#### d123

##### Advanced Member level 5

Could some-one tell me if my maths reasoning is right or wrong here, please:

I want to turn on a BJT slightly after a logic output signal or a button is pressed. I'm assuming average button press by a human exceeds 2 milliseconds.

Maths theory (of mine to solve my problem) is following:

1 Tau = RC = ~63% Vsupply

5 Tau = 5RC = ~99% Vsupply

Vsupply is 3.9V to 6.6V

BJT base voltage turn-on is approx. 0.55V

R = 22k

C = e.g. 147nF

0.55V/3.9V = 0.141 Tau

0.55V/6.6V = 0.083 Tau

22k * 147nF = 1 Tau = 0.00323s

So, 5 Tau = 0.01617s

**0.01617s * 0.083 Tau = 0.00134s to reach 0.55V at 6.6 Vsupply**

0.01617s * 0.141 Tau = 0.00227s to reach 0.55V at 3.9 Vsupply

0.01617s * 0.141 Tau = 0.00227s to reach 0.55V at 3.9 Vsupply

Is that method for the maths correct or not?

As can be seen from the schematic, original idea of one RC on an NPN base to achieve my goal didn't work, so I resorted to the '

*randomly add more components and simulate over and over until it looks right*' approach... I have no idea how to work out the time constants for the C2+R3 and C3+R4 networks yet as I haven't had time to think properly about what's happening there yet.

Anyway, is the NPN turn-on delay calculation method correct or incorrect, please?

Thanks