How about this circuit. This should work keeping only one led on at a time.

 

How about this circuit. This should work keeping only one led on at a time.

I think one will fade in while the other fades out, before each transistor reaches saturation point. I'll try it tonight...

Thanks!

 

Here's another option, using a 'long-tail pair' configuration.

 

Here's another option, using a 'long-tail pair' configuration.
View attachment 101639

I got something to work!

I put both your's and hobbyist's latest on breadboard, and they both allow both LED's to stay lit simultaneously for a certain range of input voltage.

I used a bistable multivibrator (flip-flop). R6 and Q4 represent variable voltage output of phototransistor. The value of R5 took some tweaking. Too low of a value and L1 will not turn off, even when L2 comes on. Too high a value and L2 shuts off when L1 comes on, but when the voltage at base of Q4 is taken back to zero, the flip-flop does not reset (L1 stays on, L2 stays off).

 

Nice work,
Yes, our designs used a linear transition, while yours is purely switching trasnsition, as what you were looking for, good job.

Now to increase this challenge a little, you said you had to tweak R5 a little.
Will a change of supply voltage cause R5 to be eratic again, if it does not, then you get the award for best design for this project.

But if it does, then were still in the game.

I'll try to come up with a design using a different discrete component, but lets say all this has to be done using only discrete components, makes for a more fun challenge.

please let us know what your results are in testing your circuit, for variable supply voltages.

 

Nice work,
Yes, our designs used a linear transition, while yours is purely switching trasnsition, as what you were looking for, good job.

Now to increase this challenge a little, you said you had to tweak R5 a little.
Will a change of supply voltage cause R5 to be eratic again, if it does not, then you get the award for best design for this project.

But if it does, then were still in the game.

I'll try to come up with a design using a different discrete component, but lets say all this has to be done using only discrete components, makes for a more fun challenge.

please let us know what your results are in testing your circuit, for variable supply voltages.

It occurred to me that changing some things in the circuit may invalidate the value of that resistor, but the voltage didn't cross my mind. Changing the supply voltage will probably break it. Won't get to mess with it until tomorrow night.

Yes, I stayed away from IC's so I could learn more from the challenge.

Phototransistor showing up in today's Mouser shipment, so I can throw that into the mix, too.

 

This should work for switching mode, its your design idea, about using a multivibrator,
and its configured to work as a one shot switching unit.

I have not built any of these, my bench is full with another electronics project.
So I calculated resistor values by assuming 20mA per LED, and a Vce sat. of 0.2v.
However I don't know how well it works under changing supply voltages.

 

Thanks, I'll board that tomorrow. That's a simpler design!

 

How much current can your photo-transistor produce when illuminated? It would have to be ~4mA to pull the base of Q2 low enough to turn off Q2 and so light LED2. I think you'd need very bright light to get 4mA.

 

How much current can your photo-transistor produce when illuminated? It would have to be ~4mA to pull the base of Q2 low enough to turn off Q2 and so light LED2. I think you'd need very bright light to get 4mA.

Alec, I received a phototransistor yesterday, and don't know how much current it produces. When I applied it to Q1 (eliminating Q3 and Q4), it was not enough to flip the flop. (Configuration 1)

However, when I put it to the base of Q3, it amplified it enough to work great! (Configuration 2)

In Configuration 1 I was measuring about 0.5V at base of Q1 with the phototransistor illuminated. Using the original circuit (voltage divider as input signal) I determined that the trigger point for Q1 was somewhere above 0.6V.

In Configuration 2, I was measuring well over 1V at the base of Q3, coming from the phototransistor.

So there are some things for me to try to understand here...I was looking at voltages and the solution apparently resides in current.

 

please let us know what your results are in testing your circuit, for variable supply voltages.

The circuit worked fine at 9V, too.

Then I started building a variable regulated circuit (aiming for 1.25V to 12V), and didn't complete that task.

 

I was measuring the voltage at the bases in reference to common (in post #50)...I think know I should have been measuring between the base and the emitter.

Correct?

 

The circuit worked fine at 9V, too.

Then I started building a variable regulated circuit (aiming for 1.25V to 12V), and didn't complete that task.

Congratulations on a successful design on your circuit in post #44.
When you said you had to tweak R5, to get it behaving with the rest of the circuit, I was wondering how well it would track with varying supply voltages.
Good job in your design.

Some time breadboard my latest circuit and see if it works also,
as alec t said (I think he was referring to my last posted circuit), that it may take a lot of light to bring Q2's base down through the photo trans.
If it does than you may have to put some emitter resistance on Q2, to raise its base voltage higher above ground.

 

Congratulations on a successful design on your circuit in post #44.
When you said you had to tweak R5, to get it behaving with the rest of the circuit, I was wondering how well it would track with varying supply voltages.
Good job in your design.

Some time breadboard my latest circuit and see if it works also,
as alec t said (I think he was referring to my last posted circuit), that it may take a lot of light to bring Q2's base down through the photo trans.
If it does than you may have to put some emitter resistance on Q2, to raise its base voltage higher above ground.

Further testing reveals that it breaks down above 9.8V. In low room light, L1 comes on and L2 stays on. So no congrats for me.

Will test your design for sure, thanks!

 

That's OK, you design a circuit to run properly within certain constraints that you set.

You designed it for 5v. so as long as you have a varying below and above that target value, with stability that's all that matters.

The only reason you check for varying supply voltages is only to accomodate for supply variations in a real environment.

Even, Commercial circuits only work within a certain envelope of varrying supply voltages.