• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Forcing a line down

Not open for further replies.


Well-Known Member
Most Helpful Member
I think I follow your description of the circuit, but if you show us how you have the res/LED/Transistor connected to the pin, with a simple diagram, that would help us.


Well-Known Member
Most Helpful Member
A tiny BCR148 transistor has a built-in 47k resistor in series with its base. For it to saturate with a 0.1V loss then the input to the resistor must be at least 47V or the resistor value must be reduced to 4.3k or less when powered from 5V.

Nigel Goodwin

Super Moderator
Most Helpful Member
Scarr said:
Hi all,

Thanks for the help so far

EricGibbs, I have attached a quick circuit (just hope I got it right!)

Nigel, Do you know of a small Ateml part of the top of your head that has a open drain pin? reason I ask is my compiler is for Atmel parts, I probably could port it but if we find the correct transistor might not be worth the hassle. and to trawl throught all the PDF's would take ages.
Sorry, I don't use Atmel processors - but they may well have an open-collector part?, they copied everything else from MicroChip :D

Auido Guru, your suggesting it's the transistor I'm using thats at fault and with my limited knowledge I can see why (however I originally thought it was due to limitations of the transistor) can you sugest a replacment?
Just use a normal NPN transistor, like a BC148, and add an external base feed resistor.
Not open for further replies.

Latest threads

EE World Online Articles