transistor current input from OPAMP

[flipper_md]

Hello all

I've built this motion detection circuit and it work fine,

**broken link removed**

but I want to add a status led hooked directly after the two diodes and I dunno how to calculate current there for a transistor and the correct resistor to get it saturated?..

all I know is that voltage is 3.2 or 0 volts...

Is there a way to physically measure these things or it has to be computed/simulated?


thanks[/img]

 

[IanP]

To drive an LED you need roughly 10mA (btw. you need a current limiting resistor in series with LED - see attached picture)) ..

An average NPN BJT has the gain of no less than 100, so:
10mA/100 =>01.mA – that’s the Base current ..

[3.2V – 0.65V]/0.1mA => 25.5kohm
0.65 represent the Ube voltage below which the transistor is “OFF” ..

To make sure that the transistor is going to be “saturated” – “ON”, choose a resistor below 25.5k, and anything between 10k and 20k would be a reasonable choice ..

IanP

 

[flipper_md]

thanks for the fast reply Ian!

I assume a 2N3904 would be fine?
where do the 0.65 Ube come from? is it the same as Vce on this spec sheet?
so I could recalculte using 1.0V...

**broken link removed**


no worry for the LED, I've burned enought of these to know they like to hang beside a resistor

 

[IanP]

V[ce] is the Collector-Emitter voltage ..

I'm talkig abot U[be]: Base-Emitter - look at the very bottom of the table that you provided in your post - U[be]sat ..

IanP

 

[flipper_md]

Oh right there in my face LOL

thanks!