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## Low freq 555 trigger circuit via potentiometer

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### Low freq 555 trigger circuit via potentiometer

Greetings,

I am looking for how to wire a 555 timer off of 5 volts in such a way that when I turn a potentiometer dial, the speed of the output pulse (high or low) goes up.

To better explain, when starting at midpoint on a 10K pot, at 5Kohms, the output triggers at say, once every 3 seconds. As I increase (or decrease, I can do either whichever makes the design easier) the resistance, the output starts triggering more rapidly: at 6K, the output increases to once every 2 seconds; at 7.5K, once per second, and at 10K full scale, 3 times per second. Preferably a linear rate if possible, if given a linear pot to operate with.

Those resistances and rates are just examples of how the circuits needs to operate. The output of the circuit will be fed into a different circuit and used as a replacement for that circuit's detection switch. I would like the width of the pulse to remain constant if possible, I am just trying to increase the frequency of the pulse.

Can this low-frequency circuit (zero Hz to 10Hz) be done with a 555?

ElectroDFW
"In Theory, the is no difference between practice and theory. In Practice, there is."
JMACgyver

Posts: 46
Joined: Mon May 09, 2011 7:23 pm
Location: Texas

### Re: Low freq 555 trigger circuit via potentiometer

Here is a suitable circuit.

With the wiper of P1 set to the top, the charge time will be 0.69 x R1 x C1; around 100ms. The discharge resistor R3 gives a negative pulse width of about 100µs. Changing R3 changes the pulse width pro-rata.

Pins 6&2 swing between 1/3 of supply and 2/3 of supply. Taking P1 wiper more negative reduces the current through R1 and so increases the charge time of C1. Taking Pi wiper progressively more negative will result in the oscillator running slower and slower until when it gets down to R2 (pin6 = 2/3 supply) it will stop altogether.

For long time periods, use a bead tantalum for C.
Attachments
555 widerange osc.GIF (3.41 KiB) Viewed 52085 times
pebe

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Location: Ellon, Scotland

### Re: Low freq 555 trigger circuit via potentiometer

Thank you, sir!

I will wire it up first chance I get and see what happens.
Sounds like it does what I'm looking for, but just have a few follow-up questions.
-Where would I connect R3 instead, if I wanted a positive pulse (pull-up instead of pull-down)?
- If I wanted the pulse to stop midrange of the pot, then I would have to choose R2 to be equal to half the total pot resistance, correct?

To explain why I am needing this:
I am making up the remote control prop from the Back to the Future movie. In it, Doc uses the joystick to increase the speed of the car, as shown on the digital display wired into the remote.
To replicate that, I have a remote with a center-return joystick, attached like they normally are to a potentiometer.
I also have a digital speedometer meant for use on a bicycle. This speedo has a LED readout and a sensor wire that is attached to the fork, while a magnet the user attaches to the spokes of the wheel activates the sensor. (If it is actually a Hall-Effect switch, I may be out of luck) But so far, when the magnet passes the sensor, it sends a pulse to the circuit, and the number of pulses per second are translated into a speed by the rest of the device.
The circuit I asked about is meant to replicate the magnet/sensor part of that device, and make it think that the wheel is spinning and closing the contact again and again.
I interface that with the remote's pot and the speedo input, and hopefully, it does the job.

Thanks again! I will keep you updated!

--Electro--

PS: If it is in fact Hall-Effect driven, any ideas on how to (simply) replicate that?
"In Theory, the is no difference between practice and theory. In Practice, there is."
JMACgyver

Posts: 46
Joined: Mon May 09, 2011 7:23 pm
Location: Texas

### Re: Low freq 555 trigger circuit via potentiometer

Where would I connect R3 instead, if I wanted a positive pulse (pull-up instead of pull-down)?

That particular circuit gives a negative pulse, and it cannot be changed. But you can add an inverter.
If I wanted the pulse to stop midrange of the pot, then I would have to choose R2 to be equal to half the total pot resistance, correct?

Correct. With R2 = 5k the slider at mid-travel will have the correct 2/3 supply voltage.

The Hall-effect will switch on when the magnet passes it. You will need to establish where in the circuit the Hall-effect is fitted. If one side connects to 0V (most likely) then the negative pulse is what you want. If one side connects to +5V then you will need to change it to a positive pulse. Use any NPN small signal transistor and connect its base via a 10k resistor to 555 pin3. Connect its emitter to 0V and connect a 10K resistor from its collector to +5V. You can then get a positive pulse from the collector.

If the Hall-effect is fitted to some other point that is neither 0v or +5V, then you will need to use an opto-coupler. More details on that if you need them.
pebe

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Joined: Tue Dec 09, 2003 11:12 pm
Location: Ellon, Scotland

### Re: Low freq 555 trigger circuit via potentiometer

Hi pebe!

:D Thank you so much for your help! It works exactly as intended. Once I adjusted the values to correspond with the actual devices being used, it does everything I needed it to do. Thank you again! You are amazingly knowledgeable. :D

Regards,

David M
aka ElectroDFW
"In Theory, the is no difference between practice and theory. In Practice, there is."
JMACgyver

Posts: 46
Joined: Mon May 09, 2011 7:23 pm
Location: Texas

### Re: Low freq 555 trigger circuit via potentiometer

I'm glad it worked OK for you.
pebe

Posts: 1058
Joined: Tue Dec 09, 2003 11:12 pm
Location: Ellon, Scotland

### Re: Low freq 555 trigger circuit via potentiometer

Hi pebe,

After I breadboarded the circuit with substitute components for testing, I am now having trouble putting it to PCB using the actual parts.

It turns out my variable resistor (the joystick) has a total resistance of 4.65K (normal for a 5% tolerance I'm sure) *BUT* the active, adjustable, range it has is a total of 740ohms. That is, from top to bottom travel, resistance only changes from 1.97K to 2.71K.

So, with that in mind, how much does it affect the complementary support components in the circuit you gave me? Specifically, R1 and R2.
I can wire P1 to be a rheostat if needed, or make R1 and R2 variable for adjustment (and have done so) but need to know what working ranges I need to start with. Right now, a 50K trimmer pot for R1 and 10K trimpot for R2 are not giving me the results I need.
At the 1.97K end of P1, I need the output to be the 0.0Hz, and at top end at 2.71K, I need the output to be approx. 1KHz (rough estimate)

Thank you for your help with this!

--ElectroDFW--
aka david
"In Theory, the is no difference between practice and theory. In Practice, there is."
JMACgyver

Posts: 46
Joined: Mon May 09, 2011 7:23 pm
Location: Texas

### Re: Low freq 555 trigger circuit via potentiometer

Hi David,
I am due to go into hospital for an operation and I will probably be in for the next 4 weeks.
Sorry I cannot answer your question now but I will as soon as I am able.
pebe

Posts: 1058
Joined: Tue Dec 09, 2003 11:12 pm
Location: Ellon, Scotland

### Re: Low freq 555 trigger circuit via potentiometer

The re-triggering is basically achieved by connecting the trigger input (pin 2) and the threshold input (pin 6) together, thereby allowing the device to act as an astable oscillator.

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