strange 555? tacho circuit

Discussion in 'General Electronics Chat' started by cornishlad, Aug 25, 2015.

  1. cornishlad

    Thread Starter Member

    Jul 31, 2013
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    Hi...I bought 3 tachometers from ebay. Described as non functioning samples and I was primarily interested only in the meter movements. The scales are marked TVR - a |British classic car.
    However I opened one and found the pcb (photo). The 8 pin dil chip has had its ID scaped off and I can only assume the it's a 555 and have drawn out the circuit and applied it to that. It doesn't look anything like the common monostable circuit with an integrating meter on the o/p - pin3 And I can't see how it's supposed tp work or even if it would ! Anyone seen anything like it or got any ideas ?

    EDIT go to end - not a 555,,but SN76810P !
     
    Last edited: Aug 25, 2015
  2. alfacliff

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    Dec 13, 2013
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    the 555 provides uniform pulses that are integrated into a dc voltage for the tach. normal ignition pulses are of varying lengths, and dont work as well.
     
  3. cornishlad

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    Jul 31, 2013
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    I've got a working version like that in my old Austin.
    Are you saying you can see how the attached circuit works the same way ?
     
  4. alfacliff

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    it works pretty much the same, squares up and sets the ignition pulses to a standard length so the integrator can charge up propely. older tachs not cleaning up the pulses were a bit unsteady.
     
  5. cornishlad

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    Jul 31, 2013
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    would you say that the time constant components are the supply line 47uf and the series 120 resistor ?
    With all that filtering on the input would you assume that it took a feed from the CB coil contact ?
     
  6. alfacliff

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    input comes from the points. low voltage side of the coil.
     
  7. AnalogKid

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    Aug 1, 2013
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    Nice job working out the schematic, but it makes it clear that the chip is not a 555 or 393. 7 is Vcc, 1 is GND, 5 is an output.

    BTW, where did the 555 internal schematic come from? Link?

    ak
     
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  8. cornishlad

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  9. ronv

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  10. cornishlad

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    Jul 31, 2013
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    Definitely not that one ! Nothing fiited when I tried that in my drawing ! However the mystery is now solved :)

    I have opened the other two tacho - not for the faint hearted as it means sawing through the rolled over front bezel,
    The other two circuits appear to be a re-designs as there are some differences, But the chip are clearly marked in these.
    texas I SN76810P http://www.datasheet-pdf.com/datasheet-html/S/N/7/SN76810P_TexasInstruments.pdf.html

    Now to draw it all out again ! 2 hours with a magnifying glass last time
     
  11. ronv

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    Cool. Good luck.:)
     
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  12. alfacliff

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    the flipflop turns the ignition pulses into square waves with a 50-50 ducy cycle by deviding the pulses by 2 that squares up the pulses into the integrator. the 555 does the same thing by triggering on the ignition pulses and giving constant time pulses to the integrator, making the pulse spacing and quantity average out on the iontegrator.
     
  13. cornishlad

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    Jul 31, 2013
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    I suspect these chips are obsolete, The TVR may have had 6 cylinders so I would have had to open them anyway to tweak/modify..

    This makes a lot more sense >>> SN76810P tacho.jpg
     
  14. ronv

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    I see some on e-bay 6.4 pounds.
     
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  15. Dodgydave

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    Jun 22, 2012
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    For 4cyls, to test the tacho, input a 50hz low voltage of 12v AC from a mains transformer, into the trigger wire 2, and ground 1, and set the preset resistor for 1500Rpm, or 60Hz AC and set for 1800 Rpm, you should be able to measure the signal with a DVM at the output pins 5-7.

    For 4cyls 50Hz set to 1500rpm
    For 6cyls 50Hz set to 1000rpm
    For 8cyls 50Hz set to 750rpm
     
    Last edited: Aug 25, 2015
  16. cornishlad

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    Jul 31, 2013
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    Thanks for that idea. I'll give it a go..I take it you are assuming a 4 cylinder engine.
     
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  17. Dodgydave

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  18. ian field

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    The 76810 datasheet is getting hard to find, this is the best example I could get:
     
  19. cornishlad

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    Jul 31, 2013
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    @Dodgydave ...Thanks for the updated calibration info. And the circuit of the bottom of the pdf seems to be exactly the same although the chip has a different name. (MCI7/C) Can't find details about it except as a seatbelt timer in an American car - but there it is shown as a 16 pin device. The whole pdf was an interesting read though.

    @ ian field Thanks Ian but I had already found that....in several places ! But no original even on TI site. That's what made me think it was obsolete. As it seems to date back to the 1970s it's not surprising.
    It didn't take much re-drawing because it's pretty much the same as the data sheet application example. As I've done it I attach it here.. SN76810P tacho complete.jpg
     
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  20. ian field

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    Oct 27, 2012
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    It must've been about the 70s/early 80s I worked for a firm that used the 76810.

    It was a member of the Stuart Warner group, they made dashboard "clocks" for busses and other commercial vehicles.
     
    Last edited: Aug 25, 2015
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