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Help with project

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ronwer

New Member
I am working on a small science project, and need assistance with the electronics part.

I have not been succesful finding an appropriate schematics.

It's actually very simple: I want to measure the drop time of different objects through a vertically mounted metallic tube (copper).

The objects will vary from a stone, an ordinary magnet to the strongest possible neodymium magnet.

At the top and end of the tube I want to mount IR-detectors that will respectively start and stop the counter, which result should be shown on a (old-fashioned) LED-display.

The fun part is that the droptime for the neodymium magnet is much longer (6-10x?) than for the other objects, which can be used to demonstrate some scientific principles.

I have some schematics that in part show the direction in which I want to go, but not completely. If someone could modify these?

You can get more detailed info send to your personal email address.

Any help is appreciated!

Ronald Werner
Norway
 
It sounds like you have the whole project mapped out. IR TX/RX top and bottom of tube to a microcontroller. Then count and output to a display.

Send up what you have (circuit wise) so people will give you ideas. Can you use your PC's com port over an LED display or do you already have the display?

I did a project this morning, where I just blink an LED with the count for debugging.
 
Drawing of science project

Thanks for your reply.

Here you will find a drawing of the project:

**broken link removed**

And the circuit I found that came closet to be usefull for my puropses, but one that will need dramatic re-design in order to work.

**broken link removed**

I would be grateful for any help you can give.

Regards,

Ronald
 
Hi Ronald.

You say that "will need dramatic re-design in order to work", but that is not really so. If you look at your circuit diagram again, the Switch S2 only has to be closed (on the object passing the top position) and open (on the object passing the bottom position). It helps if you get the specs for the 74C926 IC from the Web.

So, without giving you too much help; think of a way to redo your light detection circuit to functionally carry out the required activity above. Hint: A flip flop set/reset driven by your photo-cells may do the trick :)

Cheers. Grahame
 
Thanks Grahame,

How would something like that look like? I am not familiar enough with digital electronics to feel I can produce reliable results.

There is latch-enable and display-select to think of. Both of them must get the right signal at the same time.

Can you explain me why we need transistor T2?

Because, in the original description following with the circuit the author (C.H. Vithalani) explains that when S2 is closed, base of T2 is connected to GND and T2 starts conducting. (If you want the full text just let me know!)

I don't believe that at all! I believe that when S2 is open T2 is conducting and will stop when S2 is closed.

But then I don't understand the circuit anymore, and the datasheet is too advanced for me. I would need to study maybe a week in or so to understand, and in the meantime life goes on. I just want to finish this project, not using time to study electronics because my bosses don't pay me for such things.

So, therefore I would be grateful for someone who draws a simple circuit how to connect the 2 pairs of IR-LED and phototransister to a flip-flip, which in it's turn is connected to the LE and DS from IC2.

I already dug up some too simple designs. I would like to use some kind of stabilisation between photocell and flip-flop, in such a way there is no danger of statics, hysteresis or flip-flops that get stuck...

Greetings,

Ronald
Norway
 
ronwer said:
Thanks Grahame,

How would something like that look like? I am not familiar enough with digital electronics to feel I can produce reliable results.

There is latch-enable and display-select to think of. Both of them must get the right signal at the same time.

Can you explain me why we need transistor T2?

Because, in the original description following with the circuit the author (C.H. Vithalani) explains that when S2 is closed, base of T2 is connected to GND and T2 starts conducting. (If you want the full text just let me know!)

I don't believe that at all! I believe that when S2 is open T2 is conducting and will stop when S2 is closed.

But then I don't understand the circuit anymore, and the datasheet is too advanced for me. I would need to study maybe a week in or so to understand, and in the meantime life goes on. I just want to finish this project, not using time to study electronics because my bosses don't pay me for such things.

So, therefore I would be grateful for someone who draws a simple circuit how to connect the 2 pairs of IR-LED and phototransister to a flip-flip, which in it's turn is connected to the LE and DS from IC2.

I already dug up some too simple designs. I would like to use some kind of stabilisation between photocell and flip-flop, in such a way there is no danger of statics, hysteresis or flip-flops that get stuck...

Greetings,

Ronald
Norway

To help with your understanding, the author is correct about T2.

When S2 is closed, T2 will be conducting (it is a PNP) and when S2 is open, the base is pulled up to the supply and T2 will cutoff.

The direction of the arrow in the emitter of T2 indicates that it is a PNP type which operates in this "reverse" fashion.
 
Hi Ronald.

The following circuit should help you.

I don't have the means to test it as I am currently "out bush" away from home and office.

Solution users a bistable flip-flop, triggered by the voltages across the photo-resistors Ka & Kb. You should pick Photo-resistors that give about 400K on dark, and 4K at light. It is best to have Ka & Kb matched as closely as possible (+/- 5%; better at +/- 1%). Also the +5V supply should be regulated with something like a 7805 Power IC together with a 470uF cap at its output in combination with a parallel cap of 1nF, both with working voltage >10v d.c.


Take another 555 chip and call it, IC-3.

IC-3 Pin 1 - Ground
IC-3 Pin 3 - Output goes to Base of T2
IC-3 Pin 7 - LED (pointing towards 555 Pin 7) with a 180 Ohm resister to +5V
IC-3 Pin 8 - +5V
IC-3 Pin 4 - Ext. Reset goes to junction of R4 & T1 and is activated by S3 Reset switch
IC-3 Pin 2 - Goes to IC-3 circuit A below
IC-3 Pin 6 - Goes to IC-3 circuit B below

IC-3 Circuit A -
15K Ohm resistor goes to +5V
Other side of 15K Ohm goes to one side of the first Photo-resistor Ka located at TOP, call this point A1.
The other side of the Photo-resistor Ka goes to ground.

IC-3 Circuit B -
One side of second Photo-resistor Kb goes to +5V.
Other side of second Photo-resistor Kb goes to a 15K Ohm resistor; call this point B1.
Other side of the 15K Ohm resistor goes to ground.

Connect A1 to 555 IC Pin 2.
Connect B1 to 555 IC Pin 6.

Functional Operation.
=============

You Reset IC-3, timing IC-1 and time display IC-2 circuit by depressing S3 Switch before dropping object in tube.

Point A1 should be low (~1.2V). B1 should be high (~4.8V). IC-3 Output pin 3 should be HIGH (~4.8V).

----- Now drop object into pipe. -----

As object passes TOP, point A1 gives a low going pulse and the IC-3 Output pin 3 goes LOW (~1V).

As object passes BOTTOM, point B1 gives a high going pulse and the IC-3 Output pin 3 goes HIGH (~4.8V).

The IC-3 Output pin 3 will go from HIGH to LOW; then HIGH again. This drives the Base of T2 - to give the TIME measurement of that is: IC-3 Output pin 3 LOW time is the TIME between TOP and BOTTOM sections detecting object passing.

Just ensure the original 555 circuit IC-1; provides sufficient fast timing pulses to the IC-2 the 74C926 for actuate measurement.

Hope this assists.
Cheers. Grahame
 
Nice. Looks like Wildpossum is also thinking along the same line.

This is a schematic to help you along. The operation of the circuit is, 95% what wildpossum been described.

There is a fundamental fault with the original circuit you have posted because the counter starts counting as soon as the reset button is pressed. Also The 555 output period is too slow a clock speed to be useful.

In the circuit I posted, there are several modification. The 555 timer is wired to give a countinuous 100Hz output of which the period you can adjust fairly accurately using VR1, after manually blocking the top sensor beam and watch the 7-segment LED display.

Initially after power ON, pushing the reset button briefly would place the 74C926 counter and 4013 Flip-flop(F/F) into reset state. This condition will remains if both top and bottom LED is shinning directly onto the corresponding photo transistor.

At this moment, 4013 Q output is LOW, the pulses from 555 cannot clock the counter as clock signal is shunted via the diode to 4013's Q output.

As soon as the top sensor beam is blocked by a falling object, the photo transistor turns OFF and its collector voltage goes HIGH. This SET the 4013 F/F so that its Q output goes HIGH. This removes the shunting of clock pulse and allows it to clock the 74C926 counter properly. After the object has passed the top sensor beam, the photo transistor turns ON again but 4013 F/F maintains its output regardless.

During the entire passage of the object between the two sensors, 74C926 counts up at the rate of 100Hz and result is displayed on the 7-segment display.

When the object blocks the bottom beam, the bottom photo transistor turns OFF and this caused a reset signal to the 4013 F/F. The F/F Q output goes LOW and the clock pulses is bypassed again via the diode down the Q output. No more clock pulses arrives at the 74C926 and so it maintains the cirrent count value. This is also displayed on the 7-segment LED.

To repeat the experiment, press the reset button to clear the result display and drop the object into the tube again.

Some suggestions: As the powerful magnet will be dropping quite slowly, it would help to drill several through holes along the length of the copper pipe and fit several LEDs to them so that they shine through the pipe towards the audience so people can see them. As the magnet drops, the light is blocked one by one by the moving magnet.

Edited: Added description of circuit operation
 

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Both Grahame and Chung MANY thanks!

I haven't yet puzzled out the schematics Grahame proposed. I just got home. But the schematics of Chung I understand.

I already ordered the 74c926 from Germany, but it will take some time to arrive in Norway. I have some old 4013's, which means I can run a test with some old IR-LEDs and see how they react.

My first try-out will be with the 555, but ultimately I feel it would be best to replace it with something with a X-tal.

Well, now I have a result I can work with!

Many thanks again! I'll check the thread for new thoughts, but as soon as I have a prototype up and running I'll send a report and some pictures to the list as a new thread.

The thought of the extra holes and LED's is great! I will use that one!

Thanks again!

Ronald
Norway
 
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