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?
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 ..