I made this circuit for my power led. It works, but there is one problem. The led fades down to the point where there isn't enough power to keep it on at all, then fades up again. The fade effect is smooth up untill this point. Is there any way to modify the circuit to prevent the led from turning off completely?
Would that be a blue power led? That circuit dips to about 2.85v, so a blue will be gone. You need to raise the virtual earth voltage, try increasing R2 to 56k. But that's a dodgy circuit, better one here. See also comments made on another forum inc. chopsuwe's comment on running a bloo.
actually it's not a blue it's a white led. But perhaps they act the same way? Because of my lack of knowledge concerning circuitry, I don't understand why this circuit would be "dodgy". Can you please elaborate?
Yes, white & blue both work at around 3.5v so this throbber circuit goes low enough to turn them fully off. I think "dodgy" because it looks like he's taken one of the 9v circuits on the net and put in a 100R resistor to run it off 12v. (a) that won't work, as the circuit current varies as the led brightness varies, so the resistor doesn't drop a constant voltage and (b) there's no need, the chip will run fine on 12v, just need to alter the led resistor. Most triangle-wave circuits use a pair of op-amps like this one, and with a single-rail supply make a "virtual earth" at half supply voltage with a pair of identical resistors (R1 & R2 on Rabid's). The triangle-wave then cycles from around 4v to 8v (on 12v supply), but you lose 0.7v on the transistor, so the led/resistor chain only sees 3.3v to 7.3v, OK for reds but a bit low at the bottom end for blue or white. Raising the virtual earth to 7v lets you cycle 5v to 9v, which should give a better effect. Increasing R2 is the crude (but easy) way, macroman's cathode-throbber does the level-shifting with an extra op-amp.
Does anyone know where to get those parts in the UK? I know where to get the resistors and capacitor but not the op-amp and the NPN amplifier.
The 1458 may have different letters in front & behind, but ESR, Rapid, and Maplin do it. Try MC1458CN. LM358N is a straight swap, usually same price and arguably a better choice. Any low-power NPN transistor will do, Maplin do MPS3904 which is the same beast as 2N3904.
If I wanted to use 4 or maybe 6 leds for this, would I be able to do so by adjusting vr1 or vr2? Will probably be using red leds...
3 reds in series (or 2 blue) will be the limit on 12v. At best the opamp will only put out 10.5v, less 0.7v at the transistor = 9.8v, less about 4v range to fade over = 5.8v. But you can use up to around 100 parallel strings of [led(s) + resistor]
cpemma, checked out your other circuits... You have some good ideas there . Thanks for your help. I did fix my circuit by changing some resistance values. The white led that it is driving doesn't turn off fully anymore... it looks great. It would be nice to be able to change pulse speed though, like in your new circuit. Oh well, this one is already finished.
