Question on LM2907

Discussion in 'General Electronics Chat' started by cybanical, May 2, 2008.

  1. cybanical

    Thread Starter New Member

    Dec 12, 2007
    Hi there,

    I'm an ME and trying to get into electronics, but I'm basically a newb. I am trying to setup my own basic tachometer with the LM2907N-8. I'm using a channel from my optical encoder to generate the square wave pulses that I expect to arrive at frequencies between 0-2400hz. I understand how to use the equations to calculate R1, C1, C2 and I'm using a 12V supply, while my encoder uses a 5V supply. However, the signal from my encoder is always 0.21V - 4.74V when i use a 1.2K pullup resistor. I'm not getting any pulsing on my charge pump and I assume it's because my signal never touches ground.

    Figure 4.b in the AN-162 suggests AC coupling to remove the offset as Ron also mentioned. Could someone give me some direction on transistor, capacitor, resistor combinations for this ac coupling? I've had a couple electronics courses so I'm familiar with what the different items are, but as an ME i really don't have any idea what realistic values for the components would be. Or after looking at this thing more, is the transistor in the picture just suggesting that the signal comes from something like that?

    edit: I just threw together a rudimentary high-pass filter with a cutoff at about 1Hz (C = 10uF, R=16.6k) and it certainly got me some results. I guess now my question is, is there a more elegant way of doing this? or what is a standard AC Coupling circuit?

  2. hgmjr

    Retired Moderator

    Jan 28, 2005
    Greetings Cybanical,

    Welcome to AAC. I have moved your question to its own thread since you posted your question at the end of an existing thread.

    Everyone is encouraged to create their own thread rather than tacking it onto an existing thread. The purpose is two-fold. Firstly, it avoids disrupting the flow on the existing thread and secondly it provides you the poster with a dedicated thread in which to discuss you particular issue.

    I hope you enjoy and benefit from your AAC forum experience. Also, I would like to direct your attention to the AAC e-book at The e-book is a rich source of information on electronics and is well worth consulting when you need to learn more about the topics being discussed here in the forum. The e-book is a work in progress and as such it contains some sections that have not yet been completed.