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 Forum index » DIY Hardware and Software » Developers' Corner
Ultra-Simple H.F. Test Oscillator
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Tony Deff



Joined: May 25, 2008
Posts: 42
Location: Suffolk, UK

PostPosted: Sun Aug 19, 2012 8:23 am    Post subject: Ultra-Simple H.F. Test Oscillator Reply with quote  Mark this post and the followings unread

(This posting is undergoing major editing)

You (more than) sometimes find that the timing of your 7555 is not quite as you'd expected, and by more than a trifle.
Frequently, you are told that this because of your component tolerances, yet you may look at your 2% resistor and 10% capacitor and wonder. What you are seldom told is that the 555 has its tolerances too, especially in its built-in resistor chain.

Resistors inside semiconductor networks are notoriously imprecise, but they are supposed to match each other closely, and obtaining a ratio of 2/3 supply at pin 5 is what it's all about. In National Semiconductor's original LM555, this is formed by a chain of three nominal 5K resistors (in the 7555, these are 100K and in the TS555, 200K).

The LM555's spec. sheet details the accuracy of these resistors in a curiously indirect way: it details the Control Voltage Level (Electrical Characteristics) min., typ. and max. for a supply of 15V and 5V. For the LM555C (Commercial variant) at 15V, this is 9,10 or 11V, i.e. ±10%.

Because of the exponential function, a 10% deviation in the target voltage is distorted by much more in the time domain.

If you look at Graph 1, a target reduction from Y=66.7% (red line) to 60% (a 10% reduction) reduces the time from X=1.1 CR (blue line) to about 0.9 CR — a reduction of some 18%.

A target increase of 20% (from 66.7% to 80%) increases the time from 1.1 CR (Time Constants) to 1.6 CR — an increase of some 40%!

Other tolerances that affect the timing are the Threshold input current (negligible in the CMOS versions) and leakage into pin 7.

A way of increasing the time period up to double thus presents itself by simply strapping pin 5 with a resistor to supply, increasing the threshold voltage to which the capacitor must charge.


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The non-linear relationship between pin 5 Control Voltage and Time.
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