Signal boost circuit

Discussion in 'The Projects Forum' started by bobtooke, May 24, 2016.

  1. bobtooke

    Thread Starter New Member

    May 24, 2016
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    Hi, just joined the forum for a little advice.

    I've been involved in a few little projects in the past but to be quite honest, my electronic theory is very limited.

    Currently, I'm trying to get a tachometer on my boat to work and looking for some basic advice on boosting the signal.

    The tachometer works on a pulse from the ignition that is made by an inductive copper wire coil wound around the HT lead.

    This arrangement produces a pulse of around 2.5mV at a current of about 1.5mA. However, because it is a long run to the helm panel, by the time it arrives the signal is barely detectable. Because of this the tachometer doesn't work. My test meter will only go down to about 0.1mV and it is certainly less than this. I tried using a screened cable but this didn't seem to help.

    So, what is the best way to boost this signal? I was thinking maybe a pull up circuit of some type? Or can someone offer another suggestion?

    Thanks in advance. Bob.
     
  2. crutschow

    Expert

    Mar 14, 2008
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    Can you increase the number of turns of the coil wound around the HT lead?
     
  3. bobtooke

    Thread Starter New Member

    May 24, 2016
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    Yes I tried that but running out of space on the HT lead! I got up to about 20 turns of wire but despite doubling the number of turns, it didn't seem to have much of an effect on the mV output.
     
  4. crutschow

    Expert

    Mar 14, 2008
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    How did you measure the 1.5mA of current?
    What is the load provided by the tach?
     
  5. bobtooke

    Thread Starter New Member

    May 24, 2016
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    The current was measured between output and ground when disconnected from the tacho.
    Load on the tacho? Not sure if there is a measurable load as such, it just has a pulse signal input.
     
  6. bobtooke

    Thread Starter New Member

    May 24, 2016
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    The unit is similar to this:
    http://www.enmco.com/pdf/664.PDF
    Mine didn't come with a shielded cable.

    I should also mention the fact that the length of the signal cable is around 5 metres.
     
  7. crutschow

    Expert

    Mar 14, 2008
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    What instrument did you use to measure the current?
    By tach load I meant what is the input impedance of the tachometer? You can measure that with a multimeter.
    Is it a 2-wire cable with one of the wires connected to common at both ends?
     
    Last edited: May 25, 2016
  8. #12

    Expert

    Nov 30, 2010
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    How about this?
     
  9. bobtooke

    Thread Starter New Member

    May 24, 2016
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    I used a simple multimeter to measure current.
    The tach has a single wire so measuring impedance may be awkward?

    When I tried a shielded cable I grounded it to the engine and not connected at tach end (nothing to connect to).

    I guess I could try and get a ground from the -ve of the battery to measure the impendance? Since its not connected under normal use, would that help us?
     
  10. bobtooke

    Thread Starter New Member

    May 24, 2016
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    Umm... maybe?
    Can you tell me any more about the particular circuit you posted and how it operates?
     
  11. #12

    Expert

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    It's a low voltage, high impedance amplifier chip set to a gain of 40 v/v. It's internal error budget is about 0.001 volts and your signal is 0.0025 volts. The capacitor on the output blocks any DC level caused by the internal error (up to 0.04 volts) and passes pulses from your signal, amplified to 0.1 volt. The 1k resistor on the output protects the chip from shorted or corroded wires on the boat. It should drive the tach cable correctly up to about 30,000 pulses per second.

    I think the output capacitor might be too large. If the output starts accumulating a DC voltage at high revs, the capacitor needs to be smaller. Basically, it should be the smallest amount of capacitance that will respond correctly at idle speed. The 15k resistor is supposed to drain off the DC between pulses. If it doesn't, the capacitor is too big.

    Of course the pick-up coil must be oriented to deliver positive pulses to the amplifier. If nothing works, the first thing to try is to reverse the coil.
    The 39k and the 1k set the gain.

    I am imagining a co-ax cable connected to the pick-up coil. A twisted, shielded pair would work, too. Bring that to a metal box which contains the amplifier. The output cable isn't near as sensitive so it can be any old wire you have laying around.

    In the upper left, the 1k and 5.1 volt zener diode clean up the power line and provide proper voltage to the chip down to 9 volts on the battery.
    The two capacitors further clean up the power and provide local current pulses for the chip when it needs to do a fast pulse.

    I don't imagine any high voltage interference trying to get in through the pick-up coil, but if that's a problem, you could add up to 100k ohms in series with the input to protect the chip. I would use 10k for an input protection resistor like this:
     
  12. BR-549

    Well-Known Member

    Sep 22, 2013
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    Why not try 3 or 4 turns on HT wire?

    And, is the HT wire cabled? i.e. shielded, armored?
     
    Last edited: May 25, 2016
  13. #12

    Expert

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    You're supposed to read the posted information before asking for the same information.
     
  14. crutschow

    Expert

    Mar 14, 2008
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    That's likely your problem.
    With only one wire you don't have a complete circuit (to carry an electrical signal always requires two connections).
    The original cable to the tach had a shield tied to the tach, likely the tach common or case.
    As well as being grounded at the engine, you need to tie the shield of your new wire to wherever the original shield was tied on the tach.
     
    shortbus likes this.
  15. #12

    Expert

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    Connecting the wires properly sure beats building an amplifier.:)
     
  16. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    I'm not convinced that a wire coil around the HT lead as shown in the link provides efficient pickup of energy. The magnetic flux lines due to the HT lead current circle the HT lead, but ideally shouldn't they instead pass axially through the wire coil? If it were me I'd experiment* with a different pickup in the form of a current transformer comprising e.g. 100 or so turns of magnet wire on a toroidal core made of florists' iron wire or similar, with the HT lead passing through the toroid.
    [* Actually that would be difficult. The HT leads on my car are inaccessible. :D]
     
  17. #12

    Expert

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    A long time ago I built a rig to display my car ignition on a 'scope. One pick-up was nothing more than a clothes pin drilled to accept a wire and that wire was held close to the spark plug wire by the spring pressure of the clothes pin. No turns at all, just a contact point. It worked.

    The need for coupling is related to the quality of the tachometer.
     
    Last edited: May 26, 2016
  18. BR-549

    Well-Known Member

    Sep 22, 2013
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    Has no one here ever looked at their ignition coil secondary with a scope?
    It only takes one or two turns for plenty of signal.

    And when I here that he has 20 turns.........................I do not assume he tried just a few turns first.

    It was post #9 before we found out shield was not connected.

    Don't assume anything.
     
  19. bobtooke

    Thread Starter New Member

    May 24, 2016
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    Thanks all for replies, please don't get spiky over my ignorance. As I said, limited knowledge but I am trying... I'm a mechanical engineer so limited electrickery know-how.

    Yes as per instructions I tried 3/4 turns first then kept doubling them until I got to about 20 turns when I ran out of space on the HT lead. I agree my earlier post wasn't very clear on that.

    In respect of the shielding, I cannot connect shield at tach end. As I said there is no shield or shield connection on my tach unit. It just has a single wire dangling from the resin sealant at the back.

    Like the impedance test, the only way to get a 0v at the tach is to get to the tach battery -ve. Maybe this as a shield connection?

    But does it make sense to connect both engine -ve and tach battery -ve to either end of shield? Would that not create ground noise at the tach?

    On this basis, I am a tad worried the little amp may suffer the same fate were I to build it?

    With the single wire (1sq.mm copper multistrand), the tach seems to work fine up to about 6ft from the engine but then the signal starts to fade and by the time its about 10-12ft away, only a trace remains and the tach non functional.
     
  20. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    The case of the tach is more than likely the common/ground of it's internal circuit. If the boat has a fiberglass dash you need to connect some type of wire to the tachs case and the engine, to have a complete circuit, this would also be true if you make an amplifier for the tach signal.
     
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