Loco detection - Reed switch

[Julia]

I am trying to setup an automated shuttle arrangement on my layout to drive a single car DMU from bay platform of station A, to bay platform of station B. 

 

I'm using inline transformers (from cirkits), to detect block occupancy, but I'd like to be able to get the train to stop on a specific position, so was thinking of using a reed switch. The question I've got is, will the motor in your typical N scale (Actually H0e, but it's all 9mm gauge), have enough of a magnetic signature to trigger a reed switch, through ~10mm of baseboard/ballast. Or would I be better off sticking a neodymium magnet on the bottom of the DMU? 

 

J

[DCB]

You will be lucky to trigger it under 10mm of baseboard etc even with a super neo magnet. When I tried reeds in 00 code 100 I buried them in the ballast after cutting the webs between sleepers to get them as high up under the rails as possible. The Huge ring magnets in Lima class 09 and 94XX use to trigger reeds but I find the tiny super neo magnets in CD drives are brilliant for this sort of thing.

[Julia]

You will be lucky to trigger it under 10mm of baseboard etc even with a super neo magnet. When I tried reeds in 00 code 100 I buried them in the ballast after cutting the webs between sleepers to get them as high up under the rails as possible. The Huge ring magnets in Lima class 09 and 94XX use to trigger reeds but I find the tiny super neo magnets in CD drives are brilliant for this sort of thing.

 

Ah, that's really interesting to know. I'll stick the reed switch in the ballast, and see what magnet fits under the loco best.

 

Cheers

 

J

[Theakerr]

Agree with the comment about position.  Re the loco magnet or separate magnet for activation, it will be a try it and see situation for each loco.  I use reed switches extensively on my 18 track automated section.  So far only two locos will trigger the reed switch, a Hornby L1 and 04.  Note when this first happened it caused some very interesting operational sequences because I had not anticipated it.

[Peter Beckett]

Julia,

I tested one of my reed switches (spare one) and using an 8mm dia neo magnet the switch operates ie closes at 6mm gap 

 

Peter

[cliff park]

It isn't as simple as the power of the magnet. The best affect is obtained if the magnet induces opposite polarities in each of the two reeds inside the glass. So, imagine those old fashioned textbook drawings of the force fields, like fountains, spraying out of each end of a bar magnet and joining back together in a sort of loop. If the reed is placed parallel to the bar magnet it induces a North in one reed and a South in the other. So actually quite a weak bar magnet, parallel to the reed will work over quite a distance. ( I would do a drawing but am using a tablet, not my PC so a bit limited) Actually I've just had another thought. What about 2 button magnets, N to S, but with a length of mild steel rod between them, so that each magnet is roughly half way along each reed. A sort of home made bar magnet. Anyone out there got the bits. and time, to try it?

[Julia]

Forget reed switches, get some Hall sensors. Type: TLE 4905. These are small: 3x4x1.5mm typical. Requires 3 wires: +Vcc, GND and output. Due to their size you can stick them on top of a sleeper (disguised as track magnet, ironically ) Use a small magnet (I've successfully used the Herkat round ones, but others work as well) glued under the vehicle, mind magnetic polarity: Hall sensors respond to magnetic south only. Additional benefit: as there's no glass, virtually no chance of breaking the thing

 

Here's the data sheet of the little things:

153751-da-01-en-Hall-Sensor_TLE4905+35+45L.pdf

 

HTH!

 

That's certainly worth considering. The main area I want to put the sensor is going to be inlaid track, so I'm hoping a very thin layer of plaster won' effect things too much. 

 

Do you know if the output needs debouncing before feeding into a microcontroller? 

 

Thanks

 

Julia