Driving multiple Pi GPIO using Arduino GPIO

Question

I am trying to send TTL pulse from single Arduino digital pin to 8 different Raspberry Pi simultaneously. For this I have connected 8 raspberry pi in parallel to arduino digital pin.

Since, Arduino runs at 5V and raspberry Pi at 3.3V, I am using a switching diode (1N4148) connected to 3.3V pin on pi as shown in the figure for droppping voltage levels.

Now here is my question, when I check for output voltage at Arduino it shows 0.2V rather than 5V which means that arduino pin is not able to drive all the pi as they are drawing a lot of current. Also another thing, when I start removing one pi at a time from circuit, the arduino starts working perfectly for 4 pi. As far I can think of this is mostly an issue related to not enough current to drive all pi.

How can I make this ciruit work?? I read up online that a voltage buffer say something like LM110, LM741 can be used to solve this issue. Is this the right direction to work on? Can something else be done?

Answer 1

The diode circuit should work but not safe to parallel multiple loads to single drive. The voltage across the pi inputs will be diode drop plus 3.3 V (VCC of Pi). The current through the diode connected externally will be about 1.3 mA multiplied by 8 will give ~11 mA. Also, the input capacitance of all the Pi's inputs and 8 cables connected will make switching that current at high speed will get trouble with rise time and fall time too.

For your particular application, if you can buy one 74LVC245 buffer IC, please go ahead with below solution.

You can simply connect the Arduino single output to all the 8 inputs and derive 3.3 V level signals from the outputs. the advantage is that it is simple to use (Breadboard friendly) and easy to connect to.

The input voltage can be up to 5.5 V. Hence, it can accept the Arduino's input and drive the Pi's input easily.

Answer 2

The more classic diode level shift circuit is this one

^{simulate this circuit – Schematic created using CircuitLab}

The output of that can drive as many inputs as you like.

Answer 3

While your ad-hoc solution with diodes might work, you might as well use a proper voltage level converter:

^{simulate this circuit – Schematic created using CircuitLab}

^{(ground connection between Arduino and all RPi boards is assumed)}

Since you drive all 8 RPi boards with the same signal, you should convert the voltage level only once, then distribute that signal to all the RPi boards.