# LED Circuit design (4 switches, 1 potentiometer)

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:

Here is a truth table that describes the switching system:

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

• Switches or buttons? Can multiple buttons/switches be active? – A.R.C. Nov 6 '18 at 10:31
• Old radio buttons would do this with no effort. – KalleMP Nov 6 '18 at 12:26
• They are SPST switches and I don't believe they should be active but Im uncertain what that means. – Kswerb Nov 6 '18 at 20:06
• 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. – Dwayne Reid Nov 6 '18 at 23:39
• Are the buttons Normally Open or Normally Closed or SPDT? – Dwayne Reid Nov 6 '18 at 23:39

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).

• 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. – Kswerb Nov 6 '18 at 15:23
• 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. – Kswerb Nov 6 '18 at 20:05
• 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. – Transistor Nov 6 '18 at 20:12
• I appreciate your help both with the problem and with the proper way to communicate it. – Kswerb Nov 7 '18 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.

• 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. – Kswerb Nov 6 '18 at 18:34
• @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. – Transistor Nov 6 '18 at 18:40
• @Transistor Ohh apologies, I will remember that. – VicJay Nov 6 '18 at 19:10
• @Transistor my apologies as well. – Kswerb Nov 6 '18 at 19:27
• @Kswerb Although, If you found my answer helpful please do consider upvoting it. Thanks! – VicJay Nov 6 '18 at 19:35