[Arm-netbook] help needed with a ttl logic circuit

Dominique Dumont domi.dumont at free.fr
Sun Aug 23 19:14:18 BST 2015

On Sunday 23 August 2015 15:52:27 Luke Kenneth Casson Leighton wrote:
> still don't feel completely happy (esp. with the schottky diode), i

I think it can be done with 3 resistors and 1 transistor used with a floating 
collector. Using a floating collector is better because CPU voltage can be 
lower than 3.3v.

I've attached a png file with a proposal. You may need to adjust the resistor 
values depending on the current drawn from the CPU.

When the input is floating, the transistor is on because of the 100k pull-up.

When the input is 3.3, the transistor is also on.

When the input is 0, the transistor emitter is driven to ~ 0.3V which should 
switch off the transistor.

When the transistor in switched on, in a saturated state, the voltage between 
collector and emitter is much lower than 0.6V, so the CPU will see a logical 
0. When the transistor is off, the output is high impedance. You may need a 
pull-up resistor on the CPU board.

Please take this with a grain of salt, it's been a while since I played with 
transistors this way (~ 25 years...)

Hope this helps.

 https://github.com/dod38fr/   -o- http://search.cpan.org/~ddumont/
http://ddumont.wordpress.com/  -o-   irc: dod at irc.debian.org
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