left, using a 741 in place was easy. I have LM321 but I'm waiting on SOT23-5 to DIP-6 adapters to come in so that I can test everything out before commiting to a full circuit build. \$\endgroup\$ Apr 7, 2015 at 0:19

Thank you @Jodes for all your help on this matter. I didn't want to use a second op-amp I figured that adding a transistor logic inverter to the comparator output may be the solution. For the original design, it wanted to use an LM321 but I experimented using a 741 opamp instead until the SOT23-DIP6 adapters got in. I felt that buffering the output using transistors would be best after overloading an op-amp with damaging results. After some experimenting on the breadboard, I got it to work perfectly and came up with the following solution using a transistor (Q1) to buffer the output driving the Red side of the LED. When the comparator output is low, the current will then be enough to bias Q2 which is set up as an inverter driving the Green side of the LED. When the comparator output is high, it will bias Q1 allowing current to flow lighting the Red side while no longer being able to bias Q2 cutting the Green side. The comparator side of the circuit below originally came from the Forrest Mims Engineer's mini notebook - Op Amp IC circuits (Radio Shack) page 27:


For the case of a bipolar LED, I was able to devise of bridge driver on a single supple. For higher and lower voltage supplies, The resistors may need to be adjusted accordingly.


The transistors added to the comparator output in this configuration has solved the question I had in my original question for changing a single indicator to a two color indicator. Hopefully this is helpful to others.


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