You might considering ditching the transistors altogether. Having pin 3 alternate between 5V and GND (0V) in-between the two LEDs should cause them to alternate (which is what I think you're doing here). Anyway, about LEDs.... they all have a specific forward voltage drop (well this is theory) so anything greater than that will cause them to burn up, so we use current limiting resistors in series. You can size these resistors by the following:

(max Voltage in - Forward voltage) / max LED current = resistor value. So a resistor for 1 LED will not serve 4 LEDs properly.

 

You might considering ditching the transistors altogether. Having pin 3 alternate between 5V and GND (0V) in-between the two LEDs should cause them to alternate (which is what I think you're doing here). Anyway, about LEDs.... they all have a specific forward voltage drop (well this is theory) so anything greater than that will cause them to burn up, so we use current limiting resistors in series. You can size these resistors by the following:

(max Voltage in - Forward voltage) / max LED current = resistor value. So a resistor for 1 LED will not serve 4 LEDs properly.

That's not the reason that you shouldn't put LEDs in parallel with just a single resistor in series with the group of LEDs. It would be a simple matter to reduce the size of the resistor by a factor equal to the number of LEDs. The problem is that LEDs, like many semiconductor devices, have negative temperature coefficients and thus are subject to "thermal runaway" -- which ever LED has the most current in it (just due to tiny mismatches) will operate at a higher temperature than the others (because it's dissipating more power) and so it wants to pull even more current at the common operating voltage (since they ARE in parallel), and so it heats up even more. This will continue, in most cases, until one LED is hogging nearly all of the current. Depending on the specifics of the circuit, that LED may fail fairly quickly and then another LED goes through the same process. The result is a cascading failure of devices, possibly over a very short period of time. Hence each device should have at least some minimal level of individual current limiting.

 

That's not the reason that you shouldn't put LEDs in parallel with just a single resistor in series with the group of LEDs. It would be a simple matter to reduce the size of the resistor by a factor equal to the number of LEDs. The problem is that LEDs, like many semiconductor devices, have negative temperature coefficients and thus are subject to "thermal runaway" -- which ever LED has the most current in it (just due to tiny mismatches) will operate at a higher temperature than the others (because it's dissipating more power) and so it wants to pull even more current at the common operating voltage (since they ARE in parallel), and so it heats up even more. This will continue, in most cases, until one LED is hogging nearly all of the current. Depending on the specifics of the circuit, that LED may fail fairly quickly and then another LED goes through the same process. The result is a cascading failure of devices, possibly over a very short period of time. Hence each device should have at least some minimal level of individual current limiting.

I agree, but I never implied that he should put LEDs in parallel, in fact I specifically say to use limiting resistors in series. What I meant was he can't use a single resistor sized for 1 LED and use the same resistor to limit current properly to 4 LEDs in any configuration, which is what I took from the OP comments. LEDs used in parallel should each have their own resistor in series.

 

Ah, then we are one the same page.

 

Thaaanks ronv! that worked

 

Thanks! My main problem is trying to power 5 LED's (3.3 - 3.8 FW voltage
20 mAh FW current) using four AA NiMh Rechargeable batteries (1.2 V - 2500 mAh).
Is this even possible?

The four cells in series total 4.8 volts. This is more than the 3.3V of the LED. All you need to do is put an 82 ohm (or use the more common 100 ohm) resistor in series with each LED. Then connect this LED and resistor across the AA cells. You will have 5 separate pairs of LEDs and resistors connected across the AA cells.

 

Thanks acmefixer I'll test it out :]

 

The four cells in series total 4.8 volts. This is more than the 3.3V of the LED. All you need to do is put an 82 ohm (or use the more common 100 ohm) resistor in series with each LED. Then connect this LED and resistor across the AA cells. You will have 5 separate pairs of LEDs and resistors connected across the AA cells.

And how does this accomplish the flashing that the OP appears to want?

 

Pulsing or throbbing LEDs:
Not prettey but it works. Period set for 8 sec, R1 is a 30k multi turn pot, R2- 20k pot. R3 biases the MPSA14, darlington NPN, so that LED does not go completely dim. R5 represents 3 more LEDs for total load of 80 mA. Output falls off when R5= 18Ω, or total of 180 mA. Considerable interaction between period & R2- 1000 uF

 

Thanks Bernard! This is very helpful

 

Could this be done with 3.6V and 5 LED's or is that voltage too close to the 3.3 (typical) of the LED's?

 

LED specs:
3.3 - 3.8 FW voltage
20 mAh forward current

They will probably still light, but may not be very bright.
You should still protect them with a small resistor - say 15 ohms against the one that is 3.3 volts.

 

Thanks ronv! I'll test it out

 

OP, jenovauh [ this page] found a nice Boost- Buck convertor on net, 3V in 5V out @ 300mA. for just a few US $.

 

I missed that the transistors were not on the output of the 555. But they are now.

Exactly the mistake I made.