If you’ve been following along with the 555 timer series, this video describes how to derive the formula that describes the timing of the circuit. Starting with just 2 components, we’ll work our way to an answer.

If you’ve been following along with the 555 timer series, this video describes how to derive the formula that describes the timing of the circuit. Starting with just 2 components, we’ll work our way to an answer.

rittterJason,

I have a NE555P set up for 70Hz, powered by a 9v, 350mA wall jack. I want to sink it using a 2N3904 transistor. I am told you need a resistor off pin 3 to the transistor. I can’t find anywhere how to calculate the current discharging pin 3 to determine the resistor size. Objective is to pulse a coil which has a resistance of 125 ohms and an inductance of 134.5 mH at 70Hz.

Lab Seventried to run video on this page. does not exist error

Jason BairdSorry to just get around to this but the video had to be rerecorded. All is fixed now.

TristanHi Jason, 0.693 in the equation T1= 0.693 (Ra+Rb) C is part of the curve for a TC. My issue is surely the 0.693 is because of a log (LN 2). Because LN (2) is 0.693. Which is why the 0.693 is used. However how do you prove that the curve is following a natural log and therefore LN 2 ?

Jason BairdThanks for the comment. It’s been a long time since I’ve made this video, so my math is really rusty. In a way, natural logs are involved by using e in the formula. My interest in the shape of the line is more from the viewpoint of showing how a capacitor charges. Capacitors always charge in a natural log function shape. Regardess, LN(2) is in the formula presented when you consider e^(0.693) = 2 and LN (2) = 0.693. You are correct. I’m just more interested in trying to describe what physical processes are occurring that cause the number to arise in a formula on a datasheet. Have a great day!