555 component selector

This page tells you how to download and use the colour 555 timer component selector program

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Internet Explorer will respond with a dialog box. You can decide whether to save the program to disc and run it later, or to open it straight away. Either way, there may be a further dialog box, giving dire warnings about downloading software from the internet. (DOCTRONICS knows of no problems with the program, but no guarantee is implied.)

The 555 component selector can be copied and distributed freely, but remains copyright . Distribution for profit seems unlikely, but is expressly forbidden.

The program was written using Borland Delphi 2.0.

How to use the program

The program opens with the Astable tab selected:

555 component selector

Suppose you want to design a circuit to produce a frequency of approximately 1 kHz for an alarm application.

Generally, you should leave the R1 value as it is. 1 kW will give a duty cycle as close to 50% as possible.

The frequency produced by the default component values is 7.164 Hz. This is a much slower than the target frequency of 1 kHz. What should you do to increase the frequency of the astable?

Click on the C units radio button to change the C value from 1.0 F to 1.0 nF:

The pulse frequency changes immediately to 7164.169 Hz. This is faster than the frequency you want. Now try changing the C value to 10 nF by selecting from the drop down list:

The pulse frequency is now 716.418 Hz, much closer to the target value. You can get closer still by decreasing the R2 value:

With R1 = 1 kW, R2 = 68 kW and C  = 10 nF, the pulse frequency is 1051.095 Hz. This is as close to 1 kHz as you are going to get.

Remember about the tolerance of resistors and capacitors. You are likely to use resistors manufactured to 5% tolerance (gold-coloured tolerance band). Capacitor tolerance is typically 10%. If the components you use to build your astable are accurate to within 5 or 10%, you are not going to get an output frequency more accurate than this, unless you replace R2 with a variable resistor which you can adjust while measuring the output frequency.

Experiment with the values of R1, R2 and C, and note the effect on the pulse frequency. This is good way of understanding what the effect of component changes will be.

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