Any Electronic Gurus out there?

[WayneS]

I need help

I am looking to put together a cheap Tachometer for my engine and, while I have found the main unit at Maplin (http://www.maplin.co.uk/Products/ModuleInfo.asp?Addon=&CartID=030827142255160&moduleno=3169) , I need to get some sort of sensor to attach to my fly wheel.

Someone has said that I could use a Hall Effect sensor but there seems to be a problem.

The Maplin unit says that it needs the pulses to be greater than 3v, but all the hall sensors that I can find are only around 0.4v.

Anyone got any ideas?

Regards

Wayne

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[Evadne]

A Hall effect sensor is a solid-state switch. Like a transistor it needs a supply voltage and a ground, and the output is switched between the two. Most will quote a range, e.g. 3.8 - 30V for the Honeywell SR13/15 series. They usually quote a maximum current source for the output of 50mA, you will need to make sure your tacho circuit doesn't try and draw more than this (not likely if using a single IC.) Are you sure the 0.4V doesn't just refer to the voltage drop across the output? If you can't see one in Maplin try Farnell, RS or CPC.

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[Paul_H]

Hall effect device would work ok. A suitable device is the SS443A supplied by RS (http://rswww.com/). This is a unipolar device that switches on with a large N field nearby and off with a small N or no field (as opposed to bipolar that need large N to switch on and large S to switch off).
The device is made by Honeywell (http://content.honeywell.com/sensing/) search for SS443A and look at installation instructions PK88700 for SS400 family details.
It has whats called an open collector transistor output. Connect the transistor output to a +5V supply via a 10kOhm resistor. You'll then get 0V when magnet is close (transistor switched on) and 5V when its not (transistor switched off). You should be able to pick up 5V supply from the tacho module.
Another method would be to use an infra red LED and photo transistor with some arrangement on the engine that interupts the light beam. Dont recommend reflection type LED & sensor cos they have to be very close to good reflective surface to work reliably.


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[Evadne]

A photo diode would work but I wouldn't recommend optical devices in an engine's innards. They will work fine while all is clean and new but can get bits of muck (oil, fan-belt dust, etc.) in them in the vicinity of an engine.

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[WayneS]

Thanks for the help guys.

Paul, you mentioned that I should find a 5V source and connect it to the Hall sensor via a 10K resistor. Could I not connect the 12V directly? The spec says that it can take 3.8 - 24v.

As you can see, I'm not electronic...

Cheers

Wayne

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[Paul_H]

The tacho input is rated for 3 to 20V input and the Hall sensor can take up 30V supply so you should be able to power the sensor direct from 12V supply. I suggest using the regulated 5V from the tacho module cos its clean (no alternator spikes) and protects the sensor and its wiring from the 100's of Amps capacity of the battery. If it was connected direct to 12V then it would be wise to add some protection around the sensor eg. capacitor, zener, fuse.


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[idoltyme]

why not get a diesel alt (ford?) and taco

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