Hi everyone, thought I would first introduce myself considering I am new to this forum. I am Chris and primarily program using Java & C++. I am a beginner to circuits and so far, I find it to be the most interesting subject I am pursuing. I have viewed this forum as a guest for a while, after finding it to be extremely useful, I decided to register myself.

I'l start by explaining what I am trying to do here. Basically I want to make an 18v DC circuit with an objective to light an LED and power a computer fan for 20 seconds, the time supposedly be controlled with C1 and R1, C1 being a 1uF capacitor, and R1 being a 20M ohm resistor. I will hopefully be constructing this using a small project back with a pole momentary switch(on/off) as well as a push button to start the timed fan and led.

Here is the schematic:

This schematic was found on a website known as Burnt Latke, I'm not sure if the overall schematic is just hard to follow or I just don't see whats happening. I would greatly appreciate if someone could give me any suggestions, advice, or possibly another way to design this.

 

Welcome to AAC!

There is a large variation of 555 types, but most use +15VDC as the max voltage, and 4.5VDC as a minimum. A CMOS 555 has a wider range, it will handle 18VDC for the max, and 2VDC for the minimum, but has extremely low drive that would make it unsuitable for something like this.

Your circuit is a bit overcomplicated. There is a simpler version that uses the power up of the Vcc line to trigger the 555. It will react exactly as the one you have.

Since it has a relay included the only thing you need to add is the LED circuit, between Vcc and the collector of the transistor. Don't forget the resistor.

The values from the other circuit would translate well, and use 0.1µF for C1 and C2. R2 could be 1200Ω with 15V, drop the voltage with less voltage. You could always use 9V for the circuit, and drop R2 to 470Ω. If the fan is 5V, then use 5V.

In general, it is always a good idea to add bypass caps across the circuit (555). You don't always need them, but it may improve performance.

Decoupling or Bypass Capacitors, Why?