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Car Forum / Land Rover Cars / April 2006Tachymeter design
Can any electronics whizzes help me out with a simple design for a tachymeter to display the RPM of a propshaft? I'm looking to monitor the output of my rear PTO so it does not over-rev while negotiating rough terrain - mostly a creek bed I have to drive through frequently while weed-spraying. Max RPM needs to be kept below 1000, but sometimes seems to sneak up well beyond, judging by the odd sounds coming from the rear. I'm pretty handy with a soldering iron, but gave up on modern electronics when they replaced valves with these new fangled transistors :) Karen  Signature"I'd far rather be happy than right any day." - Slartibartfast > Can any electronics whizzes help me out with a simple design for a > tachymeter to display the RPM of a propshaft? 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. SignatureBlast off and strike the evil Bydo empire! >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. 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. Signature"We have gone from a world of concentrated knowledge and wisdom to one of distributed ignorance. And we know and understand less while being increasingly capable." Prof. Peter Cochrane, formerly of BT Labs In memory of Brian {Hamilton Kelly} who logged off 15th September 2005 > 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. At engine speeds, a reed switch would meet its maximum contact lifetime very quickly, so you'd need to go solid state. SignatureBlast off and strike the evil Bydo empire! > Can any electronics whizzes help me out with a simple design for a > tachymeter to display the RPM of a propshaft? Further to my earlier hint re tachs with alarms, they certainly seem to be available, for example; http://www.controlability.com/sensor/sfm130-speedometer-tachometer-with-alarm.html 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 ;-) SignatureBlast off and strike the evil Bydo empire! > Gawd knows what the prices on these things are like though ;-) 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. SignatureBlast off and strike the evil Bydo empire! >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. 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 > When Karen posted her query I thought I'd like something similar to > disconnect a tractor pto, to prevent stalling. 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. SignatureBlast off and strike the evil Bydo empire! >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 > Can any electronics whizzes help me out with a simple design for a > tachymeter to display the RPM of a propshaft? [quoted text clipped - 7 lines] > I'm pretty handy with a soldering iron, but gave up on modern electronics > when they replaced valves with these new fangled transistors :) 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. SignatureEMB > Can any electronics whizzes help me out with a simple design for a > tachymeter to display the RPM of a propshaft? [quoted text clipped - 9 lines] > > Karen 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 >> Can any electronics whizzes help me out with a simple design for a >> tachymeter to display the RPM of a propshaft? [quoted text clipped - 8 lines] >> electronics when they replaced valves with these new fangled >> transistors :) > 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 [quoted text clipped - 6 lines] > of diodes and a capacitor, with sundry resistors. Would you like to see > the circuit ? 1000RPM max of the PTO - reed sw should be fine. SignatureEMB > 1000RPM max of the PTO - reed sw should be fine. Nah, it'll bounce like the clappers. Steve > Nah, it'll bounce like the clappers. 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. SignatureBlast off and strike the evil Bydo empire! >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. 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. Signature"We have gone from a world of concentrated knowledge and wisdom to one of distributed ignorance. And we know and understand less while being increasingly capable." Prof. Peter Cochrane, formerly of BT Labs In memory of Brian {Hamilton Kelly} who logged off 15th September 2005 > Solid state, although sexier, takes slightly more than three small > wires :-) A hall sensor has exactly three wires, supply, ground and output. And they aren't (too) brittle. Steve >> Nah, it'll bounce like the clappers. > >Would the switch even have time to switch in the first place, let >alone bounce? Perhaps with a large-ish curved magnet? too big a magnet and you'll have an unbalanced propshaft... SignatureAustin Shackles. www.ddol-las.net my opinions are just that Confidence: Before important work meetings, boost your confidence by reading a few pages from "The Tibetan Book of the Dead" from the Little Book of Complete B***ocks by Alistair Beaton. > too big a magnet and you'll have an unbalanced propshaft... So another wedge of metal on the other side, or alternatively drop the reed switch and use solid state like a sensible person ;-) SignatureBlast off and strike the evil Bydo empire! >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 ? 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 >> 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 ? > 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. 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. http://www.thetaylorfamily.org.uk/cmine/displayimage.php?album=8&pos=0 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 >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. Thanks Steve but 'scuse my ignorance, what are the op amps? AJH >> 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 [quoted text clipped - 5 lines] > > AJH The little triangles Q2 on the picture, holds two of these "Operational Amplifiers" in one 8 pin box. Like I said, if you just want frequency to volts, forget them. An op-amp is just a very simple amplifier, with a lot of amplification, unless you turn it down with external resistors. Steve >> Thanks Steve but 'scuse my ignorance, what are the op amps? >> [quoted text clipped - 4 lines] >volts, forget them. An op-amp is just a very simple amplifier, with a >lot of amplification, unless you turn it down with external resistors. Sorry I hadn't realised you meant I could leave them out when you said ignore them. AJH >>> A crude RPM to volts detector needs only a single transistor, a > couple of diodes and a capacitor, with sundry resistors. Would you [quoted text clipped - 28 lines] > > Steve The picture I was looking at doesn't appear to have a pin 2 on SKT1. SignatureIf Your specification is vague or imprecise, you'll likely get what you asked for not what you wanted! He who says it cannot be done would be wise not to interrupt her doing it. > The picture I was looking at doesn't appear to have a pin 2 on SKT1. That should be pin 1. Ooops. Steve >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. if the propshaft's doing 6000 rpm, then you'll be going so fast you'd not notice the counter anyway... SignatureAustin Shackles. www.ddol-las.net my opinions are just that Travel The Galaxy! Meet Fascinating Life Forms... ------------------------------------------------\ >> http://www.schlockmercenary.com/ << \ ...and Kill them. a webcartoon by Howard Tayler; I like it, maybe you will too! >> Can any electronics whizzes help me out with a simple design for a >> tachymeter to display the RPM of a propshaft? [quoted text clipped - 21 lines] > > Steve Yes please Steve, I'd appreciate that - kareng@a1.REMOVE.com.au Signature"I'd far rather be happy than right any day." - Slartibartfast >Can any electronics whizzes help me out with a simple design for a >tachymeter to display the RPM of a propshaft? [quoted text clipped - 9 lines] > >Karen I've got a slightly different requirement, which people here may have ideas on... I need a very accurate odometer fitting to a vehicle (mine). I'm not sure how accurate car ones are, but I want 1% tolerance. I don't want a kit - I want someone to fit, calibrate and test it, and ideally give me some kind of certificate. I'm aware that the calibration will drift as the tyres wear etc... Anyone know anyone who can do this? SignatureTim Hobbs '58 Series 2 88" aka "Stig" '03 Volvo V70 '06 Nissan Navara aka "The Truck" >I need a very accurate odometer fitting to a vehicle (mine). I'm not >sure how accurate car ones are, but I want 1% tolerance. Blummie - is that actually possible without having one of those extra wheels on a frame at the back and only driving at <30mph? >I don't want >a kit - I want someone to fit, calibrate and test it, and ideally give >me some kind of certificate. I'm aware that the calibration will >drift as the tyres wear etc... Talk to your local weights and measures bods, or the nice people from HMR&C. They will know who produces such equipment. Any GPS based system will be too far out IMO. Signature"We have gone from a world of concentrated knowledge and wisdom to one of distributed ignorance. And we know and understand less while being increasingly capable." Prof. Peter Cochrane, formerly of BT Labs In memory of Brian {Hamilton Kelly} who logged off 15th September 2005 >>I need a very accurate odometer fitting to a vehicle (mine). I'm not >>sure how accurate car ones are, but I want 1% tolerance. [quoted text clipped - 10 lines] >HMR&C. They will know who produces such equipment. Any GPS based >system will be too far out IMO. I was thinking of a propshaft-based transducer, then calibrated over a measured number of wheel revs (or on a rolling road or similar). Certified for x miles until tyres wear to a certain amount. Definitely don't want a GPS-based system, 'cos that's what I'm testing! SignatureTim Hobbs '58 Series 2 88" aka "Stig" '03 Volvo V70 '06 Nissan Navara aka "The Truck" >>>I need a very accurate odometer fitting to a vehicle (mine). I'm not >>>sure how accurate car ones are, but I want 1% tolerance. [quoted text clipped - 14 lines] >measured number of wheel revs (or on a rolling road or similar). >Certified for x miles until tyres wear to a certain amount. try taximeter types, too - they're mostly driven form the speedo cable and calibrated over a measured mile. Whether they do as well as 1% I don't know. SignatureAustin Shackles. www.ddol-las.net my opinions are just that "The breezy call of incense-breathing Morn, The swallow twittering from the strawbuilt shed, The cock's shrill clarion, or the echoing horn, No more shall rouse them from their lowly bed." Thomas Gray, Elegy Written in a Country Churchyard. > Talk to your local weights and measures bods, or the nice people from > HMR&C. They will know who produces such equipment. Any GPS based > system will be too far out IMO. I had a quick google about for doppler radar systems that point at the ground, but got bored. SignatureBlast off and strike the evil Bydo empire! >> Talk to your local weights and measures bods, or the nice people from >> HMR&C. They will know who produces such equipment. Any GPS based >> system will be too far out IMO. > >I had a quick google about for doppler radar systems that point at the >ground, but got bored. Yes that was my first thought, there used to be a company called something like RDS that fitted this kit to tractors so you could measure the wheelslip (wheelslip is a function of efficiency in getting traction into the ground). Too late for me to google now but integrate the velocity and you have distance. AJH > Yes that was my first thought, there used to be a company called > something like RDS that fitted this kit to tractors so you could > measure the wheelslip That's why I had a peek at it, with the idea that if it's good enough to measure speed accurately enough for tractor applications then it should do the job. ISTR it being fitted as standard to some models. SignatureBlast off and strike the evil Bydo empire! >>Can any electronics whizzes help me out with a simple design for a >>tachymeter to display the RPM of a propshaft? [quoted text clipped - 20 lines] > >Anyone know anyone who can do this? Terratrips can be set up fairly accurately IIRC, given that they are used on competition vehicles. Not sure about certificates though. Would have thought Warren might know someone? Failing that, see if the plod know anyone. SignatureSimon Isaacs Peterborough 4x4 Club Chairman, Newsletter Editor and Webmaster Green Lane Association (GLASS) Financial Director 101 Ambi, undergoing camper conversion www.simoni.co.uk 1976 S3 LWT, Fully restored, ready for sale! Make me an offer! Suzuki SJ410 (Wife's) 3" lift kit fitted, body shell now restored and mounted on chassis, waiting on a windscreen and MOT Series 3 88" Rolling chassis...what to do next 1993 200 TDi Discovery 1994 200 TDi Discovery body shell, being bobbed and modded..... > Failing that, see if the plod know anyone. That's probably your best bet. The guys who do the NZ Police speedo calibrations will also provide the same service to anyone else - not much use to you being 12000 miles away but I expect there's someone similar in the UK. Otherwise the people who calibrate taxicab meters normally have the technology to at least test the mileage covered and give you an indication of what the error is. Incidentally, which of your vehicles do you want to fit this to? If it's the Navara (which IIRC has an electronic speedo) it should be easy to hook in to the signal wire for that with a pulse counter that can then be easily calibrated to show distance travelled. SignatureEMB >> Failing that, see if the plod know anyone. > [quoted text clipped - 11 lines] >to hook in to the signal wire for that with a pulse counter that can >then be easily calibrated to show distance travelled. Yep, it's going on the Navara. Part of the purchase rationale was a truck with plenty of room to fit 'toys'. The space under the back seat is filling up nicely.... SignatureTim Hobbs '58 Series 2 88" aka "Stig" '03 Volvo V70 '06 Nissan Navara aka "The Truck" >Otherwise the people who calibrate taxicab meters normally have the >technology to at least test the mileage covered and give you an >indication of what the error is. Acceptable margin is 5 percent, most will get away with 8, 9 or (at a push - and if they're not too stroppy) 10 percent. Not really that good for Tim (or taxi passengers!) Signature"We have gone from a world of concentrated knowledge and wisdom to one of distributed ignorance. And we know and understand less while being increasingly capable." Prof. Peter Cochrane, formerly of BT Labs In memory of Brian {Hamilton Kelly} who logged off 15th September 2005 >>Otherwise the people who calibrate taxicab meters normally have the >>technology to at least test the mileage covered and give you an [quoted text clipped - 4 lines] > > Not really that good for Tim (or taxi passengers!) Ah ok. Here in NZ the rules are <1% tolerance on the odometer part of the taximeter. SignatureEMB >>Otherwise the people who calibrate taxicab meters normally have the >>technology to at least test the mileage covered and give you an [quoted text clipped - 4 lines] > >Not really that good for Tim (or taxi passengers!) ISTR the meter being potentially more accurate than that. worth asking the taximeter makers. SignatureAustin Shackles. www.ddol-las.net my opinions are just that "Remember that to change your mind and follow him who sets you right is to be none the less free than you were before." Marcus Aurelius Antoninus (121-180), from Meditations, VIII.16 >>Acceptable margin is 5 percent, most will get away with 8, 9 or (at a >>push - and if they're not too stroppy) 10 percent. [quoted text clipped - 3 lines] >ISTR the meter being potentially more accurate than that. worth asking the >taximeter makers. They used to be very accurate - but now on installation (as new) can be within 2 percent, when tested in use, 5 percent. I'd have to dig my notes out from a training day a few years ago to pin down any references (unless the regs have reverted in the interim). Thankfully such licensing issues are now the remit of local authorities. Signature"We have gone from a world of concentrated knowledge and wisdom to one of distributed ignorance. And we know and understand less while being increasingly capable." Prof. Peter Cochrane, formerly of BT Labs In memory of Brian {Hamilton Kelly} who logged off 15th September 2005 |
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