# Using the 555 control pin

#### wayneh

Joined Sep 9, 2010
16,384
I'm trying to use the ~0.1Hz output of one timer on a 556 to induce a slow-changing shift in frequency of the second timer at ~0.5Hz, by routing the output through R5 to the control pin of the 2nd timer and C3 in the attached. The timers are both working nicely on my breadboard.

I'd like to maximize the shift in frequency of the 2nd timer, up to a factor of two in either direction from the mean, ie. from 0.25Hz to 1Hz. With the current component values, I think I can see some variation but it's hard to tell and probably not a halving-doubling.

The math on this is a bit much and I'm hoping someone has experience with this approach or could simulate this for me. How much shift can I get, and how can I get this range to be stretched out to, say, 5 seconds of increasing frequencies followed by 5 seconds decreasing?

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#### Wendy

Joined Mar 24, 2008
22,138
Your diagram shows the reset pins open, this is an illegal condition. Tie those suckers to Vcc!

#### SgtWookie

Joined Jul 17, 2007
22,210
The resets on bjt 555's have the equivalent of 100k pull-ups on the reset pins, but the CMOS versions don't. Either way, you risk getting "bitten" if you don't tie them to Vcc.

You're not going to get quite as much shift in frequency as you'd like; maybe from 0.33Hz to around 0.85Hz. The big thing that will change is the duty cycle; at the higher frequencies the duty cycle will decrease very significantly. That's because when the threshold is so low, the timing cap charges really quickly, but the discharge takes a relatively long time.

#### wayneh

Joined Sep 9, 2010
16,384
... you risk getting "bitten" if you don't tie them to Vcc.
Easy enough to do that. But does the very low frequency off my application make this less of an issue? It's obviously working fine w/o this. I suppose an occasional reset wouldn't be an issue as long as it doesn't lock up altogether.
...maybe from 0.33Hz to around 0.85Hz.
I can live with that. I just want to maximize whatever I can get with this dual-555 strategy, given the general timing goals (~1 second pulses getting faster for ~5 seconds and then slower for 5).

#### SgtWookie

Joined Jul 17, 2007
22,210
Well, I posted a week or two ago about using a 4017 to change timing caps for a 4093 NAND free-running astable multivibrator if that would be of interest.

#### wayneh

Joined Sep 9, 2010
16,384
Well, I posted a week or two ago about using a 4017 to change timing caps for a 4093 NAND free-running astable multivibrator if that would be of interest.
Yes, and that would give greater control, but I opted for the lower parts count and smaller footprint of this "good enough" approach.

#### Wendy

Joined Mar 24, 2008
22,138
The resets on bjt 555's have the equivalent of 100k pull-ups on the reset pins, but the CMOS versions don't. Either way, you risk getting "bitten" if you don't tie them to Vcc.

<snip>
Uhhh, no.

It may depend on the 555 make you are getting, but I don't recall ever seeing any schematic of a 555 with what you describe.

This is from the NE555 datasheet.

The reset pin is a digital input, you must tie it to something. Always a good rule for logic, except maybe TTL.

I admit I'm curious. I'll have to try it out sometime. I have run experiments to see how much voltage you need to turn it on, it was a minimum of 0.7VDC (call it 1.0VDC). I've never tried it totally open before.

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#### wayneh

Joined Sep 9, 2010
16,384
I just did that experiment, without realizing it. But I'll nail them to Vcc for my build.