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Salvage It! BY JULIAN EDGAR A $10 lathe & drill press tachometer In essence, this tacho is a simple frequency-to-voltage converter driving a moving coil meter. The standard adjustment pot is shown at top right. Want to know the chuck speed of a lathe or drill press? A car tachometer can easily be adapted to do the job for less than 10 dollars. A read-out of chuck speed on variable-speed drill presses and lathes can be very useful. That particularly applies if you use an electronic speed controller but even if you have to swap gears or pulleys, it’s still good to have a display showing the tool’s rotational speed. In fact, wouldn’t it be good if you could have a big dial displaying revs per minute? Hmm, cars have one of those – it’s called a “tachometer”. And all modern tachos are driven by a simple pulse input, so it’s quite easy to adapt one to do the job. It’s easy and cheap to make a tachometer that measures the rotational speed of your drill press or lathe. The project uses just a few low-cost components and a re-scaled tachometer from a car. 88  Silicon Chip The components To make this speed display you’ll siliconchip.com.au need to scrounge a tacho from a car. When sourcing many secondhand parts, you don’t want to go along to a wrecking yard and ask for the tacho from a specific model – not unless you want to pay top dollar, anyway. No, what you want is an orphan that’s going cheap or perhaps it’s part of a dash display that’s been discarded because the faceplate is scratched. The Australian-built Nissan tacho used to make this display was sourced from the shop at the local tip. In fact, I got the whole instrument panel for just a few dollars. My guess is that it is from a mid-80s Australian-built Nissan Skyline or Pintara but that doesn’t really matter. In addition to the tachometer, you’ll also need a 12V DC plugpack, a reed switch (eg, from the speedo of the same instrument panel), a small magnet, a 10kW trimpot, a 33kW resistor, a 1000mF capacitor and a box to mount it all in. To make a new scale, you’ll need a PC, scanner and printer. By the way, many older Nissans use reed switches to transmit road speed from the mechanically driven speedo to the ECU. However, if you don’t get a reed switch with the speedo, it can be picked up quite cheaply from electronics stores. Building it Fig.1 shows how it all goes together. A reed switch is briefly closed each time a magnet mounted on the driven pulley passes by. This reed switch is fed with a nominal +5V at one end, derived from potentiometer VR1 which is across the +12V supply. Therefore, each time the reed switch Fig.1: the drill press tacho uses a tachometer that’s been scrounged from a car. As shown, a reed switch is fed with +5V DC, derived from trimpot VR1 which is across the 12V plugpack supply. The reed switch briefly closes each time a magnet attached to the machine tool’s output shaft passes close by and feeds an input pulse to the tacho’s input terminal. The 33kW pulldown resistor ensures that the signal input is low when the reed switch is open, while a 1000mF capacitor smooths the signal that’s fed to the meter. closes, a +5V pulse is fed to the tacho’s signal input. Conversely, when the reed switch opens, the tacho’s input is pulled to ground by a 33kW resistor. Note that the tacho assembly shown here has a separate PC board for the electronics, which is actually a frequency-to-voltage converter. This particular one uses an LM2917N as The reed switch was salvaged from an old film processor at the local tip. Here it can be seen mounted directly above the output pulley of the drill press. siliconchip.com.au the frequency-to-voltage converter chip and the datasheets for this are available on the web. As calibrated from the factory, it’s likely that the speed range will be too high for the new application – there aren’t many cases where you want the lathe or drill press doing 8000 RPM! This means two things: first, The reed switch is triggered by this magnetic washer which is glued to the pulley. This washer was extracted from a salvaged stepper motor. March 2005  89 Here’s the original instrument panel and an early draft of the scanned and rescaled drill press tacho scale. It’s printed out 1:1 and so is a perfect size match for the original instrument. a small “washer-shaped” magnet taken from the middle of a salvaged stepper motor. Once the magnet is installed, wire up the rest of the circuit as shown in Fig.1. Trimpot VR1 should then be adjusted to provide a nominal +5V to the reed switch. Don’t forget to install the pull-down resistor. The capacitor across the meter’s drive damps the jerky movement that occurs when the input frequency is lower than it would normally be in a car. Note the polarity of the capacitor – you can work out the meter’s polarity by disconnecting it from its drive circuit and applying a low voltage (eg, 1V) to its leads. When it’s connected so that the needle moves in the right direction, take note of the polarity of the supply. Testing the frequency input range of the tacho will have to be altered; and second, a new scale will need to be made for the meter. In our case, the on-board pot gave plenty of adjustment. In fact, with just one input pulse per revolution of the drill-press, the needle could be adjusted for full scale deflection even at the slowest drill press speed. However, we’re getting ahead of ourselves. Tacho connections The tacho should have three connections: +12V, ground and signal (frequency) input. If you buy the tacho with the whole instrument panel intact (the best approach), look very closely at the tracks on the flexible PC board on the back of the panel. In many cases, +12V and ground (earth) will be marked, leaving only the third pin which must then be the signal input. Conversely, if the board isn’t marked, you may need to seek the help of an automotive instrument repairer to get the pin-outs right. Alternatively, you can usually figure it out by tracing the supply connections. Once the wiring connections are sorted, it’s best to do some experimentation. Use good quality glue to hold the magnet in place on the driven pulley or gear and mount the reed switch so that the magnet passes close by it on each rotation of the shaft. We used The revised and modified scale, printed out on orange paper and covered in clear contact. But just who is behind the “JE Instruments” company? 90  Silicon Chip Once you’ve wired up the circuit, start the machine tool and make sure you get at least some needle deflection on the tacho. If you don’t get any, experiment with the value of the pulldown resistor. This is easily done if you use a 10kW pot wired as a variable resistor and adjust it up and down. If there’s still no joy, try increasing the voltage going to the reed switch. Finally, if you still get no needle movement, add a second magnet directly opposite the first so that there are more pulses per revolution. Once the needle is registering something, run the machine at its fastest speed and try adjusting the on-board pot to get full-scale deflection. If you A reed switch from a speedo was initially used but some dummy bent a lead too close to the glass envelope and broke it! Another salvaged component was then used instead – the reed from a float switch. siliconchip.com.au Silicon Chip Binders REAL VALUE AT $14.95 PLUS P & P The rear of the modified assembly. At left is the pot providing the +5V reed switch supply, at top-right is the new smoothing capacitor, and at right is the PC-board from the original tacho. The 33kW pull-down resistor is just visible at far right and connects the signal input to ground. can’t get there with the built-in adjustment, add more magnets to the shaft of the drill-press or lathe. Calibration If you know the fastest and slowest speed of the machine, you can calibrate the scale to those revs – everything in between will then fall into place since the meter response is linear. You can use your PC and a printer to make a new scale. First, scan in the original car tacho scale and use image manipulation software (eg, Photoshop or Paintshop Pro) to alter the numbers and to delete other markings you don’t want. Of course, at this stage you can also add whatever labels are suitable. Finally, print it out at full size and it’s then just a case of sticking it over the original. We used clear adhesive film SC to protect the paper scale. Alternative Calibration Another good way of calibrating the unit is to first use a frequency measuring multimeter to measure the speed of the tool. The meter will measure in Hertz (cycles per second), so to calculate the tool’s rotational speed in RPM, just multiply by 60. Note: this assumes that you have just the one magnet on the output pulley. If the meter jumps around a lot, try temporarily adding small value capacitors in parallel with the reed switch to dampen the bounce that occurs when the switch closes. Alternatively, if you have a scope, it’s ideal for reading the frequency. Rat It Before You Chuck It! Whenever you throw away an old TV (or VCR or washing machine or dishwasher or printer) do you always think that surely there must be some good salvageable components inside? Well, this column is for you! (And it’s also for people without a lot of dough.) Each month we’ll use bits and pieces sourced from discards, sometimes in mini-projects and other times as an ideas smorgasbord. And you can contribute as well. If you have a use for specific parts which can siliconchip.com.au easily be salvaged from goods commonly being thrown away, we’d love to hear from you. Perhaps you use the pressure switch from a washing machine to control a pump. Or maybe you salvage the high-quality bearings from VCR heads. Or perhaps you’ve found how the guts of a cassette player can be easily turned into a metal detector. (Well, we made the last one up but you get the idea . . .) If you have some practical ideas, write in and tell us! These binders will protect your copies of S ILICON CHIP. They feature heavy-board covers & are made from a dis­ tinctive 2-tone green vinyl. They hold 12 issues & will look great on your bookshelf. H 80mm internal width H SILICON CHIP logo printed in gold-coloured lettering on spine & cover H Buy five and get them postage free! Price: $A14.95 plus $A10.00 p&p per order. Available only in Aust. Silicon Chip Publications PO Box 139 Collaroy Beach 2097 Or call (02) 9939 3295; or fax (02) 9939 2648 & quote your credit card number. Use this handy form Enclosed is my cheque/money order for $________ or please debit my  Visa    Mastercard Card No: _________________________________ Card Expiry Date ____/____ Signature ________________________ Name ____________________________ Address__________________________ __________________ P/code_______ March 2005  91