Tachymeter design

Do a quick google for "shaft tachometer", there's a fair few on the first page that look promising, all intended for fit-it-yourself to a nondescript shaft, i.e. not tailor made for a car so should be possible to sort it out.

I'd imagine you can even get them with alarms etc. if you try such sites.

Further to my earlier hint re tachs with alarms, they certainly seem to be available, for example;

Set the max RPM at it operates relays when it hits it, and can be used with a variety of sensors, optical, electrical, magnetic etc. Handles

12 or 24 volt.

Gawd knows what the prices on these things are like though ;-)

Shouldn't think it'd be too difficult to glue a very small rare earth magnet to the prop and use a reed relay to signal a cheap engine tacho.

Go and have a chat to your local Jaycar store. They do an inductive pickup tachometer that will probably work happily (maybe with a very slight modification) off the signal from a magnet mounted on the PTO shaft and a pickup coil. If the store has the usual resident electronics freak they will probably even help you get it all working.

Martyn suggests a reed switch as a sensor, but at 6000 RPM =10msec/rev - its pushing it for a reed to have had time to stop bouncing around before it has to move again, and a glass tube next to V8 is not going to be too happy.

It would be better to pick it off with a hall switch instead.

A crude RPM to volts detector needs only a single transistor, a couple of diodes and a capacitor, with sundry resistors. Would you like to see the circuit ?

Steve

Rats, the moment I hit "post" my eyes fall on the price tag displayed prominently at the bottom of the page, £120. Not too bad if you don't have the time or inclination to put together your own job. You could probably get something much cheaper if you don't want it to ring an alarm when the revs get too high.

At engine speeds, a reed switch would meet its maximum contact lifetime very quickly, so you'd need to go solid state.

1000RPM max of the PTO - reed sw should be fine.

Nah, it'll bounce like the clappers.

Steve

It looks reasonable to me.

When Karen posted her query I thought I'd like something similar to disconnect a tractor pto, to prevent stalling. I was thinking it should be simple, a magnet on the shaft, a Hall effect transistor to sense it and a standard rev counter to pick up the pulses, designing the electronics would cause me a problem so this device is just the job.

AJH

Yes I would, I have a small anemometer with three wires coming off it so would like to know how to connect it to a readout. I'm guessing it contains a hall effect transistor that counts the metal blades as they go past.

AJH

On or around Tue, 25 Apr 2006 12:49:43 +0100, Steve enlightened us thusly:

if the propshaft's doing 6000 rpm, then you'll be going so fast you'd not notice the counter anyway...

Would the switch even have time to switch in the first place, let alone bounce? Perhaps with a large-ish curved magnet?

Still reckon solid state is the best. Even with a 100,000,000 cycle lifetime (which is high for a reed switch), at 1,000 RPM that's a lifetime of 1,666 hours if I've done my maths right. Depending on how much the engine is run, that's a lifetime of between 70 days to one or two years.

A device like the one I posted should do it as you suggest, although if Karen didn't want the relay operation I reckon she'd be best off with an analogue display, personally I find monitoring a rapidly altering measurement on a digital display to be much harder than on a good analogue display, especially where vibration comes into it.

I shall be talking with them, one of their sensors does exactly what I want.

I agree the analogue versus digital display and this thing does vibrate!

AJH

Depends really. A simple reed and magnet is fine for taking measurements - probably not best for a permanent fix, but once you have the measurements you wouldn't need full time monitoring.

Solid state, although sexier, takes slightly more than three small wires :-)

I built a similar measure (using a s/h revcounter) for a go-cart which worked, after a fashion, for years and cost less than a fiver.

AJH wrote: > On Tue, 25 Apr 2006 12:49:43 +0100, Steve > wrote: >

Here is a circuit we use with a water flow sensor. Its a pretty useful universal frequency to voltage converter and is very linear. It drives a moving coil panel meter from SKT 2. Pin 7 on Q2 drives an alarm o/p, whose trigger point is set by R10. The output will need to be buffered to do anything but light an LED.

If you ignore everything after Q2 pin 3, volts on R2 is directly proportional to Frequency, and you can do what you like with it. Keep the load on the pin very low, or you WILL need the opamp.

The time constant of R2/C2 affects the scale and range. As shown here, it will work with the 1000 RPM signal nicely I think, but otherwise make R2 a bit bigger.

Here is the picture of the circuit.

The input is shown as +12, signal and ground. In fact the input pin SKT

1 pin 2 can be driven with anything from about 1V to 12V, or you can add another little transistor amplifier.

Steve

A hall sensor has exactly three wires, supply, ground and output. And they aren't (too) brittle.

Steve

Thanks Steve but 'scuse my ignorance, what are the op amps?

AJH