# Cut current below potentiometer level

I have this simple schematic where 2 LEDs are meant to be dimmed and brightened alternatively with a pot. What I would like to achieve is for the middle position, both LEDs to be off and then on each side to start the gradient of one of the LEDs.

So middle, both off. Left LED gradualy getting brighter up to max when I get to the left most pot position. Vice versa for the right one.

Is there a simple electronic way to achieve this? I don't know where to start looking.

simulate this circuit – Schematic created using CircuitLab

• You could use a centre-tapped potentiometer and ground the centre tap. (US English: center.) Commented Apr 12, 2022 at 18:10
• @AndrewMorton Wouldn't you get large amounts of current in that case? Commented Apr 12, 2022 at 18:39
• @VoltageSpike If the LED part numbers are correct, they have an absolute maximum forward current of 30 mA, and 2.5 kΩ (without doing actual calculations) across the power rails won't pass a large current. Did I perhaps go too far in the direction of simple? Commented Apr 12, 2022 at 18:46

You could do something like this, but I suspect more satisfactory results would be achieved by feeding the pot into a microcontroller and PWM'ing two outputs with a resistor (3 parts plus a bypass capacitor and the LEDs).

simulate this circuit – Schematic created using CircuitLab

One disadvantage of this circuit is that it wastes 18mA * 12V = 216mW with both LEDs off, and even with one full on uses 22mA to get 8.3mA through one LED.

For your calculations, the effective minimum resistance is 500 ohms (R1*R2/(R1+R2) and the effective voltage is half of V1, so with a 12V supply we get 6V in series with 500 ohms, so about 8mA with a 2V LED Vf.

You could improve that by adding voltage followers to each side.

simulate this circuit

That circuit only draws 900uA with both LEDs off. You could also use your favorite dual rail-to-rail output op-amp to replace the 4 transistors.

• Thanks, I'll try my luck with the transistor version. How would the resistor values change for 5V input and a 250K pot? Commented Apr 13, 2022 at 8:49
• 250k is a bit high depending on your desired LED current. Also 5V is not enough with this circuit even with red LEDs (LED sees 1/2 of voltage - 1.4V in series with R3 maximum). There is a price to be paid for simplicity. Commented Apr 13, 2022 at 11:14

There isn't a simple way to do this, here is a template circuit for a way to do this. Another way might be to read the voltage values with a micro and then shut the LED's off, but at that point you might as well just read the voltage from a potentiometer and do PWM control.

simulate this circuit – Schematic created using CircuitLab

When done using discrete transistors, the general idea is to use some form of feedback to increase the effect of the potentiometer imbalance.

With one transistor per branch, the current limiting resistor provides not only current limiting but also negative feedback that "flattens out" the nonlinear wiper-to-current relationship.

The current below drives the identical LEDs at 10mA at either end of the range, decreasing to 1.5mA at 10% and 90% of the range. Idle consumption is around 0.1mA.

simulate this circuit – Schematic created using CircuitLab

The transfer curves of potentiometer position to LED current, for voltages 2.0, 2.2, 2.4, 2.6, 2.8 and 3.0V:

With four transistors we can use gain to:

• decrease idle current consumption by an order of magnitude,
• widen the useful intensity adjustment range of the potentiometer,
• make the circuit maintain useful brightness over the full discharge cycle of 2 AA batteries

simulate this circuit

The transfer curves of potentiometer position to LED current, for voltages 2.0, 2.2, 2.4, 2.6, 2.8 and 3.0V:

Replacing the idle current-setting resistor with a stabilized current source, we get a degree of independence from the battery voltage:

simulate this circuit

The transfer curves of potentiometer position to LED current, for voltages 2.0, 2.2, 2.4, 2.6, 2.8 and 3.0V:

As the supply voltage is further increased past 3V, the brightness increases only moderately.