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October 16, 2010 / consort3

overclocking the 555 timer

Updated 10 May 2016

I got 9.4Mhz clock out of the Micrel MIC1557 version of the ubiquitous 555. This was with 5V supply,  no timing capacitor (relying on the internal capacitance of the device) and a 330 ohm timing resistor. This indicates that the internal delay of the device is about 55nS. Not bad, considering it is a low power design. The device is in a SOT23-5  surface mount package and needs a 0.1uF supply decoupling capacitor.

The table shows the frequencies I got with 10pF 1Kohm timing components The frequency at 5V was fairly stable from 4.6 to 6.8V.

Supply volts Frequency MHz
2 2.7
3.3 6.58
5 7.5

The 555 was designed by Hans Camenzind Fascinating interview with him here:

The output of a 555 is a sawtooth but here is a circuit of a sine wave oscillator using the 555:

The inductor has properties of 64 ohms resistance. I initially developed it using the ecircuitcenter model of the 555 then built it and finally was gratified to see it work on the LTspice model of the 555. However the real frequency was 69kHz and this model give 79kHz. Below are the waveshapes you get at the junction of  the threshold and trigger pins and the output. Unfortunately the output square wave is not symetrical. I got better results with the above circuit if I made R3 680 ohms as the output did not overshoot so far negatively. With a 15mH inductor (Farnell 186-4093)  and 27nF capacitors it should do 10kHz. To get the frequency prediction more accurate in the model you could add the parallel capacitor given by the inductors self resonant frequency.

Interesting article about modelling the 555 and others:

Useful article on a LED driver using a 555

Amusing article on emulating a 555 with a PSoC. The ground on Pin 1 appears to be the only problem


There’s life in the old 555 yet. An ingenious micropower 555 with programmable divider to extend the timing range.

This analysis of a 555 based temperature controller shows some interesting Spice techniques. Personally I would isolate the control circuit with a transformer and opto-triac


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