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How do I make a circuit that acts like a set of radio buttons, i.e only one LED can be selected at a time. Answered

I'm trying to make a timing light for use in toastmasters and would like to know of any IC's that can help me out. I've already come up with 2 possible solutions, but both seem more complicated than necessary.

All I want is to have 3 LEDs with 3 associated push buttons. Any time an LED is selected via a button, any LED that was already on turns off, so that only one LED is on at a time. A rotary switch would be perfect for selecting one LED at a time, however I am looking for a low profile solution, and also something that can be used with one hand while attached to a small light weight device.

It seems like there should be an IC for this, but I don't have much experience with them and don't know what this functionality is called. Any advice is appreciated.

Edit: I'll also mention that I'm looking for a solution under $50 and that uses parts that I can source locally (both possible solutions mentioned above meet these requirements so I'm not very flexible on this).


Sorry I messed up. For 3 buttons you would need qty 3 '2 input AND gates'. The removal of a button requires that the AND gate has 1 less input. Also the 3 input AND gate in my original post should have been 4 input AND gate.

I've attached a logic diagram of a radio button layout. This is for 5 buttons. Remove 2 sets of 3 gates to get down to the 3 buttons that you need. In your case you would need qty 3 '3 input AND gates' and qty 6 '2 input NOR gates'. The NOR gates create an 'SR latch' and the OR gates are used to turn off the other outputs when a button is pressed. I tested this using 'Logism'. This is a very useful program for testing logic circuits. Very powerful. You can find it on the web. Just search for 'logism'. Hope this helps.

radio buttons circuit.jpg

How about this guy:


It's a digital switch with an additional bus enable input. 

Put one of these on each LED circuit, and tie the other two LED circuit states to an OR gate, then feed the OR output into the bus enable of the first LED circuit.

Using a bus switch was a good idea. One of my designs was actually a pretty similar idea using a decade counter, but the problem I had with that one was contact bounce. Got me thinking about using latches instead of flip-flops and now I think I have it figured out. Got it down to 3 latches, 2 or gates and 3 transistors. Should be simple enough and does exactly what I want it to ... hopefully.

That might work, but you won't find it at radio shack!  And again, hooking up three of these along with some gates will be more complicated and potentially more expensive than a microcontroller.

Would getting it online count as "source locally"? :)

Atlernatively, instead of going with an IC solution, just look in the local junkpile for an old standing fan.  Those usually have a switch array, of which only one can be activated at any one time.

For me it counts!  I get almost everything online since the local shops don't carry surface mount parts and other exotic stuff.  I suppose I'm lucky to even have local shops!

The fan switch idea is great!

A latching relay with a momentary switch, Maybe? They have them in Many Pole, And Many Throw Configurations. Im not sure if you want 3 leds fed from the same power as whatever load the LEDS are going to represent? If so you need a Single Pole Triple Throw. If you want Leds Powered separate from the Main Load Look for a Double Pole Triple Throw Latching Relay. Put a Momentary press type button and you would just toggle through them by pushing the button. 


8 years ago

I'm not aware of a single chip that will do this.  Perhaps you could rig something up with a few logic devices (AND and OR gates, etc) but that solution will ultimately be more complex than a single microcontroller.

If you're comfortable doing the programming, you could get the programmer and microcontroller for less than $30, plus another $10 or so for switches (I recommend arcade buttons!) and LEDs.  You could even make it beep each time a speaker's time is up in case they don't notice the LED has changed.

You can buy switches that are set up like 4 in a row.  Turning any one on turns any of the other ones off.

A multi-contact rotary switch would be perfect for this.  Rotate to position 1 led 1 goes on, rotate to 2 and 1 goes off and 2 comes on, repeat for as many as you need.

What are the 4 in a row switches called?

I agree that a rotary switch would be great, but judging by the condition of the thing I'm replacing, having a knob sticking out is not going to last.

You've specifically said "analog" here. Is there a reason for that ?


Haha, I just changed that. All I meant is that I don't want to program a chip to do it.

Hmm, I'm not sure 'program a chip' is a real thing either. What I mean to say is: if there is programming involved it's probably overkill.