How to reduce the voltage needed for a photointerrupter

[kchriste]

I had no way of knowing that without reading the datasheet which was not necessary or relevant to the good advice I gave re measuring the device and choosing a specific resistor value.

Here I disagree, and that's all I have to say, about that, at this point

If you think I'm worng then just say so, I'm wrong as often as the next guy. But don't explain what my words "mean" as if I was too stupid to choose the right words myself.

I apologize for typing on your behalf or appearing to be condescending towards you. That was not my intent.

 

[Mr RB]

This is another part that I have. Its number is S7275-1 and I did not found any datasheet for it.

The circuit being more complex I just tried it without modification at 5 volts and to my surprise It is working. I then tried an unmodified one like the first one and it is working too at 5 volts. Same crisp and clean trigger.

For the usage of this part, as I understand it now, the CNC software keep the photointerrupter dedicated parallel port pin high. Since its output (of the part) is floatting when not trigered, the port pin stay high. When triggered, It connect the ground to the port pin forcing a low witch indicate to the software to stop everything. Does it make sense?

Alain

It's almost identical circuit to the other, again it si very typical of these devices. There are 2x 1.2k resistors in series (2.4k) to the power pin of the LED and photodiode (LED is on the right again) the photodiode forms a voltage divider to ground with the 30k resistor (303) and the centre of the voltage divider goes direct to the base of the output transistor which is NPN SOT-23 standard pinout BCE (top is B, bottom is E to gnd rail).

The photodiode goes low resistance where there is NO obstruction to the beam, this causes the base of the NPN to go high and it has an open collector output. So the output pulls LOW when there is nothing blocking the beam.

So now the big question... When you connect it to 5v power, what voltage are you getting on the LED and opto?

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To kchristie, sorry for jumping on you. It's just that it's a pet hate of mine when people explain what I "mean" especially when they don't get close to what I actually did mean.

 

[AlainB]

Since there was different voltages present on the donnor machine, some parts working at 12 volts, some other at 24 volts and some other at other voltages and since the first device showed here is designed for 24 volts with a 3K3Ω resistor, could we deduce that this second one, with 2 - 1K2Ω resistors in serie is probably designed for 12 volts operation?

If so then could we deduce that the resistor of the first device should be changed to something like 1K2Ω to work at 5 volts instead of the 330Ω mentioned previously ?

Is this too simplistic?

I am more concerned about the first device that can be easily modified. The second one, specially if it is effectively designed to work at 12 volts, I will keep it as it is.

 

[Mr RB]

They are both very similar, just that one has a 3.3k resistor and the other has a 2.4k resistor. Both use a sensitive NPN output transistor so the LED current is not real critical.

If they work reliably with 5v supply then they work! Otherwise change the resistor to a lower value until it starts to work reliably.

Also watch you don't get an sunlight or other ambient infrared into them, ie spotlights etc.

 

[mneary]

If you want 'OEM' operation but on 5V, I would try to determine what the original supply current was (if you only know the voltage, set it and measure the current on the bench). Then change one or both of the 122 resistors until that current is achieved at 5V.

This sensor is a little more versatile, too. If you have minor problems with stray light, you can reduce the 303 resistor (maybe as low as 4k7) to desensitize it.