Variable frequency 555 circuit

J

jonj44

New Member
Hello all,

I seem to recall having a schematic for one roughly 20 years ago, but there have been several hard drive crashes and at least one plumbing "flood" since then.. and I can't seem to re-locate it online.

My goal is to externally trigger square wave pulses (of variable frequency say from 20-22000 Hz) with a 555 timer circuit using a function generator/laptop/tablet/etc. Most of the schematics I have found show 555 pins TRIG 2 & 6 joined together for astable square waves.

Is this an internal IC connection or just a common way of externally wiring the 555 for astable mode?

Ideally, I would like to keep my output voltage fairly close to the maximum 15 VDC input (and I'm wondering how duty cycle would impact this goal). Also, I am looking to software control a 555 square wave oscillator, not "set" a hardware output frequency.
 
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Looking at the DSP freq generator board, it does look better suited to my goals. Unfortunately, looking at the photos, I'm seeing USB cables, so I'm guessing output voltage in the 3.7-5 VDC range.

I was looking for selectable (ideally sweepable) frequency square wave pulses in the 12-15 VDC range, hence a 555 approach. Perhaps the DSP board could be used as an external trigger source somehow?
 
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Changing the output signal level would not be a problem. You need to tell us what your idea of accurate is. (As a percentage or parts per million.) Do you want to control the frequency with a variable voltage or do you want to set the frequency with a digital signal of some sort ? Giving us details of exactly what the purpose of the signal is would be a great help to save us trying to imagine what you are trying to do.

Les.
 
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Hi,

There is not much problem with the output level, you just amplify it, and if it is just a square wave you can probably use one small transistor to boost the voltage up to any desired level. As a sarcastic example, you can drive a 555 with it and use the output from the 555.

The frequency gen board is capable of doing a frequency sweep. You just keep sending it digital words that change as needed. For example, let's say the codes were just 4 bits. You might send it binary 0001 then binary 0010 then binary 0011 (that's decimal 1, 2, 3), like that, only with 32 bits. You may not even need that high resolution so you may be able to do binary 0010, then binary 0100, then binary 0110 (that's decimal 2, 4, 6). You do that as fast as you feel necessary.

There are two basic modes of operation, series, and parallel. If you need to change the frequency as fast as possible you would use the parallel mode. That lets you set all 32 bits in one transfer. The serial mode requires sending 1 bit at a time until you've sent all 32 bits, so that would be slower, but it could still be fast enough for your purpose.

You still have not specified what you need as far as stability, but there is no way a 555 is going to be as stable as something crystal controlled. For one thing, the 555 uses an RC network to generate the timing and that means temperature drift. Because you have not yet specified what exactly you need though we don't know if that would be a problem or not. Since you want to sweep the frequency that may not be an issue.

There is also the problem of nonlinearity when using the control input, which I assume you would use to modulate the frequency. The frequency does not change in proportion to the control voltage. That means you cannot supply a ramp signal to sweep the frequency unless you don't mind that nonlinearity, or you can stay within a certain range to obtain quasi-linear operation. That also means you might have to do a little calibration between what voltage you use for what frequencies.

You should study the frequency gen board in detail if you would like to use it or just try it to see if it meets your needs. You do need to control it with a microcontroller or similar.
 
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I guess you need to step into your time machine, go back 70 years, and pick up some air variable capacitors because they don't make them anymore. You might be able to salvage one from an electronic scrap pile. The only other alternatives might be varactor diodes or switched capacitor banks.
 
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Variable air caps for a 20-22000 Hz solution, I must be missing something ?
 
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Last edited:
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AnalogKid said:
Digi-Key has over 1700 part numbers in their system, with over 340 part numbers in stock, spanning 0.25 pF to 600 pF.

600 pF and 60 megohm gets you 20 Hz.


ak
Click to expand...
Curious, at 60 Mohm does one have to be concerned with offset due to leakage and part no longer
triggering ? Nothing in datasheet seems to discuss range of allowed timing components.

Chatgpt gave me this :



Old Signetics ap note seems to indicate there are limitations (bipolar part) :



Regards, Dana.
 
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danadak said:
Variable air caps for a 20-22000 Hz solution, I must be missing something ?
Click to expand...

Many audio frequency generators used air spaced capacitors, but didn't cover the audio band in a single range. Another popular option was a stereo potentiometer in a wein bridge circuit, but again, not in a single range.
 
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