Electronic – Sinking and sourcing current

The '238 have only one of eight outputs logic high, sourcing current, the rest of the pins will be logic low, sinking current. The '138 is precisely the opposite, one of eight pins can be low, sinking current, the rest will be high, sourcing current.

To "invert" the function of the '138, you could use eight PNP transistors with the bases each tied to an output of the '138 with a resistor, the emitters all connected to +5 and the collectors each connected to one of your servo connectors. Or use a bunch of inverters (74HCT04 or octal 74HCT240) to change the sense of the outputs.

Your choice of 1 of 8 decoders will limit what you can do with your servos, as your circuit can only activate one output at a time. Max speed of any one servo will be limited by the number of servos that you want active. If you wanted all 64 channels on, for example, they would all be running at at average of 1/64th speed.

The PCF8575C is very handy for expanding digital IO using serial I2C protocol from devices like Arduino. You could run 64 servos using 2 pins on the Arduino and 4 PCF8575Cs. This would give you more flexibility in setting your PWM duty cycles.

You should put a resistor in series with each LED and eliminate R18. If you are just switching LEDs it's perhaps wasteful to construct a high-side switch- if you can switch the low side it only requires one NPN transistor per LED.

The high-side switch you show (top diagram) will work, however you can only switch a fairly small current due to the 10K base resistor. At 12V you'll get about 1mA base current so most transistor will be well saturated for up to ~20mA load current, but if you want to switch 100mA reliably you should reduce that resistor in most cases.

Also, it's good practice to add a resistor from base to emitter on the PNP. The reason is that the leakage in the NPN can be amplified by the PNP and result in excessive current at the output (particularly at high temperatures). Something like 20K to 100K will work fine (it's not critical). That said, you can guess that the gain of the PNP will be low at low base current and the circuit will typically work fine at moderate temperatures.