Transistor selection

[Coyoteboy]

Hi,

I'm trying to monitor the 12v(13.Xv) outputs from a cars ECU. Obviously i
dont want to disturb the signals, but i do want to light two LEDs, one for
'output on' and one for 'output off'.
Id considered a whole logic circuit to do it but then thought - can i just
connect up a set of standard transistors - an npn (bc108) and a pnp
(BC214)to each ECU pin, and when the output is low one will light, when high
the other will?
Can anyone see a problem with this? How do i calculate the resistor needed
at the base leg on each, assuming they are running 20mA/3v leds.

Cheers
James

 

[scada]

"Coyoteboy" <coyoteboyuk@hotmail.com> wrote in message
news:cao6i1$5i8$1@sparta.btinternet.com...

Hi,

I'm trying to monitor the 12v(13.Xv) outputs from a cars ECU. Obviously i
dont want to disturb the signals, but i do want to light two LEDs, one for
'output on' and one for 'output off'.
Id considered a whole logic circuit to do it but then thought - can i just
connect up a set of standard transistors - an npn (bc108) and a pnp
(BC214)to each ECU pin, and when the output is low one will light, when
high
the other will?
Can anyone see a problem with this? How do i calculate the resistor needed
at the base leg on each, assuming they are running 20mA/3v leds.

Cheers
James

Not a good idea, unless your sure what the input / outputs are! Many of the
inputs are from sensors and are high impeadance, your circuit could load
that down and change the engine settings. You may not even notice it at
first, it may be a subtle change that will effect engine performance or
worse!

 

[Coyoteboy]

Not a good idea, unless your sure what the input / outputs are! Many of
the
inputs are from sensors and are high impeadance, your circuit could load
that down and change the engine settings. You may not even notice it at
first, it may be a subtle change that will effect engine performance or
worse!

After some more in depth investigation ive found the ECU actually grounds
these inputs - they are basically just switches BTW. But this makes the
whole LED lighting issue even harder. I am pretty sure I can whap more
current thru it - it operates a solenoid which probably has more resistance
variation in manufacture than that of an LED.

J