# How do I connect this 3.3V system and 6V ish system together?

I have an old 'Cybot' robot I built from a kit. I'm trying to mount a Raspberry Pi on it to act as brains. I'm a computer programmer, but my electronics knowledge is very limited. Have access to basic tools like multimeter etc.

So far I've disconnected the bit I'm pretty sure is the motor driver board from the rest of the Cybot internals and I'm wondering if I can just wire it directly into the GPIO pins on the pi?

Schematics for the motor driver board I'm trying to connect are here: http://lpilsley.co.uk/cybot/driver.htm

I'm thinking the driver board has transistors so the GPIO pin isn't connected directly. Is 3.3V enough to trigger the transistor? Do I need to connect the grounds of the systems together?

It looks like it should work fine at 3.3V. Each connection will draw about 3 mA of current from its RPi output. Yes, you need to connect the grounds together as well.

Gotta love the low-battery indicator circuit — it just wastes power until the battery is low enough to allow the LED to light up!

The Motor Board's 7 pin connector on the left (of the schematic) is where you would connect. 1 through 4 connect to the motor drive transistors, and 7 is the ground.

Q3 through Q6 are fairly standard small signal transistors, S8050, with a 1KΩ base resistor. Considering the microcontroller is expected to run from the 7805's 5v out, I'd assume the transistors are set up for that (using Ohm's Law, I = V/R)

5v - 0.7v (Transistor's Base to Emitter voltage drop) = 4.3v
4.3v / 1000Ω = 0.004A or 4mA.

Since you would drive them with 3.3v GPIO, it's 3.3v - 0.7v = 2.6v.
2.6v / 1000Ω = 0.0026A or 2.6mA

You would be driving them with a smaller current. BUT those small transistors are used to drive bigger 2SC3279 NPN and 2SA1300 PNP transistors (2~3 Amp).

You will see minimal difference in application. You could always replace r28 - r31 with 620Ω or 680Ω resistors to make them ~4mA at 3.3v.

In short, yes, you can connect them directly. A transistor's base voltage does not need to be the same as it's collector/emitter voltage, allowing a 3.3v signal to drive a 6v or higher source.