Fun with 555 timers

[Neil Purling]

Has anybody got anything in mind that would be fun to knock up on a bread-board.
I have just made a basic oscillator so far. It has a wide frequency sweep.
Those huge capacitors are the only 0.01uf I had to hand being Soviet 0.01uf @ 700V PIO type
From looking at the trace on my scope it seems my hearing is good as far as 20Khz, as that's as far as I can hear the output.

Apparently a square-wave will get through the IF stages of a radio, even though the frequency is nowhere near IF of the radio.

I don't understand the maths behind changing the components to alter the duty cycle. It's 50-50 at the moment.


BTW: I need a potentiometer to reduce the output. The trace on the 'scope was 10V p-p.
Any ideas on a suitable value?

[kalee20]

You can't get a 50/50 duty cycle from a 555 with the circuit as drawn (the 'classic' 555 oscillator circuit). However, with the values you have, 1kΩ upper resistor and 1MΩ lower, you probably won't notice that it isn't quite 50:50.

The maths is quite simple:

Charge time constant = C x (R1 + R2); Discharge time constant = C x R2

where R1 and R2 are upper and lower resistors.

So, the ratio of mark:space is (R1+R2) : R2

And if you want mark = space, to give 50:50, then R1+R2 = R2, which is only possible if R1=0.

But if R1=0 then you'll burn out the poor little discharge transistor inside the 555, as well as 'crowbarring' the supply rail. So, it's not possible.

However, for your circuit (with the 1MΩ at maximum), mark:space = 1,001,000:1,000,000 which I make 50.02:49.98, so not surprising you can't tell from your 'scope!

[Mike Brett]

Hi Neil
Get yourself a copy of IC 555 Projects by E. A. Parr. Its only about £6 on ebay and there is about forty plus circuits to try.
Mike

[Aerodyne]

It is unlikely that you'll be hearing 20kHz, Neil. That's beyond human range. I think it more likely that you are hearing square wave harmonics.
-Tony

[Neil Purling]

I can, or at least I could hear the echo-location of hunting Noctule bats (20-25 Khz).
I am aware of E.A Parr's book & in fact I am seeing if the local library can lay hands on it.
I wanted to reduce the mark-space ratio so I have what is more like a series of pulses.
This is impossible?

How can a square wave penetrate the IF stages of a radio when the wave has a frequency far lower than 455 or whatever the IF is? Or have I got the wrong idea?

[paulsherwin]

The square wave signal generates harmonics which reach several MHz. You can do the same thing with a two transistor multivibrator. Signal injectors based on square wave generators were common beginners' projects in the 60s with the hobby mags featuring them frequently.

The harmonics will be multiples of the fundamental frequency, so I don't think Tony's explanation for the sound you're hearing is correct. Some people can indeed hear 20kHz, though it's very unusual in anybody over 30.

[Herald1360]

Originally Posted by Neil Purling

I wanted to reduce the mark-space ratio so I have what is more like a series of pulses.
This is impossible?

No. Just connect the load between output and rail instead of output and 0V and your marks will become spaces

[Neil Purling]

See post #2. Mark/Space ratio is near as dammit 50/50. What difference will the suggestion in post #7 make? Am I missing something here?

[Red to black]

There are several excellent books on the 555 and it's Dual brother the 556, I had at one time the excellent '555 timer cook book' by walter Jung, and this shows how to make a true 50:50 M/S output Astable (Kalee is correct you cannot get 'true' 50:50 output from the basic circuit).

There is huge amount of information on the net for free, on using this device, which IMO was one of the most versatile of IC's, excluding opamps.

If you prefer paper books, here is a short list, apologies if I have repeated any already suggested.

IC Timer Cookbook; 2nd Ed; Walter G Jung; Sams Publishing; 384 pages; 1983; ISBN 978-0-672-21932-0.
IC 555 Projects; E.A. Parr; Bernard Babani Publishing; 144 pages; 1978; ISBN 978-0-85934-047-2.
555 Timer Applications Sourcebook with Experiments; Howard M Berlin; Sams Publishing; 158 pages; 1979; ISBN 978-0-672-21538-4.
Timer, Op Amp, and Optoelectronic Circuits and Projects; Forrest M Mims III; Master Publishing; 128 pages; 2004; ISBN 978-0-945053-29-3.
Engineer's Mini-Notebook – 555 Timer IC Circuits; Forrest M Mims III; Radio Shack; 32 pages; 1989; ASIN B000MN54A6.

The above list is C/O wikipedia which has a page dedicated to the 555 IC.

[gezza123]

Tandy made one 1978.,I have got this one it only uses one transistor,two electrolitics, two resistors, one diode, one disc input cap, and an I.F. tuneable can, plus led.
Works a treat for fault finding,and is very rich in harmonics.
Note never use them on ac/live chassis.Gezza

[Herald1360]

Originally Posted by Neil Purling

What difference will the suggestion in post #7 make? Am I missing something here?

The mark:space ratio (per post 2) is (R1+R2)/R2 which as was pointed out can only be >1:1 (50% duty cycle). In order to get narrow pulses you need<<1:1 or <<50%.

Assuming normal "mark" is asserted when the output is high wrt 0V, in order for it to be the narrow part of the waveform, you need to reference to rail instead. The pulses will be negative going, but a floating transducer (loudspeaker or whatever) won't care about that. If the pulses have to be positive going, use an inverter on the output instead.

[Neil Purling]

Looking at the image of the 'scope trace: It is unlike that of the image in post #11.
I'll try the circuit again & display output reference to both 0V and rail.

[Neil Purling]

Any more ideas for the resistance value for a output level control?

[dim-bulb]

If your e-mail can take a 3 megabyte file, send me your address. I have a small book on 555's in pdf format I will mail you if you're interested. it has a bunch of basic circuits in it.

Bob

[Aerodyne]

The harmonics will be multiples of the fundamental frequency, so I don't think Tony's explanation for the sound you're hearing is correct. Some people can indeed hear 20kHz, though it's very unusual in anybody over 30.

Quite right, Paul. Rather illogically I was thinking of sub-harmonics, which would not be present in a square wave. I still find it hard to believe anyone can hear 20kHz, however, being partially deaf myself - 5kHz is tricky for me.
Neil: If you are feeding a small loudspeaker, fit an output capacitor from pin 3 - positive to pin -of around 1 - 100uF, followed by a 1k-10k pot to ground and tap the level off the slider to the LS.
That should work.
-Tony

[Neil Purling]

dim-bulb: You have a PM

Aerodyne:Thanks for the info on the additions I needed to make. I was using an output capacitor anyway. 220uf actually. But as I was only messing about with a bread-board assembly substituting components is a piece of cake.

[Meat Pye]

Try this site for a pdf on the 555 timer:-

http://www.talkingelectronics.com/te...ive_index.html

There's a couple of pdfs on transistor circuits that are worth a read as well as one on leds.

Despite having been discontinued there is still a good supply of 555s to be found out there. I got 10 555s along with 10 DIL sockets for £2 a couple of weeks ago.

[kalee20]

Discontinued? The 555 DISCONTINUED? When?

I routinely use National Semiconductor's version, LM555CN (now Texas), as I tend not to trust analogue CMOS and prefer things less prone to ESD. No supply problems -unless somebody know something that I don't!

[llama]

For a true* square-wave, probably the simplest way is to follow your 555 with a divide-by-2 flip-flop CMOS chip (D-type should be suitable). Double the 555 freq of course.

*The rise-time and fall-time could be marginally different but that's in the nanosecond region.
Graham

[Red to black]

For a true* square-wave, probably the simplest way is to follow your 555 with a divide-by-2 flip-flop CMOS chip (D-type should be suitable). Double the 555 freq of course.

Whilst this is one way, the circuit I saw used steering diodes to achieve this, and this was a bit simpler than driving a bistable.

There may be a circuit on the net.