Controlling an analog automotive tachometer

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)!

https://www.fairchildsemi.com/application-notes/AN/AN-8208.pdf is a good read nonetheless

The Tone() library cuts out around 30hz so that is likely why you are having issues at that level.

Bill

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.

tone(9, 29); gets it up to 1500 and tone(9, 14.5); seems like it's maybe half, but other multiples don't work. Any ideas?

Switched to a 2SC1624 with a 1k resistor to the Arduino PWM and I can get it up to 3000 now, but can't reach 4000 or 5000.

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.

I’m surprised you can fry a 2N3904 with a current limited pwm signal at the base and 10mA max on the collector - how exactly are you wiring this ? ( please post a diagram)…

Some modern revcounters ( and other analog readouts) use quadrature meters - these have 2 coils at right angles. You need to drive them with sin and cos of the required angle of the meter.

Rather different…

I’ve done this for a revcounter on my daughter’s boat using the ripple of the alternator as the input signal , and a pro-mini with a bit of signal conditioning. Works fine.

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 .

A bit like the ancient Smiths chronometric speedos from the 1950’s.

Again, rather different…

regards

Allan

This gauge is vintage, not modern, pretty sure it’s off of a diesel power boat and is marked as 8 cylinder. You can see the exact same gauge here,

You can see the gauge being driven by the circuit here,

https://www.instagram.com/p/BOA7U58hUDo/

The circuit is attached. The gauge has a ground and lead for the lamp and two leads for the signal. Thanks.

gauge_circuit.jpg

You'll need a current limiting resistance in series with the movement. Your direct connection probably explains why you blew up 2N3904's.

And may not have done the meter itself much good ,through overcurrent

And you will probably need a regulated + 12 or other voltage.

Measure the meter resistance with an ohmeter - what does it say?

regards

Allan

You'll need a current limiting resistance in series with the movement. Your direct connection probably explains why you blew up 2N3904's.

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.

Does that mean I should be running between one of those gauge leads and the gauge lamp ground instead?

Hooked up to the boat's control panel this gauge had a yellow wire running across both of the leads and a white to both lamp leads.

See the enclosed.

The value of R2 depends on the meter sensitivity ( how many mA for fullscale ) and it’s own internal resistance, and the supply volts…

regards

Allan

meterdr.pdf (18.7 KB)

allanhurst: The value of R2 depends on the meter sensitivity ( how many mA for fullscale ) and it's own internal resistance, and the supply volts.

When I add R2 the only time I get meter movement is when the resistor starts to smoke.

Then your meter is faulty - see my post 26

I’d expect it’s resistance to be a few hundred ohms - not <1.

regards

Allan

allanhurst: Then your meter is faulty - see my post 26

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.

I came across this,

http://www.4crawler.com/Diesel/CheapTricks/Tachometer/index.shtml

Where the fellow seems to only have one signal out to a deisel tach, I'm assuming the ground was that just left out of the diagrams?

Hmmm....

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

Allan.

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

I found the linked PDF which is about repairing this company’s gauges, page 20 talks about tachs. I’ll have to digest this. The back looks like the other attachment.

http://faria-instruments.com/site_manuals/IS0100_L_Warranty_Repair_guide.pdf

Two long screw terminals, which are also used for mounting. When I removed these from the boat panel these terminals each had a yellow wire attached.

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.

Operation

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).

Application

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.

a) The alternator tachometer: which is also called a variable ratio tachometer is hooked up to the AC output terminal on the alternator. This terminal can be tracked in a variety of different ways: AC. AUX., S, R, TACH or nothing at all. Once installed, the tachometer is then calibrated to that specific engine by using a shop tachometer or a known “no load” governor speed. The white label on this tach gives the formula: [Crankshaft pulley diameter divided by the alternator pulley diameter times the number of Alternator Poles = N]. “N” is used to determine the correct switch setting. Another adjustment on the back allows for fine tuning.

b) The Switching Diesel Tachometer: is hooked up to a tachometer signal generator which is spun by the engines’ mechanical take-off. One of the signal generator’s wires is grounded to the engine and the other is connected to the tachometer’s signal terminal. The white label on this tach is marked: 1/2:1, 1:1, 1.5:1, 2:1, which corresponds to the different mechanical take-off ratios.

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.

Calibration

Set up a calibrated “shop tachometer” or “strobe tachometer” to monitor the engine’s true RPM. Start the engine and (after an appropriate warm-up period and with the shift in neutral) increase the engine speed to the boat’s normal cruising RPM read on the shop tachometer. Set the coarse adjustment switch to the proper position described on the label. Remove the stop-plug or paper label corner (at the 8 o’clock position on the rear of the case for most) and insert 5/64” Allen wrench into the “Fine adjustment” trim pot, rotating it CW or CCW as necessary to indicate the true RPM.

$_12.JPG

I can't be 100% sure it's off of a diesel, could be gas. The sticker on the gauge up on eBay, which looks exactly the same reads,

Connects in series with the ignition switch so coil current flows thru tach.

OK - I was wrong.

It isn't a straight analog meter.

Quote from the faria instruments datasheet.... who seem to be professional respected providers.

'Tachometers will zero when the key is turned on. It doesn't matter what the tach reads with the key off.'

A straight analog movement would return to zero with it's spring.

So this must be a modern quadrature or pulse counting type movement as described in my earlier post #24, driven by internal electronics.

  • either way you can't drive it with PWM from an arduino.

Only with a variable frequency signal of various possible types..

regards

Allan

'

Seems like a square wave of some sort should do it. Perhaps if I switch to a 555 circuit.

To quote you quoting faria ...

Connects in series with the ignition switch so coil current flows thru tach.

Implies this is for a gas engine, detecting pulses from the ignition coil....

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

I've got confused over the thread - is this for a vehicle? if so does it have an old fashioned contact breaker/ coil type ignition?

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?

regards

Allan

is this for a vehicle? if so does it have an old fashioned contact breaker/ coil type ignition?

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.

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?

The wires are all cut, however there are two yellows coming of each ignition switch which could have been the two yellows that went to each RPM gauge.

The sensor leads on the gauges are open loop to the lamp power and ground on the gauge.