so im helping someone with a project. they have this heart thing: http://www.vellemanusa.com/products/view/?id=350676 and normally it blnks because of the two resistors and caps connected to the two NPNs that control two different sets of LEDs on the heart. he wants to control it with his Arduino. So i told him to take out the two caps(22uf) and resistors(33 and 100k) which make it blink and then connect the base of each NPN to a Arduino pin with a resistor. It normally runs off of a 9v battery so he shared the grounds between the arduino and heart and still used the 9v. so since its a NPN, its should activate when held HIGH? that didnt happen…when given +5v or 0v to the base pin(triple checked !); it would be on, but if he completely unplugged the base wire from the arduino, the LEDs would shut off. we used a 47 kohm resistor on the base pin, i now realize this is too high. it should be more like ~13K. would this cause the issues i describe?

First test is to take 10k resistors high by disconnecting from arduino and connecting them to +9v rail. This should activate LEDs so confirming that transistors are still working.
Assuming each transistor has to pump say 20ma and has a gain of at least 10, then you are looking for say 2ma of base current. With 5 volts from arduino and 0.7v junction loss, base resistors should be around 2k2. OK if transistor gain is higher, you can increase base drive resistance, but 2k2 will definitely get you going.

jackrae:
First test is to take 10k resistors high by disconnecting from arduino and connecting them to +9v rail. This should activate LEDs so confirming that transistors are still working.
Assuming each transistor has to pump say 20ma and has a gain of at least 10, then you are looking for say 2ma of base current. With 5 volts from arduino and 0.7v junction loss, base resistors should be around 2k2. OK if transistor gain is higher, you can increase base drive resistance, but 2k2 will definitely get you going.

the website said the whole thing draws 8ma. so each NPN pumps 4ma about. i was using the following equation: resistor=(VC*hFE)/(Load Voltage* Load current)
so VC = 4.25 (arduino 5v - voltage drop .75)
hFE for the NPN is 110
load voltage is 9
load current is 4ma or .004A

and some people say to do (Hfe/Load current)*2 just to be safe, so npn doesn't get to hot

so thats 4.25v*110/9v*.004A
then (110/.004A)*2 = 55000
so (4.25v/9v)*55000=25972ohm

so 26K or so? did i do the math right? if i didnt double (hfe/Load current) it would be 13k obviously.

is it supposed to be in amps or ma(.004A or 4ma) in the equation? if i do 4ma instead of .004A in the problem i get the resistor value in Kohms not Ohms; i got 12.9K and 25.9K. so the same thing...

sirbow2:
the website said the whole thing draws 8ma. so each NPN pumps 4ma about. i was using the following equation: resistor=(VC*hFE)/(Load Voltage* Load current)
so VC = 4.25 (arduino 5v - voltage drop .75)
hFE for the NPN is 110
load voltage is 9
load current is 4ma or .004A

and some people say to do (Hfe/Load current)*2 just to be safe, so npn doesn't get to hot

so thats 4.25v*110/9v*.004A
then (110/.004A)*2 = 55000
so (4.25v/9v)*55000=25972ohm

so 26K or so? did i do the math right? if i didnt double (hfe/Load current) it would be 13k obviously.

is it supposed to be in amps or ma(.004A or 4ma) in the equation? if i do 4ma instead of .004A in the problem i get the resistor value in Kohms not Ohms; i got 12.9K and 25.9K. so the same thing...

Two points:

Only one set of LEDs is on at a time. So the current through each NPN transistor is around 8mA, not 4mA.

You are looking to saturate the transistors, so you want much more than 2x the calculated base current. Try somewhere in between 5x the calculated base current (at minimum hfe) and one tenth of the required collector current (i.e. 0.8mA). So between 4k7 and 12K.