Hello my name is Tim. Im in need of some help. I apologize if this is not the right place.
So my father in law had his boat restored at a local marina and they replaced the guages on the dash. The tach they installed is a sierra 68399p. The problem is the motor is a 1986 force 125hp. It has a 20 pole stator. The Guage says to take the number of poles and divide it by 2 and adjust the tach to that number. The tach only has options from 2 to 6. I currently have it set on 5 and the tach reads double the RPM. Is there a way i can make a circuit to divide the number of pulses to get this to display correctly?
If You can present the electrical characteristics of the signal, a divide by 2 ought to be easy.
Is an oscilloscope available?
The technical manual of the motor might tell.
Maybe a DMM in AC would give an indication.
This can be done with a flip flop IC such as the 74HC74. Hear is a simple circuit.
You will have to connect the preset and clear signals, not shown, to there inactive state.
@Railroader Im not sure about characteristics. But ik how its weird if that helps. There are 2 wires coming from the stator to the first cd box for cyl 1 and 2 and 2 wires coming from the stator to the cd box for cyl 3 and 4. And 2 wires going to the rectifier. One of those 2 wires has a purple wire attached to it at the rectifier goes to the input on the tach.
I do not have an oscilloscope. But i do have a mac tools em721 DMM. I havent used it in AC so not sure what to set it for to test this.
@gilshultz im alittle confused by You will have to connect the preset and clear signals, not shown, to there inactive state. How do you do that? Besides that. Im assuming you put this inline? CK would be from the motor Q would be to the tach?
You look at the data sheet and if it shows for example clr with bar over it or a circle where it enters the chip that indicates it is active low so you would tie it high. If there is no bar or circle when the pin enters the chip it is active high. https://www.ti.com/lit/ds/symlink/sn74hc74.pdf Look at page three.
@gilshultz. First off sorry for the terrible artwork lol. This is where im at for understanding. (At least i hope im right) where would the clr pin connect to?
Pins 1 and 4 need to be high (inactive state). The HC parts are 5V parts. There is 74C74 and maybe some in the 4000 series that will withstand the voltage. You can always use a voltage divider for the inputs. None of the parts will withstand the transients in the vehicle over time. The signals will need to be conditioned if you want it reliable.
Again really sorry im new to parts like this. So 1 and 4 need to be high. That means what? Jump it to vcc? And by 5 volts you mean the vcc needs to be 5v?
They are very sensitive to the magic glitch. If they are not anchored they will float between on and off and noise will also trigger it. That in turn will either turn the latch on or off referencing "Q". When it does that it will add a pulse.
Lots of information, maybe too much.
If you don't have access to an oscilloscope, I suggest you perform a quick "proof of concept" test.
You can get CD4013's on Amazon for only a few bucks.
Wire the U1A (see post 14) using a breadboard or even a solderless breadboard. Don't worry about the input filters but don't connect to the vehicle battery until the engine is running. HOWEVER YOU MUST PUT A 5000 OHM RESISTER BETWEEN THE INPUT SENSOR AND THE CD4013. Since I posted #14 I read some on you tach and it seems the input signal goes both positive and negative. Without this 5000 ohm resistor the CD4013 will likely be toasted on the first cycle.
The 74hc74 is a 5v device, you will have to create 5V power and if the tach needs more than 5v for drive you will have to put another device (mosfet or transistor).
Have you asked the mfg if they have any suggestions?
