• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Photo electrice eye question

Not open for further replies.


New Member
I have a photoelectric sensor with a gap. I have it hooked up as follows:

Now here is the problem. There is not enough difference between the on and off state: The relay sometimes makes a buzzing sound, for it is not switching completely.

How can I amplify it?
What do I need to put in yet?


Active Member
Hi goodpickles,

A darlington pair should have more than adequate gain.
Is it possible that they are not wired correctly ?
i have assumed they are NPN types.

I don't see why any buzzing or rattling should be put
down to the relay not closing fully ?

Maybe the light is mis-aligned ?

If the relay and the sensor are physically close or
mounted on the same board, is it possible that the
action of the relay is causing some movement of the
sensor, and making it buzz ?

Could you give some details of the components and
the relay please.

Regards, John :)



New Member
It is a Motorola 8844 K TIP-145 I am not sure if the NPN or PNP. Here I how I wired them:

The light is not misaligned for it is a gap sensor that is put together from the factory.

I am using a 12 Relay.

Maybe I should tell you what I am trying to do. I is for a label machine to sense where the edge of the label is (see picture). My problem is the difference on how much light the backing and labels let through is not very much. So I only get a difference in voltage of about 1 v.

It does work if I use card board instead of labels.



Active Member
Hi goodpickles,

I found this,
it certainly looks like the component you're using.

It actually is already a 'darlington pair' encapsulated as
a single transistor.

You may not have realised that when you arranged these two units
as a darlington pair.

Have you tried the system using just one of these ?

Also it is apparently a PNP unit, so i will amend my drawing

After a bit of searching, i found a PDF for TIP145 on this site:
This PDF includes the pin connections, something which is missing
on the other PDFs that i have found. I consider this an omission
of such magnitude that the people responsible should be punished
by being publicly ridiculed on the internet by anyone who wanted
that information, and downloaded their PDF for it.

Could you also give the details on the photo sensitive gap unit
you're using, it might be that choosing an appropriate resistor
would 'bias' the signal to the optimum level.

On the face of it this should be straight-forward.

I have corrected my little copy of your drawing.

Please check with the PDF mentioned that i have it correct.

John :)



Active Member
Hi goodpickles,

Also they are being used in an 'emitter-follower' configuration,
that is probably fine where you have a good signal, but as you
want a response through the semi-transparent backing, then i think
that it might be a good idea to re-arrange the TIP145 into a more
traditional amplifying set-up.

This would simply mean putting the relay in line with the collector
instead of in line with the emitter, as it is now.

And as you want the relay to be energised when the sensor is
obscured you will need a resistor to operate the TIP145 in the
absence of signal. I would suggest 10k to 15k, you would want it to
be as high a value that will comfortably operate the relay.

As can be seen from this PDF:
this unit has built-in base-emitter shunt resistors, the additional
resistor would need to be chosen carefully.

Cheers, John :)


Not open for further replies.

Latest threads

EE World Online Articles