I need to design an LED circuit that acts as a switching system. It must meet these specifications.

  • Case 1(Press Button 1): LED 1 ON, LED 2 ON, LED 3 OFF
  • Case 2(Press Button 2): LED 1 ON, LED 2 OFF, LED 3 OFF
  • Case 3(Press Button 3): LED 1 OFF, LED 2 OFF, LED 3 ON (and controlled by a Potentiometer)
  • Case 4(Press Button 4): LED 1 OFF, LED 2 OFF, LED 3 OFF

Each time a switch is pressed (or changes state), the circuit should behave according to one of the four cases. In case 3, LED 3 is controlled by both the switch and the potentiometer. LED 3 is turned ON when a switch is pressed and the Potentiometer is used to adjust the brightness. We can use no more than 4 switches in total and the circuit may be designed with only resistors, LEDs and switches. I've been stuck for some time on this and any help would be appreciated.

Edit 1: These are momentary tactile switches (or buttons). Of the 4 buttons that can be used, each time a button is depressed it exhibits 1 of the 4 cases.

Edit 2: The voltage is being supplied by a function generator but this can be changed to an alternative method if needed. The type of diodes are pictured below:

enter image description here

Here is a truth table that describes the switching system: enter image description here

enter image description here

Ultimately, this is what I created. Case 4 did not have to be included as this state exists with no action by the user.

  • \$\begingroup\$ Switches or buttons? Can multiple buttons/switches be active? \$\endgroup\$
    – A.R.C.
    Commented Nov 6, 2018 at 10:31
  • \$\begingroup\$ Old radio buttons would do this with no effort. \$\endgroup\$
    – KalleMP
    Commented Nov 6, 2018 at 12:26
  • \$\begingroup\$ They are SPST switches and I don't believe they should be active but Im uncertain what that means. \$\endgroup\$
    – Kswerb
    Commented Nov 6, 2018 at 20:06
  • \$\begingroup\$ Case (1) and case (4) seem to conflict with what you said: Case (1) says "button 1 or no button pressed whereas you say that Case (4) is no buttons pressed. Please edit your question to correct the confusion. \$\endgroup\$ Commented Nov 6, 2018 at 23:39
  • \$\begingroup\$ Are the buttons Normally Open or Normally Closed or SPDT? \$\endgroup\$ Commented Nov 6, 2018 at 23:39

2 Answers 2


Step 1. Make a truth table.

        LED1  LED2  LED3
Case 1: ON    ON,   OFF 
Case 2: ON    OFF   OFF
Case 3: OFF   OFF   ON (and controlled by a  potentiometer)
Case 4: OFF   OFF   OFF

You should find this easier to read and to convert into a circuit.

Step 2. Decide what a "Case" is.

Edit your question to explain what you mean by "Case". Is it pressing a particular button or what? Also, we need to know what kind of switches they are: how many poles and how many ways (one-way or two-way).

  • \$\begingroup\$ These are momentary tactile switches or buttons. So I would say that each time a button is depressed it exhibits the required action of each case. \$\endgroup\$
    – Kswerb
    Commented Nov 6, 2018 at 15:23
  • \$\begingroup\$ my switches are SPST. I made a truth table and determined that Case 1 will exist when no switch is depressed and a function generator supplies a voltage between 1 and 4 volts. \$\endgroup\$
    – Kswerb
    Commented Nov 6, 2018 at 20:05
  • \$\begingroup\$ There are a couple of problems with your last comment. (1) You haven't shown your truth table so I can't see if it's the same as mine. (2) There is no previous mention of a function generator. What's this about? (3) You won't light an LED with 1 V. What type of LEDs have you got? Put all the information into your question. \$\endgroup\$
    – Transistor
    Commented Nov 6, 2018 at 20:12
  • \$\begingroup\$ I appreciate your help both with the problem and with the proper way to communicate it. \$\endgroup\$
    – Kswerb
    Commented Nov 7, 2018 at 2:11

Your choice of wording for the question makes it really hard for someone to comprehend exactly what you want . Assuming you meant it in the simplest of ways you can use the below. But i have a feeling that's not what you want. B1- B4 are the switches. L1-L3 are the LEDs along with the current limiting resistances.

enter image description here

  • \$\begingroup\$ I apologize for the wording. I've edited a little be so I hope that helps. Your design works well except that it doesn't follow the cases exactly, mostly because of my wording haha. \$\endgroup\$
    – Kswerb
    Commented Nov 6, 2018 at 18:34
  • \$\begingroup\$ @VicJay: Welcome to EE.SE. As this is obviously an assignment for the OP we don't give complete solutions to the problem - for obvious reasons. Instead try to help the OP figure it out for themselves. There is a schematic editor button on the editor toolbar. \$\endgroup\$
    – Transistor
    Commented Nov 6, 2018 at 18:40
  • \$\begingroup\$ @Transistor Ohh apologies, I will remember that. \$\endgroup\$
    – VicJay
    Commented Nov 6, 2018 at 19:10
  • \$\begingroup\$ @Transistor my apologies as well. \$\endgroup\$
    – Kswerb
    Commented Nov 6, 2018 at 19:27
  • \$\begingroup\$ @Kswerb Although, If you found my answer helpful please do consider upvoting it. Thanks! \$\endgroup\$
    – VicJay
    Commented Nov 6, 2018 at 19:35

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.