# Having trouble making a 555 timer circuit for a 0.5ms LED flash...

#### SpielbergRules

Apr 18, 2021
7
Hello, new to this forum. Sorry for the odd subject. Sometimes I have a hard time describing what I'd like to do in words. Please bear with me.

So, let's say as an example I have a pizza cut into 20 equal slices spinning on a 45 RPM vinyl record turntable.

I'd like to flash an LED on the center of each piece of the crust every time it passes.

I tried some different math, asked a few folks and seemed to have gotten different answers from all.

I know the formulas and have seen the 555 calculators online. I wish there was a "reversed engineered" site where you can put the Th and Tl in and it would give you the values. Has anyone found that anywhere?

The issue is that the 1M linear taper pot I have has such a small range of "fast flashes" near the end.

Can someone help me with the values of resistors / capacitors I'd need so that I can flash the LED for 0.5 ms, then have it off for the balance of time until the next 'center' passes by?

I have attached a pic of what I have come up with so far.

Thank you all for any help. This has been driving me crazy!

Mike

#### Attachments

• Frequency Slice Sample.jpg
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#### AnalogKid

Jun 10, 2015
2,922

"Engineers don't take a dump, son, without a schematic."

Be sure to include a reference designator and part value for each component.

A common way of getting better adjustability is to use a fixed resistor in series with a variable one. For example, if calculations for a timing resistor indicate that the optimum value is 900 K ohms, rather than use a 1 M pot and try to adjust it to 900 K, use an 820 K resistor in series with a 200K pot. Now the optimum value is near the center of the pot, and the adjustability ( timing change per degree of rotation ) is much smaller.

ak

#### SpielbergRules

Apr 18, 2021
7
Thank you for that instantaneous reply! The schematic is the same one everyone uses. I actually have it in the ancient Mini-Engineer's notebook from Radio Shack back in the 80's.

I am still looking for something to 'reverse engineer' the frequency to values part.

Have you seen something like that online? My math is terrible to try and figure that out myself. I graduated in 1982 and forgot more than I learned!

Mike

#### AnalogKid

Jun 10, 2015
2,922
Still not clear. Are you looking for a circuit that senses when each piece goes by? Or one that takes an input from a sensor and flashes an LED? Or a free-running oscillator to flash the LEDs that is times such that is is synchronized to the rotation?

Still not clear. Is the 555 circuit supposed to be a monostable or astable?

AND - the 555 datasheet has a chart for selecting R and C values with zero math.

AND - do you even need a 555 circuit? If a contact under each slice brushes a stationary contact as it revolves by, that would flash the LED with zero electronics.

ak

#### SpielbergRules

Apr 18, 2021
7
Thank you for the reply. Sorry I am not good at explaining...

Astable, free-running oscillator that flashes the LED that are synched to the rotation.

45RPM 20 'slices' per [full] revolution.

Thank you!

#### AnalogKid

Jun 10, 2015
2,922
Flash all 20 LEDs together, or one at a time as they pass some reference point?

ak

#### AnalogKid

Jun 10, 2015
2,922
The schematic is the same one everyone uses.
That does me absolutely no good.
I actually have it in the ancient Mini-Engineer's notebook from Radio Shack back in the 80's.
I don't.

ak

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,769
This calculator gives you the component values when given the frequency and the duty cycle. Both to be calculated by you from Tl and Th.

#### SpielbergRules

Apr 18, 2021
7
Flash all 20 LEDs together, or one at a time as they pass some reference point?

ak
Thank you for the reply. Actually the dots on the diagram were just 'centers'.

There would be only one LED.

#### AnalogKid

Jun 10, 2015
2,922
So - and I'm just GUESSING here:

1. There is some kind of sensor that is tripped or triggered 20 times per revolution of something, and you want to flash one LED (voltage? current? power?) with each sensor activation - ?

2. There is no sensor. You want a free-running astable oscillator to flash an LED at 20 times the RPM of something, and you manually adjust the frequency of the oscillator ro match the rotation speed of the something - ?

If 2, then the 555 datasheet has the equations for this in the Astable Operation section. There are two equations, one for the output high period, and one for the low period. You can start with the Monostable Operation component value chart to select one capacitor value that works for both the high and low output periods, then re-arrange the two equations in the Astable section to start with the time and capacitor values, and solve for the resistors.

A long-standing problem with the 555 circuit is that when using "the same one everyone uses", the minimum output duty cycle is 50%. IOW, the high part of the output cycle never can be shorter than the low part.

Depending on the power source and the LED, this might be a problem, solved by adding an external transistor to invert the 555 signal for a short, constant high output, followed by a longer, adjustable low output. Or, it might be nothing. Or you can use another 555 astable circuit; not the one everyone uses, the one only some use.

What is the LED. what is the circuit power source? Does the circuit interact with other electronics, requiring that they share a common ground?

ak

Last edited:

#### SpielbergRules

Apr 18, 2021
7

To AnalogKid: Thanks for the info. I was using the 555 calculator from DigiKey's website, but never tried the Mono mode first.

I was HEAVILY into electronics in the 80's and 90's. I'm 57 now and have forgotten a LOT of what I used to know 30-40 years ago.

But I think you pretty much summed up the issue by saying that the Duty cycle is 50%, which definitely won't work for this circuit.

I would have needed 0.5ms on and the balance of time off...

So now I am wondering if there is a way to use two 555's or a 556 and after that 0.5 ms flash, can that trigger the other half to wait whatever the delay is until the next 0.5ms flash is required... hmm...

Thanks again.

#### AnalogKid

Jun 10, 2015
2,922
If you dill in more details about the LED. it can be done with one 555.

Available voltage to run the circuit
Vf of the LED
Desired LED current
Does the LED have to be tied to either Vcc or GND

ak

#### Bluejets

Oct 5, 2014
7,041
What happens if your 45 rpm turn table is a bit off in speed..?
Seems to be an idea application for a pro mini or similar.

#### SpielbergRules

Apr 18, 2021
7
You know what Bluejets, I didn't think of that! I have a Sony PS-LX230 Turntable that I've had since the 80's. If it IS off, then I will have other issues!

I am unfamiliar with the "Pro Mini" - what's that?

Available voltage to run the circuit: I have an old PC power supply rewired, so +5 and +12
Vf of the LED: 3.2-4.0V
Desired LED current: Current listed as 25ma (For me, whatever it is, it is.)
Does the LED have to be tied to either Vcc or GND: The LED is tied to GND right now, but doesn't have to be. I'm flexible!

This is the LED I'm using:
https://www.jameco.com/Jameco/Products/ProdDS/2290247.pdf

Thank you folks!

Mike

#### Bluejets

Oct 5, 2014
7,041
Turntables driven by synchronous motors should be correct speed most of the time BUT if the drive mechanism slips a bit or the frequency of the mains is off a bit, then it will not be exactly your required rpm.
When listening to a record one possibly would never know the difference.

Arduino Promini is a cheap microcontroller module with inputs and outputs you can easily program via a usb lead, usb to ttl converter and a pc.
Well, the Clone versions are cheap, around \$3 and the usb/ttl programmer probably about the same.

Masses of example programs and help if one runs into trouble.

For example, with you application, you could check the switching point with a reflective optical device and if you had say, a black background with a thin white stripe on the circumference of the turntable, when the optical device trips, the LED would flash.

Another optical device which is very accurate on my grinding machines, is the slot type sensor where a vane of some description passes through a tiny slot and trips again as the above.

Would be possible I think with a 555 timer also with the opto device on the reset line.
Long time since I played with 555 timers.

#### SpielbergRules

Apr 18, 2021
7
Would a stepper motor be better? That should be pretty precise... I was just using the turntable, well, since I had it forever and it's here.

I don't have the Arduino ProMini, but I do have an Uno. Rev. 2. (Haven't used it n maybe 10 years or so)

I looked on Arduino's website - they didn't seem to have the ProMini at all. Amazon had the clones.

I really wanted to get a Raspberry Pi - think that would be better?

Thanks again.

#### Bluejets

Oct 5, 2014
7,041
Rasberry Pi totally unnnecessary for such a menial task.
Uno would be ok but plenty of clone ProMini's on Ebay etc.
Promini is a fraction of the size.
Stepper ok, just different, not necessarily better. Depends on your mechanics I guess.
So long it is designed in such a way as you don't loose any steps.

Did you look at ways of resetting a 555...?? (pin 4)

Jun 10, 2015
2,922

#### AnalogKid

Jun 10, 2015
2,922
If you connect the LED anode to Vcc, and the cathode to the 555 output through the current limiting, then the LED is driven by the 555 low output, not its high output. The low output has no minimum duty cycle percentage. It is set by Rb only, so it will remain fixed as Ra is adjusted to match the turntable speed.

I'm not a "555-for-everything" kind of guy, but it is the right part for the job in post #1.

ak

#### Bluejets

Oct 5, 2014
7,041

Thought that would have been obvious given the possible inaccuracy of the turntable speed.

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