The requirement

The boat engine does 500rpm at tickover and 2500rpm at full throttle and drives the propeller through a 2:1 reduction gearbox, so the propeller rotates at between 250rpm and 1250rpm, and it was propeller rpm I wanted on the boat's instrument panel.  Converted to frequency, these rotational speeds become approximately 4Hz for 250rpm and approximately 21Hz for 1250rpm. So the requirement electronically was to derive an input from a shaft rotating at between 4Hz to 21Hz and convert this input into a readout of 250 rpm to 1250 rpm.  Now, OK, there is a standard circuit using the DATEL DMS-30PC-1 3½ digit 7 segnment LED module and the 14 pin LM2917 tacho chip, (right), which will give you a ready-made 2000rpm tachometer with a direct reading input, (Freq in of 33.33Hz gives an output reading of 1999 (rpm)).  However, the DATEL module is quite expensive at around £45 and you may have difficulty finding the 14 pin version of the LM2917, which seems to have given way by and large to the 8 pin version, so I decided to do my own thing.

Generating the input

There were only two practical ways to take an input from the rotating propeller shaft: a magnetic pickup or an optical pickup.  Both devices were readily available from mail order suppliers with the Omron range of reflective photomicrosensors, (left), coming just a little cheaper than the magnetic pickups, (right), but both at under £10.  However, the bilge of a boat where the input device was to be mounted is quite an unfriendly place for electronics, so I decided that an optical solution was not very practical down there, and went for the more robust magnetic pickup solution.  The magnetic pickup requires a piece of ferrous metal to pass it in order to produce a pulse output, and I could achieve this easily by attaching a small piece of steel to the coupling between the propeller shaft and the engine with epoxy resin or one of the 'liquid metal' cements, and mounting the pickup within a few millimetres of these 'poles'.

The output

Like generating the input there were two practical ways of presenting the output: an analogue display or a digital display.

The analogue display.  This route was by far and away the easiest.  The LM2917 frequency to voltage converter chip, or Tachometer chip as it's known, was built for the purpose and would provide a very simple output to a moving coil meter which could be calibrated in rpm.  The difficulty here was that the basic LM2917 chip gives an output of 1 volt per 67Hz of input, so my input of 4Hz to 21Hz would give me only 0.06v to 0.3v of output to work with - not much.  The answer to this was to attach more 'poles' to the propeller shaft and I concluded that 8 poles would increase my input frequency sufficiently to give me an output from the LM2917 going from about 0.5v to 2.5v, (I'll let you check my arithmetic to arrive at that rather than give it here), so that a 1mA FSD moving coil meter with a 5kR trimmer in series would allow me to set the FSD of the meter to 1250 rpm.

The digital display.   The digital display solution was a more robust one for the environment it was going into, but more involved electronically.  As with the analogue solution, there were ready-made digital devices to do the job, namely, the 74LS90 BCD counter chip and the 74LS47 seven segment display driver chip. These could be configured to drive a 7 segment, 4 digit LED display to show the output frequency, but there were difficulties here too.  This arrangement would display the actual input frequency, so the 32Hz to 168Hz frequency spread I achieved from the 8 pole magnetic pickup was awkward to deal with and would require a frequency multiplier at the input to the BCD counter to make the digital display right.  The thing to do here ideally would be to attach 60 poles to the propeller shaft so that the pickup produced the same number of pulses per second as the rpm of the shaft, (240 rpm = 4revs/second, 4revs/second with 60 poles = 240 pulses per second). The coupling from the propeller shaft  to the engine was big enough to attach this number of poles to, but the thought of doing it didn't appeal to me much.

Decisions decisions!

I eventually decided to try the analogue solution first, mainly because I had all the components in my workshop. The logic here was that if the moving coil meter failed to withstand the environment of an open fishing boat with the salt air, high frequency engine vibration and lower frequency shock pulses of the boat slamming into heavy seas, then I hadn't lost much and could turn to the digital solution without changing the input device.

The workshop phase

I was lucky enough to have an old sewing machine motor and foot pedal control lying around, so I pressed it into service for my tachometer test bench.  There was no way to attach 8 poles to the small pulley on the output shaft of the sewing machine motor, so just one sliver of ferrous metal had to suffice.  The magnetic pickup was then mounted close to this single pole, the LM2917 was mounted on a bit of veroboard with its components, the meter was connected, and an oscilloscope was connected across the magnetic pickup output to measure the input frequency, (unfortunately I haven't got a frequency counter).

The test bench setup circuit diagram, above, looked like this in practice, (right), not a pretty sight but functional, and you will notice that I used my analogue voltmeter to display the output rather than the arrangement in the diagram.  The LM2917 performed exactly as expected and gave a nice smooth output which increased linearly with input frequency.   So everything was working and the only thing which remained was to rig up a simulation of the boat in the workshop, namely an 8 pole input, so that the tachometer could be calibrated in the workshop before being transferred to the boat.

To cut a long story short, I used a variable speed electric drill with a circular grindstone in the chuck to simulate the boat.   I glued 8 poles of ferrous metal to the grindstone, and had a mate work the drill while I marked the meter scale.  This arrangement worked well and I was able to calibrate the meter quite accurately, (well as accurately as I needed for a fishing boat), from 200rpm to 1500rpm.  This transferred to the boat very well, and when I checked the calibration on the boat using a borrowed stroboscopic tachometer, it was as accurate as I could hope for, and all for less than a tenner.

This was an interesting little project, and I hope it proves to be of interest to someone.