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Portable Low Frequency square wave generator circuit - Solved - page 3

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soldar:
I would appreciate a little more politeness and less rudeness. I make a habit of trying to ignore rudeness because I prefer not to escalate.

I have been working with 555 timers since before most people on this forum were born.  In 1977 I was already using 555 timers and bought The IC Timer Cookbook (pdf) which I still have on my shelf.  Around 1980 I was running a small home business with a friend building and selling timers for photography enlargers. A lot of people were developing their own photos at home and needed a timer for the enlarger. We sold maybe a thousand or two. I still have the circuit and PCB. Two scales: up to 60 seconds and up to 3 minutes.

The LM555 (pdf) is bipolar and the mother of all later CMOS.

Zero999:

--- Quote from: spec on January 31, 2019, 02:32:32 pm ---
--- Quote from: Zero999 on January 31, 2019, 11:43:49 am ---The original poster wanted 12V peak to peak, not 24V, but that's an easy mistake to make, since it wasn't clear and it's trivial to reduce the supply voltage to +/-6V.
--- End quote ---
No mistake. The output impedance is 50R forward terminated and the output voltage is +-6V8, as the OP informed me that he wants a little over 6V. The output voltage can be adjusted to any voltage from 2V to 12V by changing the two zener diodes at the output.

--- End quote ---
The original poster didn't actually say 12V peak to peak into a 50 Ohm load. They said 12V peak to peak with an output impedance of 50 Ohm. It's understandable you interpreted it as 12Vpp into a 50Ohm load, with an output impedance of 50Ohm. Indeed you could be right, as the original poster wasn't very clear.


--- Quote from: spec on January 31, 2019, 03:40:43 pm ---What on earth is all this mark to space ratio stuff all about? The circuit as presented will meet the OPs requirement and that is it.
There is no more to say.  By the way the 555 is a CMOS type.

--- End quote ---
Except the mark space ratio will not be 50% at 1kHz, because the value of R1 is so low the on resistances of the 7555's internal MOSFETs will skew the duty cycle considerably. The output stage inside the 7555 is not perfect. The output transistors have asymmetrical on resistances. The high side transistor's resistance is between 1k and 150R and low side transistor's resistance between 143Ohm and 16Ohm, depending on the supply voltage to the 7555. The higher the supply voltage, the lower the on resistances.

I worked out the resistances, using the figures given on the graphs on pages 6 and 7 of the datasheet.
https://www.nxp.com/docs/en/data-sheet/ICM7555.pdf

The on resistances with a supply voltage of 12V are not specified. Suppose high side is 200R and low side 30R. The capacitor will be changing via R1+200R = 1k2, which will take longer than discharging via 30R+R1 = 1k03, so the duty cycle won't be 50%.

I understand you felt personally attacked when I made this assertion but it was not my intention. It is something which is easily overlooked. Please build a 7555 oscillator, with 50% duty cycle configuration and a 1k timing resistor and you'll see what I mean. Note how the duty cycle is nearer to 50%, when the power supply voltage is 18V, than it is at 2V, because the on resistances of the output transistors are lower and contribute less to the timing.

A much higher value for R1, say 10k, could be selected to make the duty cycle much nearer to 50% and use a much smaller capacitor, but then the potentiometer would need to be 20M, to get down to 1Hz and the current so low it would be prone to noise, hence why I suggested different ranges.

iMo:
This is probably the simplest wiring which could work.
It gives you aprox +/-6V at the output, you may set the frequency and 50% duty by the 2 pots.
You need a switch for frequency range selection.
You may play with the values, I think 50k lin pot in freq and duty is ok, the values selection for the capacitors - the calculation is left to the readers..
You need the NE555 (not the CMOS) when required 50ohm output.
C1-C4 should be foil capacitors (for highest stability use quality one, ie. WIMA for 63V).
The C7 is an elyt 100uF/25V.
No warranties of any kind, use at your own risk :)

soldar:
Zero999, the OP is kind of confusing and contradictory even but I get the idea that this is to simulate some speed signal sent by some transducer on a train and I feel quite safe assuming there is quite some latitude in voltage, duty cycle, etc.

imo, that looks good.

iMo:
I do not know how is it with trains in UK, but the trains in EU are mostly fully controlled by computers. Last summer we were standing in the middle of nowhere for 3 hours because a faulty "door sensor" in one of carriages sent an "open door" signal. And the loco driver did anything with that as the computer said "no-go". So I can hardly imagine you can fool an locomotive such easily :)

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