fmgst@unix.cis.pitt.edu (Filip Gieszczykiewicz) (08/29/90)
Greetings. I got my simple (+/-1999 mV) meter to work just like the "real thing" I have 0-500 VAC, 1-500 VDC, 0-20Kohm (no amperes, that's what I use my analog for... :-) Now, I would like to add a frequency counter and, perhaps, a crude (key word) capacitance meter. Is there a chip or hybrid that will convert frequency (hopefully linear) to volts (or ohms)? It can be made of discrete components as long as the price is not "hubble" (hey, new word for "astronomical" ;-) Also, the capacitance meter seems to be a "can of worms" if one is to make one from discrete components - is that true? I would not like to buy a chip that measures capacitance but drives a SEPERATE display. On the other hand, if that's the way to go, I'll take it. Finally, I would like some sort of overvoltage protection for my meter. Short circuit would be nice but I'm not sure if it's needed. Take care. I will summarize and make public whatever I accumulate. (Please use e-mail) P.S. By the way, this is going into a 19" rack! Size of your circuit is no problem! ;-) Power is neither - I have +/- 20VDC @ 5A and +/- 5VDC @ 10A (No, I won't take tube circuits - I'm not that crazy... :-) -- _______________________________________________________________________________ "The Force will be with you, always." It _is_ with me and has been for 10 years Filip Gieszczykiewicz "A man without a dream is like a fish without water." FMGST@PITTVMS or fmgst@unix.cis.pitt.edu "My ideas. ALL MINE!!"
kenny@m.cs.uiuc.edu (08/31/90)
> Is there a chip or hybrid that will convert frequency > (hopefully linear) to volts (or ohms)? Drive a 555 timer wired as a monostable. Each positive-going transition of the signal gets a HIGH output for a specific amount of time. Integrate the result, and you've got a frequency meter. For a capacitance meter, put the capacitor under test into the control circuit of a 555 astable. The output frequency is in inverse proportion to capacitance, and you can take it from there. Kevin, KE9TV kenny@cs.uiuc.edu but moving to ke9tv@nrtc.northrop.com
kenny@m.cs.uiuc.edu (08/31/90)
Earlier today I wrote: >For a capacitance meter, put the capacitor under test into the >control circuit of a 555 astable. The output frequency is in inverse >proportion to capacitance, and you can take it from there. A better idea is to use the 555 as a monostable, and trigger it with a fixed frequency clock. Duty cycle will be proportional to capacitance. Circuit will look something like: +5 ---+---------------+---+ | | | R +----------+ |(see below) | 8 4 | +---+------+-----|7 3|---------------^v^v^v---+------- Vout : | | LMC555 | | C to test +-----|6 | ----- : | | ----- ground +-----|2 5|----+ | | | 1 | | ground | +----------+ 0.1 uF | | | Clock ----+ ground ground (see below) The ON time for the monostable is about 1.1RC, so component values that should work would be a 50 Hz clock, say a 1 Hz low-pass filter on the output, and R = 9.09K, 1%. That combination will give an output of one volt per microfarad. Switch R in decades for smaller capacitors. Trim R for calibration. Kevin, KE9TV kenny@cs.uiuc.edu ...but moving to...ke9tv@nrtc.northrop.com
whit@milton.u.washington.edu (John Whitmore) (08/31/90)
In article <33045@unix.cis.pitt.edu> fmgst@unix.cis.pitt.edu (Filip Gieszczykiewicz) writes: > > Greetings. I got my simple (+/-1999 mV) meter to work > just like the "real thing" > > Now, I would like to add a frequency counter and, perhaps, > a crude (key word) capacitance meter. > > Is there a chip or hybrid that will convert frequency > (hopefully linear) to volts (or ohms)? Try a phase-locked loop; for few percent accuracy, you can use a CD74HC4046 (the data sheet claims 0.1 %; that seems perhaps optimistic to me), with a single resistor and capacitor. This includes a high-impedance preamp, works to circa 20 MHz, and runs on a few milliamps of +5v. Cost under $2. > Also, the capacitance meter seems to be a "can of worms" > if one is to make one from discrete components - is that > true? Not necessarily: get a clock and a counter; set up an R/C timer with a good accurate timer chip (XR320 from Exar, or LM322 from National, is more accurate than a straight '555); drive the counter with the (constant) clock and gate it with the timer output. This scheme is actually simpler than the A/D converter your DMM has in it. Alternately, hook the capacitor into a PLL chip (like the CD74HC4046), and observe the output voltage when the PLL locks onto a fixed frequency. This produces a voltage proportional to C. Change the fixed frequency for range-switching. John Whitmore