Single LED brightness adjustment

[HelpLess]

If you use a 555 to make pulse-width-modulation to adjust the brightness of the LED, then with it turned down so the LED is turned off the 555 consumes a fair amount of battery power, unless it is a Cmos 555 driving a transistor to boost its power.

Kchriste's circuit draws nearly no battery current when it is turned down.

OK then I will scrap the 555 and go with the original idea of using a pot. Thank you for the advice.

 

[HelpLess]

Is R2 supposed to be 220K? As soon as I put that resistor in, it's lights out

 

[audioguru]

If the pot is 1M then with it turned to max it forms a voltage divider with the 220k resistor and tries to make 3.6V at the base of T2. But the base of T2 is about only 1.85V when the LED is bright.
Make sure the pins on the transistors aren't connected backwards.

 

[kchriste]

Is R2 supposed to be 220K? As soon as I put that resistor in, it's lights out

As AudioGuru noted, R3 (The pot) has to be 1MegOhm if R2 is to be 220K. If R3 is 100K, then R2 should be 22K. Just make sure that R3 is apx 4.5 times the value of R2 for the circuit to work right. Don't eliminate R2 as it protects the transistors against excessive current when the pot is at max.

 

[teeb61]

re:
The only problem with a 9V battery is that it won't light your LED for very long. (About 1/20th the time as 3AAs). Here is a circuit that'll work from apx 3.5-5V. It's a little more complicated that the previous one but will give you better brightness over the life of the batteries. The max LED current is apx 20-26ma (At low currents Vbe will be 0.5-0.7V) and is set with R1.



I wasn't 100% clear with this circuit where the battery connections - + go?
https://www.electro-tech-online.com/attachments/led2-gif.10571/

Thx.