5V TTL to GPIO

[m3istr]

Hi guys,
I know that I can't connect a 5V TTL to the GPIO without a series resistor. But to calculate the resistor, do I need the input resistance of the GPIO. I can't find this specification in the datasheet of the RPi or the BCM2835. Can somebody tell me, wher I can find this spec. Thank you.

[pcmanbob]

You don't want a series resistor you need a potential divider , that is 2 resistors connected in series between the 5v TTL output and ground with the pi gpio connected between the 2 resistors.



This is an example of a potential divider



there are several sites that will help you select the resistors

https://www.electronics2000.co.uk/calc/ ... ulator.php

https://www.random-science-tools.com/el ... ivider.htm

[rpdom]

No, you can ignore the input resistance of the GPIO, it is so high it doesn't matter. What you need to do is use two resistors to form a divider to bring the voltage down to about 3V.

5v -[2K2]---GPIO---[3K3]---0V

That should provide sufficient voltage on the GPIO pin to register from the 5V input. Total current flow through the resistors will be about a little under 1mA maximum.
[edit]
pcmanbob did a better diagram than me
His resistor values will work too, with an even lower current.

[pcmanbob]

pcmanbob did a better diagram than me

only because in true Blue Peter fashion here's one a did earlier... showing my age now..

[rpdom]

only because in true Blue Peter fashion here's one a did earlier... showing my age now..

I still remember my 1967 Blue Peter Annual

[hippy]

I know that I can't connect a 5V TTL to the GPIO without a series resistor.

We have been told the Pi does not have the input pin characteristics which many microcontrollers have so current limiting an over-voltage input using an in-line resistor is not suitable for a Pi.

That is what we've been told so I am not going to pick a fight arguing otherwise.

[LTolledo]

A level shifter would be nicer...

level shifter.jpg (125.15 KiB) Viewed 1125 times

[mahjongg]

Bidirectional level shifters are only needed for bidirectional signals (like I2C, but only if the I2C peripheral device demands 5V levels, most don't, and work fine with the 3V3 levels of an RPI).
for most other uses the resistor divider is the simpler, thus preferred solution.

[boyoh]

I would be very much inclined to use a OPTO ISOLTOR to switch
The 5v TTL to the Pi 3.3v IN/PUT, and not use a divider
This will keep Bothe voltages separate, there will be no common
Connection, TTL 5v--------( R )-------( a-led k )----0v-TT
Pi 3.3v Rail--------------------------------( c T e )------3.3v( GPIO)----( 10k )---0v
R = 470r
a— led—k =.opto I/R
c T e = opto transistor
10kr Pulldown