# Detecting a magnet passing through a coil using a darlington pair?

Discussion in 'Electronic Basics' started by Kieran Simkin, Aug 4, 2006.

1. ### Kieran SimkinGuest

Hi Guys,
I'm trying to build a simple project to get some pratice in analog
electronics. The intention is to make an electronic cat flap that only
allows entry to a cat wearing a magnetic collar (yes, you've seen them in
pet shops).
I a 403Ohm coil around the entrance to the cat flap, passing a magnet
through this coil generates approximately 1-20mV and varies the resitance of
the coil by about 20Ohms in either direction. I need to turn this signal
into a digital on or off, with on indicating that a magnet is moving through
the coil. I'm not concerned about directionality, and the more sensitivity I
can get the better. Neither does the signal need to remain on for very long,
as I have a latching circuit in place that will work with a very short
signal from the detector.

I attempted to built an A.C. coupled darlington pair as described on the
following page (following the resistor and capacitor values exactly):
http://www.talkingelectronics.com/Projects/Inductor/Inductor-3.html
However, this did not work as the output remained on in every instance.
Measuring the coil and the 22nF capacitor in series gave me a resistance of
infinity - I don't know why because I'm not quite sure of the function of
the capacitor in this circuit.
I tried removing the coil+capacitor pair and replaced it with a normal
potential divide with the coil on one side and a fixed resistor and a preset
on the other side of the divide. Even with the resistance on either side of
the divide matched, I'm still unable to get any negative output from the
circuit.

Can somebody point me in the direction of a circuit that should work for my
project? I'm not sure whether I should be using a darlington pair or an
op-amp, or whether I need to A.C. couple the coil to the collector in my
first transistor, if so, I'm unsure of the reason for A.C. coupling and I'm
also unsure of why it does not work for me.

I've tried doing a search of the internet but cannot find anything more than
a basic description of a darlington pair and how to wire one to an LDR,
presumably a coil is significantly different to an LDR?

Any advice you guys could give me would be greatly appreciated.

Best regards,

~Kieran Simkin
Digital Crocus
http://digital-crocus.com/

2. ### Guest

The collector value is wrong try 10k instead.

The capacitor blocks the dc current, the text tells you that, so it
You will upset the dc conditions.
You will only get a very small voltage so it will need a lot of
amplification. The cicuit gives you an ac singnal which you will need
to process in some way before you can operate a latch. Overall it's a
scheme that is unlikely to work very well if at all.
indeed it is.

Considerably more research effort, read some books.

Hi simkin,

Did u try magnetic switches? these tiny things could work for you...
just put a few as reqd under the catflap, or somewhere it can catch the
cat's collar!

prays

4. ### MikeGuest

It sounds like maybe the coil voltage would be changing fairly slowly,
so You may need to increase the value of the 22n cap to something in
the range of 1uf-10uf. Also try values in the range of 10k-33k instead
of the 100k from the collector of the 1st transistor. The transistors
are not biased very well so the actual resistor values needed will
vary from those shown. If you are consistantly getting 10mv-20mv from
the coil then it might work. If you don't consistently get enough
voltage you add some turns to your coil, but too many turns and you'll
have trouble with false triggering from stray magnetic fields mainly
from AC power wires.. Keep in mind that the 47k from the collector of
the 2nd transistor is a fairly high value so you won't be able to put
much of a load on it. When you get it working, try lowering the 47k to
a 4.7k or 10k. FYI, those 2 transistors are not configured as a
darlington pair. It is a 2 stage direct coupled common emitter
amplifier with the input AC coupled. I'm not sure what you mean by
"negative output". The output should sit at approximately 9v and go
toward 0 then back to 9v as the magnet passes through the coil.
Hopefully this is what your latching circuit requires to operate. The
input capacitor is required so that the low resistance of the coil
does not short the emitter-base junction of the 1st transistor which
would keep it from operating. One other thing, The resistance of the
coil does not change as the magnet passes through it. The coil just
generates a voltage as the magnet moves through it.

Mike
"The scientist is possessed by the sense of universal
causation...His religious feeling takes the form of
rapturous amazement at the harmony of natural law,
which reveals the intelligence of such superiority
that, compared with it, systematic thinking and acting
of human beings is an utterly insignificant reflection."
Albert Einstein (theoretical physicist)