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ZeGas

Photo Transistors In Series

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I am trying to rig up photo transistors in series for section timing and its doing my head in.

 

Initially I set up a trial circuit on the Arduino Uno board with +'ve going to collector leg on first transistor, emitter leg on first to collector on second and emitter to ground on board.

I am sure that worked fine. I then put them all in place on the track and low and behold, nothing.

Thought I might have got the polarity wrong so tried the other way and laps went by one after the other without influence, so I guess that was wrong.

 

Question is, phototransistors are a closed circuit with a light source aren't they?

 

The thing that gets me is if I pull the +'ve out of the board (circuit not closed) the laps tick on by on their own again. with only ground connected, doesn't happen if I leave +'ve in and ground out though.

Where is the power coming from, I checked and double checked that there was no short in the circuit, it is a pretty simple circuit.

 

I can get each one working individually OK, just not in series.

 

Mind you it was late last night so I'll check again today.


Cheers Grant

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Yes they will work in series, but there are a couple of things to note.

 

Polarity is KEY. With 2x sensors, there is only 1 correct wiring out of 4 possible ones.

Make sure that each sensor has an adequate light source to work correctly.

Check your minimum laptime setting. If it's too long, only one sensor will 'work'.

 

Now, it's also possible that you need a pull up resistor (5k to 10k) between the input pin and +5V too...

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Yes they will work in series, but there are a couple of things to note.

 

Polarity is KEY. With 2x sensors, there is only 1 correct wiring out of 4 possible ones.

Make sure that each sensor has an adequate light source to work correctly.

Check your minimum laptime setting. If it's too long, only one sensor will 'work'.

 

Now, it's also possible that you need a pull up resistor (5k to 10k) between the input pin and +5V too...

 

This is the polarity I set up.

When I say +'ve, it is actually pin 5 on the digital part of the UNO board.

20160830103549-01ce19ed.jpg

 

Check on Light source

Check on lap timing

 

Not sure on where or why to place resistor, what is "pull up" sounds like trying to raise the voltage with a resistor, is that right?

Edited by ZeGas

Cheers Grant

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I'm assuming that "Posative" is actually the input pin on the arduino...

So, a pullup resistor sits between the input pin, and +5v.

 

Also, my brain is 'rusty' right now, and I can't remember which way arround the sensors need to be, but the orientation does not change from the single sensor setup.

 

But, other than that, it looks correct...

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Hey Michael,

I watched this fantastic video and understood 90% of it.

Towards the end he writes code to release the pullup resistor built in to the UNO board.

I cannot see in Dave's Sketch "LAPCOUNTER" where I can change the setting to release this or if it has already been done.

The on off thing going on with him is exactly what I experienced so am guessing a pull-up or pull down would fix the problem.

I have not got into the intricacies of Arduino yet so may ask insomniac Dave if he can help, unless you know where to find it in the sketch..

 


Cheers Grant

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As we are switching the inputs between 'floating' and ground, the internal pullup resistor should be on for the inputs we use, but I have found that sometimes it's a bit 'weak', and needs external assistance.

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That's, dear I say it, understandable.

So one phototransistor is enough for the internal resistor to divert the power but putting two lowers the effectiveness of it, so adding an additional one external helps.

Bloody hell if I'm correct I have had an epiphany, if wrong ....

 

So just to be clear, I put the resistor between the red line (input pin) on my sketch above and connect to the 5v pin on the board, is that correct?


Cheers Grant

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Cheers Grant

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Don't worry just reread your original post Michael, in one ear out the other.

I'll give it a go and see what happens.


Cheers Grant

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Thanks SR the resistor did the trick.

All go now.


Cheers Grant

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Great to hear. Have fun.

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