Challenge: can you light these LED's?

Tonyr1084

Joined Sep 24, 2015
7,852
REALLY??? In high school electronics class we built a hotdog cooker that consisted of a couple of nails, some hot dogs and some prohibited from this website connections.
 

Tonyr1084

Joined Sep 24, 2015
7,852
OK, lets not take this thread off topic. The question is can you light two LEDS one at a time using a single push button, unlimited lengths of wire and unlimited numbers of resistors. To answer that question - as has been answered, "Yes".
 

Thread Starter

xox

Joined Sep 8, 2017
838
OK, lets not take this thread off topic. The question is can you light two LEDS one at a time using a single push button, unlimited lengths of wire and unlimited numbers of resistors. To answer that question - as has been answered, "Yes".
Ok, forget about the hardware. Can we make this thing with nothing but hotdogs?
 

Tonyr1084

Joined Sep 24, 2015
7,852
Well, I never tried sticking the leads from an LED into a weenie. So I couldn't answer that question. But let's not get off the subject. We're going to get in trouble if we keep this up.
 

jpanhalt

Joined Jan 18, 2008
11,087
LTSpice simulates. Closing the switch, LED1 lights and the battery for LED2 shorts out making LED2 off. Otherwise LED2 is on and LED1 is off.

Unfortunately, I am lousy at LTSpice and don't know how to simulate a push button switch. One could play with serial resistors to the batteries to prevent a dead short circuit. But as some might say, short circuiting a battery was not excluded in the problem definition.
 

Thread Starter

xox

Joined Sep 8, 2017
838
LTSpice simulates. Closing the switch, LED1 lights and the battery for LED2 shorts out making LED2 off. Otherwise LED2 is on and LED1 is off.

Unfortunately, I am lousy at LTSpice and don't know how to simulate a push button switch. One could play with serial resistors to the batteries to prevent a dead short circuit. But as some might say, short circuiting a battery was not excluded in the problem definition.
Rule bender, eh? Submission rejected!

Here's my version:

lit.png


It's hardly perfect, but I have used both high and low side switch versions in a few circuits without any issues to date. It's also fairly easy to "balance". My main complaint is that it's just inherently wasteful, energy-wise. So maybe not so great for circuits powered by small batteries.
 

jpanhalt

Joined Jan 18, 2008
11,087
I thought about substituting bananas for hot dogs, but the calculations got too complex. I am sure that is the solution. Now, if you allow cabbage, the solution is quite easy, They are naturally rectumfiers.
 

WBahn

Joined Mar 31, 2012
29,976
Here's the setup. You have an unlimited number of resistors, two LED's, a single push button switch, and plenty of wire. Your task is to have one LED light up when the switch is closed and the other to be lit when the switch is opened. In other words, at any given moment exactly one of them will be lit.

Can you solve the puzzle?
What kind of push button switch is it? SPST? DPDT? Something else?

Since there was no requirement that they be lit equally, there's a few simple ways to do it. One uses just two resistors in addition to the LEDs and a SPST switch. You could eliminate the resistors altogether if you are willing to use enough of the "plenty of wire".
 

Thread Starter

xox

Joined Sep 8, 2017
838
What kind of push button switch is it? SPST? DPDT? Something else?


Since there was no requirement that they be lit equally, there's a few simple ways to do it. One uses just two resistors in addition to the LEDs and a SPST switch. You could eliminate the resistors altogether if you are willing to use enough of the "plenty of wire".
Schematic?

Or how about pickles or some other food that you can get to fluoresce?
I was kidding! Don't get me wrong though, I'd still love to see someone pull something like that off.
 

WBahn

Joined Mar 31, 2012
29,976
Schematic?
Didn't know if you wanted schematics or other outright spoilers.

I don't have a schematic capture package installed on this machine, so Paint will have to do.

I'm assuming that the diodes are characterized as follows:

Vf = 2 V
If = 10 mA
Current changes by one decade every 120 mV.

Although I think that reducing the current in one diode by 90% would be good enough (especially when the other diode is on right next to it), I'll set the goal of reducing it by 99%.

TwoDiodes.png

With the switch open, we'll size the resistors to yield an I1 of 10 mA. When the switch is closed, the voltage across the other diode will be around 2 V and we want to drop about 240 mV across R1 at 0.01 mA. That means that R1 has to be 2.4 kΩ. When the switch is open, it needs 10 mA flowing in it so the voltage across it will be 24 V. Let's pick Vcc so that the voltage across R0 is 10x this, so it will be 24 kΩ and Vcc will be 266 V.

Since no constraint was placed on the supply voltage, I can pick whatever I want. I will need to dissipate nearly 3 W of power, but that's not outrageous in the pursuit of a brute force circuit like this.

When the switch is closed the nominal 2 V across the right LED would result in 11 mA flowing in it, which would increase the voltage across the diode by a pretty negligible 5 mV, making it about 2.05 mV.

If we want to call 10% of the current sufficient to be "off", then we can work with a much more reasonable supply voltage. Now we can use 120 Ω for R1, 1.2 kΩ for R0, and use a supply voltage of around 15 V.

Similarly, if we use diodes that are "on" with 1 mA of current we can scale things down (though not as much as you might think -- but we can get the power down to well under half a watt).

A much more elegant solution is a bridge arrangement, variations of which others have already posted.


TwoDiodes2.png

Again assuming a desired diode current of 10 mA at a forward voltage drop of 2.0 V, I chose R1 = 100 Ω, R2 = 300 Ω, and R3 = 550 Ω.

EDIT: Forgot to specify that I chose 11 V for the supply voltage in the bridge circuit. That should result in 10 mA @ 2 V in each diode when on.
 
Last edited:

Thread Starter

xox

Joined Sep 8, 2017
838
Didn't know if you wanted schematics or other outright spoilers.

I don't have a schematic capture package installed on this machine, so Paint will have to do.

I'm assuming that the diodes are characterized as follows:

Vf = 2 V
If = 10 mA
Current changes by one decade every 120 mV.

Although I think that reducing the current in one diode by 90% would be good enough (especially when the other diode is on right next to it), I'll set the goal of reducing it by 99%.

View attachment 224919

With the switch open, we'll size the resistors to yield an I1 of 10 mA. When the switch is closed, the voltage across the other diode will be around 2 V and we want to drop about 240 mV across R1 at 0.01 mA. That means that R1 has to be 2.4 kΩ. When the switch is open, it needs 10 mA flowing in it so the voltage across it will be 24 V. Let's pick Vcc so that the voltage across R0 is 10x this, so it will be 24 kΩ and Vcc will be 266 V.

Since no constraint was placed on the supply voltage, I can pick whatever I want. I will need to dissipate nearly 3 W of power, but that's not outrageous in the pursuit of a brute force circuit like this.

When the switch is closed the nominal 2 V across the right LED would result in 11 mA flowing in it, which would increase the voltage across the diode by a pretty negligible 5 mV, making it about 2.05 mV.

If we want to call 10% of the current sufficient to be "off", then we can work with a much more reasonable supply voltage. Now we can use 120 Ω for R1, 1.2 kΩ for R0, and use a supply voltage of around 15 V.

Similarly, if we use diodes that are "on" with 1 mA of current we can scale things down (though not as much as you might think -- but we can get the power down to well under half a watt).

A much more elegant solution is a bridge arrangement, variations of which others have already posted.


View attachment 224922

Again assuming a desired diode current of 10 mA at a forward voltage drop of 2.0 V, I chose R1 = 100 Ω, R2 = 300 Ω, and R3 = 550 Ω.
The bridge design is definitely much better than the one I came up with.

That high-voltage circuit is a little crazy. It does however work flawlessly in the simulator so your logic is obviously spot on.

All in all, very impressive examples. Well done!
 

Thread Starter

xox

Joined Sep 8, 2017
838
FWIW here's a slightly improved version. Both high and low side switching shown, along with estimated current consumption.


offison.png


One specific application would be an indicator to show that a circuit is "on standby" (off but connected to power).
 

Tonyr1084

Joined Sep 24, 2015
7,852
Top