# 555 tone generator

Discussion in 'General Electronics Chat' started by ssherwood, Dec 12, 2008.

1. ### ssherwood Thread Starter New Member

Aug 25, 2008
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0
I am trying to build a tone generator, and have been changing caps and resist. in the circuit to change Hz.
But when I get near my target of 1400 Hz the output distorts and jumps around on the oscilloscope. The closest I get with a good signal is 1370 or so by changing the 10k to about 2k. If I move my POT even more, it distorts, then will clear up again at around 1530 Hz (sort of clear). Any ideas?

Sorry, I am new at this, and the mathematics didn't work out for me...if anyone could help with my issue and/or a formula for finding a specific Hz. it would be greatly appreciated. Thanks so much to you Gurus who take your time to help us newbies out. I also posted this as a reply to thread titled tone generator, but thought it best to post a new thread. Circuit is at http://hop.concord.org/s1/core/s1ca1.html#top

Added info: Thanx to SgtWookie for the link to the 555 design pro--I will construct my circuit with the new values and see if it works for my Hz requirements. http://www.schematica.com/555_Timer_design/555_Timer_PRO.htm

Last edited: Dec 13, 2008
2. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
That's OK.

Pin 7 is an open-collector transistor that can sink a maximum of 15mA current. You shouldn't try to sink more than 1/2 that much - so, about 7mA.

If you're running from 4.5v, the smallest that the resistor can be between the supply and pin 7 is about 640 Ohms. R=E/I, so R = 4.5/.007 is about 640.

Your 0.1uF cap might be out of specifications - you should get a 1.4kHz output at about 55% duty cycle when the upper resistor is 1k and the lower resistor is 4.6k with a 100nF (0.1uF) cap.

You might have a bad pot; they get "dirty" easily. Try another one.

3. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
Or you could stick another resistor in series with the pot to move the noisey spot on the pot. Not ideal, but functional.

4. ### ssherwood Thread Starter New Member

Aug 25, 2008
9
0
Thanks again Bill & SgtWookie, great information. I'm away from my test bench, and not happy about it , but will post the results in a couple days!!

Best Regards,
ssherwood

5. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
Ssherwood,
Try using the "wide range astable" configurations. Unfortunately, the freeware version won't show much in the way of resistor values, but if you're generating tones, you'll probably want close to a 50% duty cycle.

Try the 2nd version with a 5k pot for P1, use another pot for R1 wired as a rheostat (one end tied to the pot's wiper).

6. ### ssherwood Thread Starter New Member

Aug 25, 2008
9
0
I will do that. About the Timer Pro software, why are there so many configurations for the same 1400 Hz, and what is it about the second schematic that makes you recommend it? Just more flexibility for adjusting the resistance with the pots? Also, they get increasingly elaborate, even using 2 555's. Why is that?

Thanks!

7. ### SgtWookie Expert

Jul 17, 2007
22,183
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There are many ways to skin the proverbial cat.

I suggested the 2nd version in the wide-range astable because it has a reasonably low parts count, yet allows you to easily adjust the ON-time vs the OFF time along with the frequency.

BJT 555's outputs will go within a few mV of ground, but when sourcing current (output high) it's about 1.7v below Vcc. Without the capability of adjusting the current for charging/discharging paths for C1 (like this schematic provides) the output waveform would be asymetric.

CMOS 555's outputs can sink about 10x the current that they can source. This will also lead to asymetric output waveforms.

You may like the sound of the asymetric waveforms more than if it's perfectly symetrical. But without having an easy method of adjustment (such as P1 provides) you'll be doing lots of fiddling around changing resistors. Besides, unless you're using diodes you're really limited to >52% duty cycle. With the version I suggested, you can vary the duty cycle from nearly 0% to nearly 100% by adjusting P1.