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October 16, 2010 / consort3

overclocking the 555 timer

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:

http://www.semiconductormuseum.com/Transistors/LectureHall/Camenzind/Camenzind_Page2.htm

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 233-3679)  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:

http://www.edn.com/design/analog/4348119/Modular-macromodeling-techniques-for-Spice-simulators

Useful article on a LED driver using a 555

http://www.edn.com/design/led/4363950/Power-an-LED-driver-using-off-the-shelf-components

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

general__http://www.planetanalog.com/author.asp?doc_id=564526&section_id=3356_simulating_a_555_timer_with_psoc_tm____an2286_12

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

http://www.nutsvolts.com/magazine/article/february2016_CSS555TimerICs

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

http://www.planetanalog.com/author.asp?doc_id=564526&section_id=3356

Battery operated sine generator article with missing circuit

http://www.electronicdesign.com/components/battery-powered-sine-generator-covers-100-hz-10-khz

fungen

Inspired by a couple of threads on EEVblog about function generators, I have come up with this design which is more cost effective than the hooky XR2206 clone. I was surprised how good the simple diode sine shaper was and have made it work with a 9v battery supply.
I used LT1057 op-amps to simulate the design rather than the simple ones shown here. The intention is to use TL072 devices. Also I would use the CMOS variety of the 555 timer. With a 250k pot in series with R1 the circuit will go from 700 to 20kHz. With a 82nF capacitor switched across C1 the generator will cover 20 to 700 Hz. The supply pins for the op-amps are not shown but go across the battery supply. The ICs need 0.1uf decoupling capacitors at their supply pins. U2 and U3 are combined in one dual opamp and U4 and U5 in another dual op-amp. U3 is a rail splitter.

The design relies on the rail splitter voltage being midway between the output limits of the 555. If the 555 output swing is not symmetrical between the battery rails, the duty cycle will not be 50%. Then it would be necessary to tweak the rail splitter voltage by changing the potential divider R4/R5. If you want a rail to rail output 555 the MIC1557 referred to at the beginning of this page fits the bill. The use of the MIC1557 means that R1 can be 1.5k and the pot in series can be 50k. C1 can now be 470nF for 20Hz to 680Hz, 15nF for 625Hz to 21kHz and 470pF for 20KHz to ~470kHz

fungen1
fungen2

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