# 555 timer frequency

#### MorganJohan

Joined Mar 3, 2022
17
I have a steeper motor that needs a slower step-pulse to get it moving then a faster pulse to increase speed. It needs to start at 1.25hz then increase to 10 hz in a short amount of time. The 555 circuit will be powered on with a trigger limit switch. Then unenergized once the stepper motor reaches a certain position. Then once it’s triggered again, starts low again at 1.25hz then to 10hz in a second or two. I have two 555 timer circuits. One set at 1.25hz output and the other at 10hz output. Is there any way to start one and delay the other? Or merge the two together? And have the frequency increase but stay in these parameters??? Help please

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#### boostbuck

Joined Oct 5, 2017
492
something like this?

#### DickCappels

Joined Aug 21, 2008
10,123

This is the circuit referenced in post #2. It does not reset to a slow speed immediately after the pulses are terminated. If this is a problem you can probably place a resistor across the 3 uf (3.3 uf?) ramp up capacitor to shorten the discharge time.

#### crutschow

Joined Mar 14, 2008
34,047
A 555's frequency does not vary much with the voltage applied to the timing circuit, as shown in the simulation below.
I doubt the person who posted that circuit ever built it.

I'm working on some alternate circuits that may work better.

Last edited:

#### crutschow

Joined Mar 14, 2008
34,047

#### MrSalts

Joined Apr 2, 2020
2,767

#### MrSalts

Joined Apr 2, 2020
2,767
A 555's frequency does not vary much with the voltage applied to the timing circuit, as shown in the simulation below.
I doubt the person who posted that circuit ever built it.

I'm working on some alternate circuits that may work better.

View attachment 261984
Do you think the delay will happen the second time this circuit is started? Thst is, will cap C3 hold its charge if the supply is turned off for a short period? A long period? Both for a standard and CMOS version of the 555? Assuming the switch is between V+ and R2.

#### MorganJohan

Joined Mar 3, 2022
17
A 555's frequency does not vary much with the voltage applied to the timing circuit, as shown in the simulation below.
I doubt the person who posted that circuit ever built it.

I'm working on some alternate circuits that may work better.

View attachment 261984

#### MorganJohan

Joined Mar 3, 2022
17
Time would be 2 seconds or less from the 1.25hz to 10hz. And I tried that circuit, like you said. It did not work

#### MorganJohan

Joined Mar 3, 2022
17
Time would be 2 seconds or less from the 1.25hz to 10hz. And I tried that circuit, like you said. It did not work
I’d also need it to repeat once it is activated again. About 5-10 seconds later would be the soonest. Up to hours

#### crutschow

Joined Mar 14, 2008
34,047
I looked at some ways to get a 555 VCO but was not able to get the full frequency range you want as a near 10:1 range is difficult to achieve.
Can you tolerate a higher start frequency?

#### MorganJohan

Joined Mar 3, 2022
17
What’s the lowest we can get?

#### MorganJohan

Joined Mar 3, 2022
17
What’s the lowest we can get?
Could there be two timers? One at 1.25hz the other at 10hz and transition between the two?

#### crutschow

Joined Mar 14, 2008
34,047
What’s the lowest we can get?
Could there be two timers? One at 1.25hz the other at 10hz and transition between the two?
Define "transition".

How about a 555 circuit with switchable frequencies from 1.25Hz to 10Hz?
What's the minimum number of frequency steps do you think are tolerable, and what frequencies would they be?

#### MorganJohan

Joined Mar 3, 2022
17
Define "transition".

How about a 555 circuit with switchable frequencies from 1.25Hz to 10Hz?
What's the minimum number of frequency steps do you think are tolerable, and what frequencies would they be?
say the first timer at 1.25 starts immediately and then delay turns off while the second timer of 10 is a delay on and takes over.

#### MrSalts

Joined Apr 2, 2020
2,767
Just for practical purposes, you are asking for a 1.25Hz signal to ramp to 10Hz in 2 seconds? That's not even three full pulses at 1.25Hz.

#### MorganJohan

Joined Mar 3, 2022
17
Just for practical purposes, you are asking for a 1.25Hz signal to ramp to 10Hz in 2 seconds? That's not even three full pulses at 1.25Hz.
That’s true. I wasn’t thinking from the math side if it. Basically I have a 3 foot sliding door, so I was just imagining physically how I’d imagine it. And 3 pulses will move the door very little. But the goal is to get the door to open and close as fast as possible. Waiting 10 seconds for the ramp up stage would we inconvenient.

#### MrSalts

Joined Apr 2, 2020
2,767
The spdt switch charges the cap in bottom left. That cap keeps the transistor "on" and connects the second capacitor to keep the pulse train slow. Once the cap discharges to a point, the transistor turns off so only the smaller cap is connected to the 555 timer and the timer jumps to 10Hz. The cap gets recharged when the switch turns off the 555 timer.

#### MrSalts

Joined Apr 2, 2020
2,767
Actually, a smaller cap and larger base resistor give a more linear ramp to slowly cut the larger capacitor. It takes about 5 to 6 seconds to get to full speed - your mileage may vary depending on the gain of your transistor and the tolerance of your capacitor. Best to have some extra values on hand to tune as you need.

#### MorganJohan

Joined Mar 3, 2022
17
Actually, a smaller cap and larger base resistor give a more linear ramp to slowly cut the larger capacitor. It takes about 5 to 6 seconds to get to full speed - your mileage may vary depending on the gain of your transistor and the tolerance of your capacitor. Best to have some extra values on hand to tune as you need.

View attachment 262042
Wow that’s perfect. Now, sorry to complicate further. Is there anything to implement a short single pulse on that ramp up cap/transistor? I’m trying to eliminate all physical means of actuation. Say once the timer circuit is enabled, a short but long enough pulse charges that 22uf cap. Again sorry to complicate. Or maybe it’s fun for you.

View attachment 262043