I want Vout to be logic 0 when Vin is logic 1 (+5V), and Vout to be high impedance otherwise. I designed the circuit like an open collector switch. But how can I design R1 value ? Usually in transistor switches, I calculate the base resistance value by calculating Ic(sat), putting Vce = 0V in the collector circuit. Then I divide it by *Beta* to find the minimum base current needed to switch on the transistor. Here there is no collector resistance or collector current. So I can't figure out what should be the minimum value of base current to turn on the transistor.

![schematic](https://i.stack.imgur.com/oXJya.png)

[simulate this circuit](/plugins/schematics?image=http%3a%2f%2fi.stack.imgur.com%2foXJya.png) – Schematic created using [CircuitLab](https://www.circuitlab.com/)