Outboard Tachometer Application Chart
Germaine Greenweig
The purpose of the tachometer is to measure the rotational speed of a machine. For example, within the automotive industry, a tachometer is typically displayed on the dashboard of the car. This informs the driver when to shift gears taking the rotational speed of the shaft into account.
Although there are various types of tachometers, most instruments work in a similar way. They can be categorised in several different ways, but the key types to be aware of in terms of operation are electronic and mechanical tachometers.
Most modern tachometers are electronic instruments featuring numerical readings and data storage capabilities. With electronic tachometers, the ignition system triggers a voltage pulse whenever the spark plug fires, at the output of the electromechanical part.
In turn, the electromechanical part corresponds to the average voltage of this series of pulses. It shows the average voltage of the pulse train as directly proportional to the engine speed. The signal is then transmitted to the indicator via a twin screened cable and displayed to the user via an LED or LCD readout.
There are several main types of tachometers, each best suited to different uses and applications. In addition to electronic and mechanical tachometers, as explained above, the two principal types are contact and non-contact tachometers. These can then be broken down further as detailed in the sections below.
Contact tachometers require contact with a rotating object to measure its RPM. They work by using an optical encoder or a magnetic sensor, depending on the particular model. Direct contact with the shaft or rotating component must be maintained in order to achieve an accurate reading.
Non-contact tachometers can perform measurements without the need for direct contact with the object. They commonly work with lasers or infrared light and are ideal for use with hazardous, hard-to-access, or mobile objects.
Laser tachometers feature a laser on the end of the device which can be pointed at a rotating object to measure RPM via a direct reading function. They are a type of non-contact tachometer and are ideal for use in difficult to access areas, such as tight spaces or working with industrial machinery. Laser tachometers feature varying detecting distances depending on the particular model, but 1-2m is standard.
Optical tachometers include an optical sensor which is used to measure the rotational speed of the wheel, motor, or shaft. They use a laser or beam of light to measure the RPM and are also able to measure any differences in frequency of the reflected light.
A digital tachometer is relatively simple and straightforward to use. With a non-contact electronic tachometer, operation can be completed simply by pointing the instrument at the motor or engine. These instruments typically feature a laser on the end of the tachometer for this purpose. Once pointed at the turning object, the device will measure the RPM (revolutions per minute) and display the results for the user.
Various tachometer accessories are also available to assist with installing and working with these devices. Accessories can include measuring wheels, adapters, and fittings, and they are designed to work with tachometers to provide a comprehensive solution.
Installing a tachometer to a coil can be a useful way to monitor performance by gauging engine RPM within a vehicle or automotive environment. The process of wiring a tachometer to a coil is as follows:
As this task involves electrics, it is highly recommended to only carry it out if you are qualified and fully knowledgeable about the process. An expert auto electrician will be able to install a tachometer to a coil within a vehicle on your behalf if necessary.
Tachometers used within vehicles allow the driver to choose the most suitable gear and throttle settings for the current conditions. They are beneficial for optimising performance and protecting the engine from overheating, insufficient lubrication, and causing unnecessary wear and tear to components within the system.
Tachos on a lot of older (pre 1990's) vehicles such as mine take a feed from the low tension side of the ignition coil and the waveform of the 0v/12v is a function of the type of coil and ignition controller being used (optical, points or points assisted), you may find some feed 0/12v when the starter is turning and 0/9v running (ballasted coils)!
I'm attempting to accomplish the same thing here with a Faria 5000 RPM 8 Cylinder Marine Gauge, however I can't get the gauge to read more than 1500. I'm using a 2SC3503 transistor and a 4148 diode. I've tried a number of sketches using tone(), analogWrite() and digitalWrite() with delayMicroseconds() but I've not been able to get the gauge higher than 1500.
I've fried a couple 2N3904's already. If I lower the resistor value between the PWM and the base to 680ohms I can get the gauge to max, but I think it's frying the circuit in the process. With 1k things seem stable but I can't max the gauge.
Another sort have a similar quadrature motor, but then a gear train to the needle spindle, such that so many pulses from the '0' position are required to get the needle to the required position - ie a little stepper motor. 1/4 turn per step.
This sort are driven negative for many pulses at start-up to get the meter to a known '0' position against a mechanical end stop, then up/down as required from there to get the wanted display position .
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You mean between the transistor and the gauge itself? After the diode or the other lead? Resistance of the gauge leads is nil -- or less than 1ohm. That's the same as another of the exact same gauge I've not touched.
I have another of the exact same gauge which exhibits the exact same behavior, even though it's only been tested once with R2 in place, so had no potential for overcurrent. I don't think the gauges are damaged as they both climb from 0 - 3000 when I used the other transistor, no R2 and the tone() function. However the only way I've gotten it over 3000 is to lower the R1 value which seems to destabilized/slow short the circuit.
Could you post a full datasheet on the meter you have. And pictures of it's conections ( how many?) and any instructions on a sticker.... it may well not be a straight analog meter, but have some internal electronics
The circle is the lamp which has a clamp terminal to the casing of the gauge and one into the lamp. These both had white wires attached. I have tested that 12v to the lamp leads does turn the lamp on.
Electronic tachometers work by counting pulses generated by the ignition system, alternator, tachometer signal generator, or magnetic pickup sender. The tachometer is hooked up to +12 VDC, Ground, and one of the signal sources listed above.
By selecting the right tachometer and setting the switch on the back to the correct position, you let the tachometer know how many pulses are sent per each engine revolution. From this information, the tachometer displays the correct engine speed. See Appendix I for tachometer dimensions. Instrument part numbers are located on a label attached to the outside of the case (i.e. TC000A).
4 cycle engines: The tachometer signal terminal is connected to the negative terminal on the ignition coil or to a transistorized tachometer driver circuit connected to the ignition system. This circuit will have a wire (usually gray) for connection to the tachometer. The correct tachometer will have a white label on the side indicating which switch position is for each engine type. This label will include 4, 6, and 8 cylinder engines for positions 1, 2 and 3.
Outboard engines: The tachometer signal terminal is usually contacted to the unrectified AC output of the alternator/lighting coil. Sometimes it is hooked directly to the stator output wire (usually yellow) other times a gray tachometer output lead is provided. The correct tachometer for this application will have a white label on the side with switch positions 4, 6, 8, 10 or 12 pole alternators. The number of poles on the alternator can be determined by checking the Faria Outboard Tachometer Application table. (See Appendix V)
Diesel engines: The tachometer signal terminal is hooked up to 1 of 3 things: a) the alternator, b) a tach signal generator that is spun by the mechanical take-off, or c) a magnetic pickup sensor which counts gear teeth.
c) The Mag Pickup Tachometer: hooks up to a magnetic pickup sensor which counts gear teeth. Here neither of the wires is grounded to the block. They are both routed up to the tachometer as a twisted pair. One hooks to the signal terminal and the other to the ground terminal on back of the tachometer. The switch is set to the approximate number of teeth that the sensor sees on each engine revolution. Another adjustment on the back allows fine tuning to the exact number of teeth. The label is marked in ranges generally from 30 to 160 gear teeth.
So you'll need several amp fast pulses to drive it ... there's probably an internal small pulse transformer to extract this. And yes eg a 555 could in principle provide this. And an arduino better, with a muich larger frequency range
there's a way to simulate this in hardware otherwise ..... but are the sense wires you're connecting to totally isolated from all other connections? - to confirm my guess of an internal pulse transformer?
I got a pair of them, along with a bunch of other gauges (fuel, pressure, amps, water temp) from a boat console I salvaged, twin engines, I don't see any switches for glow plugs and I do see a switch for a blower which I guess tells me it's probably gasoline rather than diesel as I had first thought.
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