I just read your blog and found it very helpful – thank you.
I am working on a new toy project and have a problem I think you can help with.
I am turning an LED (see attached) on and off from a small taiwanese MCU (like a PIC chip). I am using a PNP transistor for this.
My application needs constant current to the LED from 3xAA batteries down to about 3.6v-3.8v. Since it’s a toy, the circuit has to be very very cheap! So far I have only managed ‘constant’ to be about a 4% drop, but I need to do better.
I need a circuit which includes the PNP from the I/O pin of th MCU. Help!
Mail from AB USA in 2006
I am trying to understand your need. All you need is a PNP transistor driving a LED. A series FET constant current element has to be added.
The circuits below can show parts of the circuits. If you need something special, i can design for you.
In Lecture 1 we introduced the idea of a current source, and mentioned that these were slightly more difficult to implement than the perhaps more familiar voltage source.
With Constant Duty Cycle and, Constant Current Drive. The circuit is composed of an oscillator, a pulse slicer, and a gated constant current source. All of the circuits except the anode of the LED receives power from an LM78L05 5 volt regulator.
delabs – 2006