Are you ready for a challenge?

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I am going to have to read through the last few posts to digest all the information but let me say this first. I am not concerned with the width, weight or speed of the pendulum. Because I will have different users they may lift the pendulum higher or lower than another user making the speed faster or slower at the bottom of the arc. I know already the pendulums being measured will have different thickness and it makes no different what so ever. I know the mass of the pendulums will all be different but length not mass determines the length of the period. All I am trying to do is measure the different in time when the pendulum's leading edge passes the start position and when the pendulum's leading edge passes again for the second time. That is one period. Yes if I time 10 periods of the pendulum and divide by 10 I will have my time in thousands of a second and that is what I would do if my circuit was not capable of measuring past hundreths of a second.

I think modifying an existing stop watch with an electronic trigger is a great idea. There may even be something like that out there. I will do some due diligence on that point.

Thanks for the ideas guys keep them coming.
 
mbarazeen said:
when we consider a full oscillation then the interrupting edge would be the same. i don't think any additional calculations are required in this case.
Click to expand...

Hello again,


Oh sorry there, for some reason i thought you wanted to measure the width of the thing
You seem to be on the right track.
 
Sounds you need to run the length calculation too on the processor, may be a difficult task.

if you only need to trigger start & stop of the watch, simple counter can do it. for sensors IR interrupt sensors can be an option, the transmitter can be opposite side of the receiver, may need some line of sight guide arrangement for the sensor only to see the source in straight line. the pendulum will interrupt the IR and will trigger the counter input.
 
hi,
Thinking about it, perhaps a UV sensitive detector could be the way to go, a smear of UV reflective paint along the bottom edge of the leaf pendulum would be all thats needed for a reflected signal.

All the 'bold' points in your post could be easily handled by a PIC, as you will be driving say a 7segment 3 decade LED display I would choose a PIC like a 28 pin 16F886 with a built in oscillator.
Datasheet attached.[tried too anyway...the site seems to be asleep'!!!]

If you mean to buy a PIC with a Basic compiler built there are types like the PICAXE [ I have never used one].

This air damping factor on a leaf type pendulum is still bugging me with regard to calculating the pendulum length by measuring its swing period.
 
Thanks for the info mbarazeen' and 'ericgibbs' but I think there may be some missunderstanding about the length of the pendulum. Think of this like a cuckoo clock. There is a pendulum hanging down out of the clock with a bob that can slide up and down the pendulum rod. I can measure the length of the rod with a ruler but that will not do me much good and I don't want to take the clock apart. The rod may be a piece of wood with a hole drilled in one end for a pivot. Somewhere down the rod you will have a bob (the leaf in the picture) that can be moved up and down the rod for speeding up the clock or slowing it down. See image below

**broken link removed**

I could take the clock apart and measure the length but that will not tell me (relative to the pivot point) how far down the rod the center of mass for the pendulum is. I can guess the weight of the stick and the weight of that bob (leaf) and try to figure it out but I do not want to do that. I want to measure how long each period of that pendulum is. Knowing that, I know the distance from the center of mass from the pivot point.

How would you like to find some UV reflective paint on your cuckoo clock that was never there before. The weight of that pendulum is very small and without the hanging weights continually dropping the pendulum would stop so you have to keep lifting the weights when the get near the bottom or the pendulum stops. Am I making any sense?

If I wanted to adjust the clock I should have a pretty good idea of how long each period should be to make the clock keep time.
 
'ericgibbs' I got "Programming and Customizing the PIC Microcontroller" by Michael Predko and starting to read it. It's bringing back a lot of old memories. Is that a good book and other there any other references you might suggest?
 
You can have a small hole above the pivot point on the pendulum rod, via which an IR source will excite a sensor that is very closely mounted to the pendulum rod. Or you can use the idea of angular reflection (can make magnification / sensitivity) so that the source and sensor would be mounted on the same plane of oscillation.
 
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Too much blah blah blah

M10SINV said:
'ericgibbs' I got "Programming and Customizing the PIC Microcontroller" by Michael Predko and starting to read it. It's bringing back a lot of old memories. Is that a good book and other there any other references you might suggest?
Click to expand...

Spent many hours of useless reading of long / unnecesary text. Predko tries to say everything that comes to mind.

To explain technical matters you do not need writing long. Just precise and to the point. Exaclty as Eric does to name one who is quite close.

"On writing" (Stephen King) should be the first reading of technical wirters.
 
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Please consider this

M10SINV said:
I would like to build a circuit that can detect a pendulum passing by, measure the period of a pendulum, and calculate the pendulum’s length.
Click to expand...

Hola M,

After I finish my current project something like yours is in my list.

I quoted what I believe is the essence of your requirement. It looks that so much info obscured a little your intentions (if I understood you corectly). I know I could be wrong.

Could you consider the following?:

Metal made pendulums ARE affected by temperature. Could you seclude your set up to control somehow the temperature? That would also help in controlling air flow around/through.

To measure the period add kind of small piece of regular section at the lower end.

Measure the time elapsed between XX (subsequent) passages in the same direction. Divide time / XX / 2 accordingly. That is your period.

No matter how complex you calculations could be. You can do using integers.

One second in any PIC at 4 MHz is a LOT.

I will follow this with interest.
 
How would you like to find some UV reflective paint on your cuckoo clock that was never there before
Click to expand...
UV paint is invisible to the human eye, thats why I suggested it.

BTW:
I agree with 'atferrari' that micro book IMHO is most suited as door stop.

Look at Nigels PIC tutorials, link near my signature on this post
 
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Well I hate to tell you guys but I have already given up on the microcontroller. Too involved just to make one relatively simple circuit. Here is the direction I am going in now I think. I would like to use a Schmitt-triggered photogate as my trigger to reset the circuitry and start a timer that drives a 7 segment display. Then I could use a flip-flop to identify the second pendulum pass through the gate. When the pendulum's leading edge passes the photogate the second time the circuit stops until I start over. What do you think?

I am assuming I can put the LED / Detector about a foot apart and they will work. I am also assuming a timer circuit could be tuned to emit a pulse every millisecond. If I use a 4 digit 7 segment display and most of my periods are about a second I will have 1.xxx for my time.
 
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What follows after that?

M, may I ask you to acknowledge my post whether you agree with it or not? Not that I want you to do that (rest assured) but simply to know in what direction you creativity is flying now.

M10SINV said:
one relatively simple circuit.
Click to expand...

I do not feel that sure...It much depends on how you follow after the detection. Just a detail: you have to calculate afterwards, what, if it is to be automated, means a micro. Or you will be doing something manually?

Definitely interested in the final outcome.

Ah, Nigel's are the best tutorial I ever run across. Concise and to the point.
 

Well I have found out that the photogate may not work at the distance I wanted it to but that is not a problem. I can buy a kit for the photogate for $7.25 but the emitter and detector are housed in a single interrupter-style unit that has an opening of 1.5 cm. I can live with that because I would mount something centered on the pivot point. As long at it is center where there is the same amount of weight above and below the pivot it will not effect the period.
 

I may take a look at Nigel's tutorial but not until I am certain I may go for the PIC. I can use the book as a door stop because it was an online version through our library here. LOL
 
Here is what I want, I could use something like this to measure the period of the pendulum. I am sure you know the little 'U' shapped device is the photogate which contains the IRLED.

**broken link removed**
 
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Microcontroller could be the simplest solution. You could consider something like Teensy USB Development Board or Arduino - HomePage. Ready made circuit and easy to program (Arduino - Software).. lots of example code availabe. Much simpler than a bunch of timers, flip-flops, display drivers etc.

Using a "photogate" and a microcontroller you are able to measure the period of one swing and the instantaneous speed of the object passing the gate. I have also some points about calculating the sine function with 8bit microcontroller if you need help with that.
 
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That may be the simplest solution but it may even be easier to connect up a photogate to a PC and do the programming in the pc.
 
M10SINV said:
That may be the simplest solution but it may even be easier to connect up a photogate to a PC and do the programming in the pc.
Click to expand...

Yes, of course. But you still need some electronics between the PC and the photogate. I don't have any real-time DAQ in my PC readily available..
 
M10SINV said:
Here is what I want, I could use something like this to measure the period of the pendulum. I am sure you know the little 'U' shapped device is the photogate which contains the IRLED.
Click to expand...

hi,
Dont wish to sound 'picky' but this simple wire pendulum is a far cry from the pendulum descriptions posted in earlier posts

If you decide exactly the range/shape etc of the pendulums and any restrictions regarding attachments to the pendulum I am sure a simple solution to measuring and displaying the period could be suggested.
It could easily done via the PC's parallel or serial port.

Its the permitted detection method and access that needs to be decided.
 
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