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IS IT POSSIBLE TO REVERSE A SWITCH? Answered

Im building a multiphase coilgun but encountered a problem. In order to activate secondary phases i need a photoelecric switch. In my case it is TCRT5000 photoelectric switch. The problem, however, is that once the ''beam of light'' is shining on it, the switch is turned on, but i need it to be turned off when ''beam'' is directed at the diode and turn on when the ''beam'' is not directed at the diode. How do I do this? And in this case changing polarities wont change anything.

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gmoon

3 years ago

More info would be helpful -- switching details, for instance, but here goes...

Usually a phototransistor / photodiode (probably for switching a mosfet for your application) can be placed differently in relation to a load...load on the top it's a standard common emitter switch, load on the bottom it's really more of an emitter follower. Those configurations yield opposite results. An example...

Failing that (if it's unreliable in your app), an additional transistor switching stage will invert the signal -- single (common emitter) transistor stages always invert. Or something more complicated with an inverting op amp, which would offer finer control of the levels.

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mondeluzgmoon

Reply 3 years ago

Very helpfull, but im not sure that you fully understood the problem at hand. The thing is that I poses a switch that turns on when the light is being shined upon it, but i need it to turn on when the light ISN' T shining upon it

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gmoonmondeluz

Reply 3 years ago

So assuming you're using a transistor for switching (nothing else would be fast enough), then inverting the signal from the phototransistor will do that very thing. Right? Inverting is the opposite behavior...

What my original post is about; the two phototransistor configurations or adding another inverting stage.

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Downunder35m

3 years ago

The best way is to use the right sensor to start with - either with alternating outputs or configured as normally open.
Everything else is only a substitude and can mess with signals and timings.
This also poses the question if your sensor are actually fast enough and "solid" enough for the task.
You have quite high speeds to deal with, even an electronic sensor without a relay in the output stage will only detect up to a certain speed.
If your projectile travels faster than the minimum response time of your sensor it will simply fail.
Same if the produced output signal is not long enough for the rest of your electronics to be used for proper switching.
This means you either have to expand the ON state long enough or compensate in your electronics with a seperate timer for each sensor.

So you see the right selection at the start can be the difference between an expensive failure or at least some fun.