Choosing right values of resistor for transistor as switch

Question

I've built something like this

for using MCU pins as a joystick controller for a Sega Master System (and the pins are driven from a USB HID device connected to the MCU, but that doesn't matter).

The question is: what is the lower current to drive T1 (I'm using a C1815 because I had this one but I can change it)?

Is it better to keep current as low as possible?

Do I need R2 at all (the standard button seems to have 0-20 Ω resistance)?

The posted solution is working fine as is; I just want to know if I can make it better.

edit: MCU side is 3.3V, max pin current is 16mA Errata: R1 is 20k not 20M

Answer 1

• assume internal pullup is 10k to 5V = 0.5 mA
• assume switch gain = 10 to 20 MAX for Ic/Ib when saturated as a switch or 10% of linear hFE Max
• choose Re=0
• compute Rb = 10% of 0.5 mA = \$ I_B= 50 uA => R_B= \dfrac{3.3~V - 0.6~V_{=V_{be}}}{0.05 ~mA }=54 ~k\Omega \$ will probably work but with capacitance on cable, use 1k to 10k

Answer 2

R2 can be 0 ohms so removed and replaced with a short circuit.

R1 is at least 1000 times too large and can be one 10000th in value. Just use something like 2k to 10k.