I have bought all the necessary parts and would start building the project shortly. However, I have some confusion on using BC557 in place of BS250 (unavailable here). Is the base resistor 1K as suggested or 100k? The latter value has been used in all other circuits that I have come across.
Anticipating your expert advice,
Asoke Kumar Bose.
It depends on the required current. You can use 10K, if your target has only the IC. If your target board required more current, you need to change the resistor or even transistor.