Can anyone tell me if this germanium transistor magnetic field detector is likely to work - assuming the pick-up coil can be made or acquired? It looks a bit over-complicated to me for what it is supposed to do. I have a use for such a design and am thinking of using it as a demo of analogue design from the past.

Thank you.

Y.R.

 

Depends on your application.
The circuit is a sensitive sensor that triggers a relay. The relay in turn applies power to a buzzer.

Is that what you need in your application?

OC71 and OC72 are obsolete.

Tell us what you are attempting to do.

 

You could replace D1 and the germanium transistors with silicon devices, reverse polarites where appropriate, and the circuit should behave much the same as before.

 

If you wired it exactly as shown, it would not work. The ground is missing from the emitter of Q4. I also wonder about the biasing of Q1, it doesn't look like it would be very stable. Q1 is normally on, probably near saturation, and turns off to generate a pulse into the darlington pair Q2 and Q3.
Try the circuit in LTSpice using more modern components and see what you can get. You will have to experiment with L1 to determine a good value to simulate for the input.
FYI This circuit senses a changing magnetic field and latches the relay/buzzer into the on state. I has to be manually reset with switch SW.

 

Thanks for the replies.

Yes, I realise the earth is missing in the diagram from emitter Q1 etc.

It's meant to sense a 'relatively strong' (no units quoted) changing magnetic filed. I wonder if it would be sensitive enough if moved by hand to react the the Earth's magnetic field. It's an old circuit and there are plenty of better ways to do it now using some of the modern Hall effect devices. But the point is to see what germanium transistors could do in this configuration. There's no problem with tin whiskers with these germanium transistors, unlike the AF117 etc. and I have plenty of good OC71/72s.