Hi everyone.

I'm looking for a circuit for 3-step level indication similar to this, but with only one LED on at a time (like traffic lights).
And hopefully using discrete components.
TIA

 

The circuit you posted can have only one LED on at a time. It just depends on the signals M2-4.

How about drawing a block diagram of what you want instead of posting a "similar" circuit?

 

I don't see that.
When the level hits M2, Q1 turns on and stays on as the level increases. And so on.

 

When the level hits M2, Q1 turns on and stays on as the level increases. And so on.

It wasn't clear to me that the tube containing M1-4 was a container containing liquid. I interpreted it as a weird representation of a cable...

 

There is a way that is very similar to the circuit you have, but it requires careful LED selection to achieve approximately equal brightnesses.

Also, it uses a trick that works better with higher voltages. What is the value of Vcc?

ak

 

Equal brightness is not a factor. This is not the problem.
Vcc is anything I want.
Tks
Marty

 

Try adding two Schottky diodes as shown:

 

But what is to stop the first led from turning back on when D3 LED lights, and shunts the base of Q2, turning it off, resulting in the shunt comprised of D4 becoming inactive, allowing D1 to illuminate again?

 

Add another diode from the collector of Q3 to the base of Q1.

 

Try adding two Schottky diodes as shown:

Brilliant. but like Hamlet said, there is a bug.
How about 3 Schottkys?

 

Add another diode from the collector of Q3 to the base of Q1.

Exactly. Tks

 

Hi everyone.

I'm looking for a circuit for 3-step level indication similar to this, but with only one LED on at a time (like traffic lights).
And hopefully using discrete components.
TIA

View attachment 356763

Are you allowed to use microcontroller?

 

Add another diode from the collector of Q3 to the base of Q1.

Thanks. I'm going to file this away, useful.

 

Below is the circuit LTspice sim with the bug corrected:
I increased the value of the collector resistors to reduce the LED current below 10mA for the 5V source I used.

 

Try adding two Schottky diodes as shown:

Below is the circuit LTspice sim with the bug corrected:
I increased the value of the collector resistors to reduce the LED current below 10mA for the 5V source I used.

Right. The 3rd diode should do it. The next step is a bread-board test to determine resistor values.
Thanks for your help. Much appreciated.

 

Are you allowed to use microcontroller?

OP said he wanted to use discrete components in the original post...

And hopefully using discrete components.

 

Are you planning to build it? If so, you must be aware of an issue with this circuit.

In the early 1980s, I used to have a fiberglass shop, where I made water storage tanks which sold as pancakes in the drought-stricken city where I lived.
To differentiate my products from the competitors, and being an electronic engineer myself, I would include a similar water-level indicator that I had designed and built.
Even with a sensor current below 1 mA, the electrodes would corrode quite rapidly due to electrolysis.
To resolve this issue, I used mosfets instead of bipolar transistors, to reduce the sensing current to about 5 microamps, which would allow the electrode to last well over 2 years.

 

Thanks for the info.
I will be using aluminum electrodes in distilled water, so I didn't think this would be a problem.

 

Hello,

Please read this:
Despite its benefits, distilled water is particularly corrosive. With no minerals to give the water pH balance, distilled water acts like a magnet, absorbing chemicals (phthalates and bisphenols) from plastics, nickel from stainless steel, aluminum from aluminum containers, and carbon dioxide from the air.

https://www.corrosionpedia.com/definition/407/distilled-water

Bertus

 

Aluminum DOES degrade with electrolysis.