1. Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
    Dismiss Notice

Comparator help please -tach/shift light project

Discussion in 'Automotive Electronics' started by mojozoom, Jul 9, 2018.

  1. mojozoom

    mojozoom New Member

    Joined:
    Jul 15, 2012
    Messages:
    28
    Likes:
    0
    My old car uses a 1mA current driven tach movement and the stock circuit isn't compatible with the MSD ignition box I use, and the adapter they sell to make the tach work buzzes like the dickens.

    So I worked up a tach circuit based on the current tach diagrams in the LM2907 datasheet that'll work off the nice 12V square wave tach output connection that MSD provides. To go along with that I'd like to include a 2 stage shift light that lights 2 separate leds at a 400 rpm interval, and when the second led lights I'd like the first to go off.

    I've got that that all worked up using an LM358 as a comparator, but I'm stuck using two pots and I really don't need independent adjustment of the two shift light turn on points - making them fixed at 400 rpm apart would be fine and hopefully minimize components. I was trying to use a voltage divider to fix two reference voltages and compare those to the tach output voltage, but everything I try seems to either work backwards or pulls down the current that the LM2907 is sending to the tach movement.

    Can someone with more experience in these topics please advise if they see a better way to do this? Here's the circuit - ignore diode D2 hanging in the breeze there as it's just like that for troubleshooting. It connects to the - input of U2 above it in order to turn off the first led when the second turns on.

    Thanks!

    tach2.JPG tach1.JPG
     
  2. eTech

    eTech Active Member

    Joined:
    Apr 25, 2012
    Messages:
    646
    Likes:
    73
    Hi

    Probably do this with a single LM339 chip.
    How much output voltage at each of the first and second shift points?
    What is full scale output?

    eT
     
    • Agree Agree x 1
  3. mojozoom

    mojozoom New Member

    Joined:
    Jul 15, 2012
    Messages:
    28
    Likes:
    0
    Ok, the range the 1mA tach movement is reporting is from 0-8000 rpm, and V3 corresponds linearly to RPM and ranges from 0-5.86V with my current set of component values. So V3 = 0.0007 X RPM.


    I'd like to set a target RPM for the shift point, but to have one light to come on at 400 rpm before the target RPM as well. So if you want to use a 5500 rpm shift point, the lights would trigger at 5100 rpm and 5500 rpm, correlating to V3 voltages of 3.57V and 3.85V respectively. Since I'm looking for a constant 400 rpm difference between the lights regardless of the setpoint, then the difference in voltage between the two references needs to be a constant 0.28V.


    Hopefully that all makes sense. I'd like to only have one light trigger at a time, so I suppose it'll either need a window comparator for the first LED or we need something sneaky like the diode setup.
     
  4. dave miyares

    Dave New Member

    Joined:
    Jan 12, 1997
    Messages:
    2
    Likes:
    -10


     
  5. mojozoom

    mojozoom New Member

    Joined:
    Jul 15, 2012
    Messages:
    28
    Likes:
    0
    I should clarify that I'm not 100% sure that the tach movement is 1mA for full sweep, but the original 1958 patent for this 1968 Faria tach (the patent number was actually stamped on it) was based on a 1mA movement. But the patent was also describing a 4000 rpm tach for a 6-cylinder, and I am working with an 8000 rpm tach for a V8. I don't think they'd use different movements though, just different components on the circuit board. On my ohmmeter it measures 725 ohms and that runs the tach up to about 1300 rpm.

    Maybe what I'm trying to do would be easier using an LM2914 in dot mode. Maybe span it over 2.8 volts so each division would be 0.28 volts, and then just use the top two LEDs? Or maybe use it in bar and connect a bunch of the comparator outputs together to one led so anything over a certain point always lights the same led.
     
  6. eTech

    eTech Active Member

    Joined:
    Apr 25, 2012
    Messages:
    646
    Likes:
    73
    Hi

    An LM3914 might be doable. I was going to suggest an LM339 comparator chip. It has four comparators that could be easily used to create one or two window comparators.
    That part it easy. But it also has to change its references according to the selected shift setpoint so the LEDs go on/off correctly.

    How do you intend to set the target RPM? Using a rotary selector mabe?
    You could also have both LEDS flash when the setpoint is selected, then light steady as each is reached. However, it does make the circuit a little more complicated.

    Operation could be something like this, but its up to you:
    1. LEDs dark. Make selection with button(if lighted selector LEDs) or rotary switch for static selection (like a silkscreen image with markings)
    2. Press "go" button. Setpoint LEDs begin to Flash
    3. As each setpoint is reached, each corresponding LED changes to steady lit.
    4. To "restart" (loop back to step 2).
    5. To "finish" Press reset button. All LEDs go dark.

    eT
     
  7. mojozoom

    mojozoom New Member

    Joined:
    Jul 15, 2012
    Messages:
    28
    Likes:
    0
    Yikes - that all sounds very complicated! It'd probably need an Arduino or similar micro for that too.

    For these old cars there's usually very little action going on at the dashboard, so if anything does light up or flash it's usually very very bad. So that's why I was thinking of just a simple shift light to start, and not an F1 bar graph style with an led every 200 rpm or something like that. But it is really nice to have a warning that you're approaching the redline, so two lights it is. The 400 rpm difference is something I chose just based on gut feel, so a little more wouldn't hurt anything and I expect I'll have to tweak that number once it's running on the car.

    My plan to set the rpm for the shift light was to use a switch to divide the setpoint in half in some way, so for a setpoint of 6000 rpm I could flip the switch to "SET" and run the rpm up to 3000, then adjust a pot until the high side light turned on. I suppose that could be done by doubling the input signal voltage, cutting the LM3914 span in half at R(hi), or maybe even switching to different LED outputs for the setting process if that made sense with how this worked. Setting the shift point at the full 6000 rpm on my car would probably have my neighbors calling the cops on me for excessive noise.

    Right now my biggest hiccup is that my simulation iterates endlessly when I have both the LM2917 and LM3914 in play. It's like I can use one or the other, but not both. I saw your posts on the tach for the V12 Jag from a while back and plan to grab some of those details for this too.
     
  8. dave miyares

    Dave New Member

    Joined:
    Jan 12, 1997
    Messages:
    2
    Likes:
    -10


     
  9. mojozoom

    mojozoom New Member

    Joined:
    Jul 15, 2012
    Messages:
    28
    Likes:
    0
    Well here's where I'm at with it now. It seems to work as expected, but I still need to iron out the setpoint mechanism. What seems to make sense to me would be to take the 7.3V that is set by the LM2917 zener and reduce that as needed to establish the setpoint voltage of the LM3914. Currently I would need exactly half of the 7.3V for a full scale 8000 rpm setpoint, and I'd like to use a pot to adjust that voltage down lower for lower setpoints.

    But then the trick is how to cut that voltage in half again so the light comes on at half the normal rpm so you can adjust the setpoint. So I need to divide the divider??

    tach3.JPG
     

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice