help a newb. need explanation of LED17 circuit.

242ATL · Post by **242ATL** » Sat Jun 17, 2006 4:05 am

Gang,

first post.

I have what I think is a very simple question concerning the LED17 circuit. I have a basic understanding of transistor circuits from school years ago, but I need a bit of help. I'm intending to use it (and LED19) for spark output to drive 2 vb921's. This is with a 2.2 board, MSnS-E and the error daughter card.

http://megasquirt.sourceforge.net/extra ... rd-d17.jpg

The Q9 transistor drives LED17. The "distributor only" and EDIS instructions call for adding a 1kohm resistor in parallel to LED17 and then taking the output from pin3 (the collector) of Q9 in order to use LED17 for spark output.

The toothed decoder instructions call for taking the spark output from the right hand side of R25. This is where my confusion starts.

Is the 1kohm resistor still necessary to use LED17 as spark output to drive a vb921? This isn't explicitly mentioned in the toothed decoder instructions.

Can someone help explain why we take the spark output from one side of Q9 in the distributor/EDIS variation and from the other side of Q9 to drive a vb921?

Again, I think this is a very simple question... I'm a little embarrassed... I think I'm looking at the circuit where LED17 is the "top" of the circuit, and Q9 is the bottom... so the toothed decoder instructions appear that we are taking our spark out put from "before" the transistor whereas the distributor/EDIS instructions appear to take the output from "after" the transistor.

your help in increasing my understanding of this circuit would be greatly appreciated.

regards,

Mike

JSchlappi82 · Post by **JSchlappi82** » Sat Jun 17, 2006 4:41 am

Not sure, bit isn't this the difference between connecting to ground or feed?

woh · Post by **woh** » Sat Jun 17, 2006 8:30 am

The signal after the transistor is inverted compared to before the transistor.

The output timing will be different depending on which one is used. The MegaTune settings may be different as well depending on the application and jumper settings.

Post by **jsmcortina** » Sat Jun 17, 2006 11:47 am

The VB921 circuit takes its input direct from the CPU, leaving the LED17 to do its original function. Be sure to follow the instructions with wiring and Megatune settings.

James

242ATL · Post by **242ATL** » Sat Jun 17, 2006 2:37 pm

[quote="jsmcortina"]The VB921 circuit takes its input direct from the CPU, leaving the LED17 to do its original function. Be sure to follow the instructions with wiring and Megatune settings.

James[/quote]

James,

thank you kindly!! Life is beautiful again... the birds sing, and the sky blue.

Thanks all!

Mike