Question about lighting and propeller

gambrino

Hello forum ,
i want to attach to the propeller a phototransistor.I draw the figure 1 in the attachment.The Vout is connected to the propeller . Light input at the base causes the output Vout to decrease from high to low.Can the propeller count the number of ( light off ) ? or i should change the figure?
Thank you in Advance.

Post Edited (gambrino) : 11/29/2008 10:29:59 PM GMT

Mike Green

Yes, the Propeller can count the pulses from this circuit. Remember that Vcc would need to be the 3.3V supply used for the Propeller. You will need to choose the load resistor value (RL) and any base resistor value (RBE) for the particular phototransistor and light levels expected. Often there's no base resistor and a good load resistor would be anywhere from 1K to over 10K.

Mike Green

In terms of programming examples, refer to the application note on the cog counters (AN001: www.parallax.com/tabid/442/Default.aspx) and the BS2 Compatibility Library (obex.parallax.com/objects/30/) which has some routines for counting and timing pulses.

gambrino

Thank you Mike,
Hello again, my goal is to count the number light off and throught propeller making some mathematical equations to control another device on the output of the propeller.I connect Vout of the phototransistor to an oscilloscope, when the light is on , the frequency was 50Hz and when was off the frequency was near 0 . Can the propeller count the number of time off ? Do i need the freq. count from the AN001? or i need to convert the analog signal to digital ( first step ). i can't imagine all steps to achieve this goal :-(

StefanL38

hello Gambrino,

the goal is to find something that can surely divide between light ON and OFF.

What do we have as the light is ON? 50 Hz on/off at the IO-PIN
What do we have as the ight is OFF? 0 Hz on/off at the IO-PIN

I think the 50 Hz come from the AC 50 Herz of the light.

If the maximum frequency of the ON/OFF-switching of the light is lower than 10 Hz.
You can use a low-pass-filter.

If the frequency is REALLY low ON/OFF for minimum 10 seconds the most simple "circuit" would be
to connect a capacitor of maybe 1-10 µF parallel to the transistor.

As you have a oscilloscope you can evaluate the right combination of RL and capacity with the oscilloscope

Another way:
Again depending on the ON/OFF-frequency you could poll the IO-PIN by a simple SPIN-program

Light On means 50 Hz ON/OFF. So you check for an OFF-Time bigger than 1/50 = 0,02 seconds
or to make it more reliable OFF-Time bigger than 0,04 seconds

The counters used in ANN001 are for higher and highest frequencies. As these counters are doing start/stop of
counting by hardware.

Now it's up to you to provide some more details about your project and ask more questions

best regards

Stefan

gambrino

The idea of putting this phototransistor : i have a motor and i want a signal from this motor to know the rotation per revolution to control the on/off of one device connected to the output of the propeller , on the uper side of the motor is a metal or maybe mirror can reflect lights.So i connect a laser , and the reflected laser beam to the phototransistor , and i put a black dot on the metal. The metal part rotate same speed like the motor.My problem is that i don't know the first step to count the rotation per revolution of this motor.

Ken Peterson

gambrino: Are you measuring 50 Hz from the laser, or from ambient light (perhaps a fluorescent fixture)? Using the laser, your should get a pulse every time your black dot interrupts the laser light. Use counter mode %01010 Positive edge detect (see table in AN001)

while loop-condition
  phsa~
  waitcnt(count-interval + cnt)
  speed := phsa

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- Bjarne Stroustrup

Post Edited (Ken Peterson) : 12/1/2008 3:58:18 PM GMT

StefanL38

Hello Gambrino,

it's like a thought. Knowing the details of your project offers new possabilities to solve the problem:

A common way to detect RPM is to use a little magnet and a hall sensor. This solution would be
completely independent about ambient light or dirt. But the hall-sensor has to be very close to the magnet
1-2 mm.

The software on the prop wpuld be the same as with the phototransistor

best regards

Stefan

Leon

Or, use a combined IR LED and photo-transistor (reflective). Paint the shaft black and put a dab of white paint on it or a strip of reflective material. That's how many tachometers work.

Leon

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Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle

Post Edited (Leon) : 12/1/2008 8:59:36 PM GMT

gambrino

Hello again ,i have a phototransistor, and i want to read the rotation per revolution of a motor . is it good idea to write like this ? now i have the result in measure

CON
_clkmode = xtal1 + pll16x
_xinfreq = 5_000_000    
PUB go
  cognew(@asm, 0 )
DAT
        org
asm
        movs CTRA, %0                   {Monitor pin 0}
        movi CTRA, %01010_000           {Set CTRa mode to POSEDGE detector}
        mov  frqa, #1
:CountA
        mov PHSA, 0                      {Resets PHSA to 0}                                
        waitpne 1, |< 0                  {Wait till pin 0 in not High,when light is obscure}
        mov measure, PHSA                {Store PHSA in measure}
        JMP #:CountA                     {loop endlessly}
 
measure long

StefanL38

Hello Gambrino,

is there a special reason why you code in assembler?

The counter is a hardware that works very fast
If you don't need a maximum latency of microseconds
a program in spin will be fast enough.

The value of the counter will be quite accurate as it takes only a few nanoseconds for the
hardwarecounter to switch and capture values.

To make a definite decision about SPIN or PASM: What is the maximum speed in RPM that this motor can have ?

best regards

Stefan

Post Edited (StefanL38) : 12/3/2008 8:25:26 PM GMT