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Thread: Engine tach pulse division

  1. #1
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    Default Engine tach pulse division

    Does anyone know much about pulse division?
    In a nutshell I bought https://www.ebay.ca/itm/4-LED-Digita...1DAHKJGeLlU5QQ and assumed that it would just work when attached to the TACH wire in a six cylinder engine. Well after blowing the input resistor I realized the TACH consists of six pulses per crankshaft rotation (six firing instances of the coil), so my digital readout is off by a multiple of six. How would I go creating a 6:1 pulse divider circuit to compensate? A lot of the examples I am finding online so far deal with other ratios involved with moving between four, six and eight cylinder engines and other pulse ratios.
    = Excellent space heater

  2. #2

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    It looks like that Tach module has a variable rate setting where you program in a number to control the division rate ?

    If that doesn't work you could use a divide by three circuit with two FF's and gates and then a divide by two with another flip flop to get a divide by 6. One example of divide by 3:

    https://www.theremin.us/Circuit_Libr..._dividers.html

  3. #3

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    Or make a "tickler coil" around any one spark plug wire.

  4. #4
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    According to the Toptop site, you can set the thing up:

    Tachometer Rate Setting Instructions:
    1. Rate range: 1-99(default 01: display value is same with actual value; if the rate is 10, display value*10=actual value);
    2. When power on, the meter display "0";
    3. Press "SET" button, display "1"; press again, display "01"...
    4. When display "1", is measuring RPM; press "ADD", from "1" to "2", when display "2", is measuring frequency; display "01" is for rate setting, short press "ADD" for +1, long press for +10;
    5. After setting the values, it will be stored if no operation in 10s, return the selected function interface;
    6. With 1 decimal point, that is when rotate speed is <1000RPM, the rotate speed corrects to 1st digit after the decimal point; no decimal point when >1000RPM.
    Tips:
    User can set the appropriate rate so as to measure frequency, less than 100KHz (999.9*99=98990.1Hz).
    Uses an STC 8051-type cheap MCU.

  5. #5

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    Quote Originally Posted by Chuck(G) View Post
    According to the Toptop site, you can set the thing up:



    Uses an STC 8051-type cheap MCU.
    With such clear instructions, how could anyone have a problem?
    Dwight

  6. #6

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    I'm not sure what Chuck's instructions meant but another way to deal with things is to change the crystal frequency to change the reference. You don't need to make any special divider circuit.
    Dwight

  7. #7

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    A 4 bit reloadable counter and a few gates can be made to divide by any ratio from 1 to 16. It needs to have a TC ( terminal count ) pin. There are a few TTL or CMOS parts one could use ( too lazy to look stuff up ). You could then use it with your 12 cylinder engine as well with just a dip switch selector.
    Is it a points or solid state ignition?
    Dwight

  8. #8

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    Solid state.

  9. #9

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    Quote Originally Posted by KC9UDX View Post
    Solid state.
    I realize, many older capacitive discharge used the mechanical points. I consider that to still be points( although solid state firing ).
    Newer cars no longer use points. There are variations, depending on how new. Some have magnetic pickups in a distributor, some have multiple coils, some have crankshaft pickups and some sync these with cam shaft pickups ( some don't ). These that don't have points I'm lumping together as solid state.
    If it doesn't have points, it has to be some form of solid state.
    If it has points, it usually charges the coil and uses flyback voltage to create the spark. The primary can then spike from 300V to 600V someplace. The input circuit needs to be protected from that. If it has points but also a capacitive dicsharge, and you are taking from the points, you don't have to protect it from the high voltage. If taking it from the coil, it will have high voltage. Most all forms of solid state use a capacitive discharge. In this case, it may or may not still have a distributor. He'd be taking the signal from the coil. It would be the same as from the points as it would be about 300V to 450V someplace. It needs a different input circuit. Most new cars use a separate coil for each sparkplug and are also capacitive discharge.
    He stated he was burning out an input resistor. That is why I was asking about where he was getting the signal from.
    Dwight

  10. #10
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    The thing uses a commodity 8051 type MCU and I suspect the setting instructions reference setting a divisor. As far as the input goes, trace the circuitry; it doesn't look to be terribly complicated.

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