Changing LED color on a flasher circuit

[fireman1333]

Hello all,
I was asked by a friend to find a simple flashing LED circuit for his project. I found a cheap kit on ebay, and here is the schematic for it. **broken link removed**
However, I want to change the LEDs to the SMD type, for their small size, and also change one LED to blue. I have the LEDs, but I was wondering what I needed to change in this circuit. I know I need to change 1 resistor because blue LEDs require more voltage, but I don't know which resistor. I also assume SMD LEDs draw less current, so that may be another issue I haven't considered.

Thanks for any help

Jeremy

 

[Audioguru]

Don't you think that when a transistor turns on then the current in the LED attached to it is determined by the voltage across the resistor in series? The resistor is R1 and R4.

You did not say your battery voltage. The kit says 3V to 15V. If you use a 9v battery then it is small so it will not last long.

You need to look on the datasheet for your blue LED to see its rated voltage and current. It might be 3.2V at 10mA. If you use a new 9V battery then the voltage across the transistor is about 0.1V, the voltage across the blue LED is about 3.2V then the voltage across R1 or R4 is 9V - (0.1V + 3.2V)= 5.7V./ Ohm's Law calculates R1 or R4 to be 5.7V/10mA= 570 ohms which is not a standard value, use 620 ohms.

 

[fireman1333]

Thanks.. Transistors always confused me, NPN, PNP, .. one is reversed as the other if I recall.. Anyway, yes- 9v battery will be used. I'll check the specs on the LEDs. Many Thanks for your help.

 

[c_mitra]

If you have a multimeter with a diode test function, you can determine the forward drop of the LED. Usually the diode test function is implemented at 5mA current and the LED will glow clearly. You can safely use 10mA forward current for a blue smd LED.