Looking for Simple Discrete Frequency to Voltage Converter Circuit. - Page 1
Forum section: electronics / beginners

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 22, 2012, 11:08:43 pm
content: as the title says. i know of chip that can do this LM2907/2917 NJM4151 but i dont have them. i would like to ask is there a simple discrete version that i can build right away? i unsuccesfully googled it. application... for reading car tachometer (square pulse) and power supply is from car's battery. thank you.

author: Skimask
author_rank: Super Contributor
timestamp: December 23, 2012, 12:47:11 am
content: http://homemadecircuitsandschematics.blogspot.com/2012/05/make-this-simple-tachometer-circuit.htmlFor one thing, define tachometer (square pulse)... Like off the coil directly? If that's the case, I don't think you're gonna get a discrete version because of the possible voltages involved. Tends to blow up chips ya know.

author: Fraser
author_rank: Super Contributor
timestamp: December 23, 2012, 01:06:10 am
content: The chip you mention is designed for just this purpose and importantly provides good linearity in its response to the input frequency. I built a frequency adapter fo my multimeter when I was a kid. It worked well enough for what I needed. The 2917 is the best long term solution. I don't know your exact needs at present but have you considered using your multimeter on its frequency range and just doing some simple maths to provide the RPM ?The after market car tachometers that I have dismantled have been based upon a couple of transistors so it must be possible to build a simple circuit but I cannot vouch for the accuracy of the f to V ratio. I will do some searching for you as I know I have seen simple tacho circuits for driving moving coil meters.Update:What did you search for ?I just entered car tachometer circuit and got buried with schematics, both simple transistor designs and IC based (even the 555 may be used). Look for images and you will find what you are looking for.http://www.google.co.uk/#hl=en&sugexp=les%3B&gs_rn=1&gs_ri=hp&cp=17&gs_id=1t&xhr=t&q=car+tachometer+circuit+diagram&pf=p&tbo=d&biw=1214&bih=539&sclient=psy-ab&oq=car+tachometer+ci&gs_l=&pbx=1&bav=on.2,or.r_gc.r_pw.r_qf.&bvm=bv.1355534169,d.d2k&fp=def9f3f538eb70d&bpcl=40096503The NE555 has an application note for being used as a car tachometer.Take a look here for some simple designs and how to feed them the required signal:http://www.circuitstoday.com/tachometer-circuithttp://www.hazelden.ca/austinhealey/RebuildingBJ8Tach/http://homemadecircuitsandschematics.blogspot.co.uk/2012/05/make-this-simple-tachometer-circuit.htmlFraser

author: casinada
author_rank: Frequent Contributor
timestamp: December 23, 2012, 01:11:07 am
content: Learn from the master Bob Pease that passed away last year, he invented the LM331 V/F converter.http://www.wisewarthog.com/electronics/pease-vf-converter-ics-handle-frequency-to-voltage-needs.html

author: ivan747
author_rank: Super Contributor
timestamp: December 23, 2012, 02:48:33 am
content: What about a phase-locked loop with a linear VCO in the loop? There's a 4000 series chip that does that.That chip is the CD4046. You can use the voltage controlling the VCO and connect an op-amp as a buffer (or as an amplifier if needed). That is the output of your circuit.

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 23, 2012, 05:58:21 pm
content: Quote from: Skimask(darn i always missed important thingthe tachometer output is from the ECM, square wave pulses(i should have included the plural "s")i think i'm going to tap that line which goes directly to the combination (speed) meter in front of the driver. i havent probe the pulses but i believe its somewhere 0V - 12V square wave and the frequency is propotional (or equal) to the crankshaft speed. tracing down my car's ECM wiring, i believe the tachometer pulses is calculated in the software since the only input i can see from engine to the ECM for this matter is crankshaft position sensor. i'm not interested at position, i only want the engine speed.Quote from: Aurorawhen i got the speed in term of voltage (converted from the square wave) i'll feed the value to a comparator. above a certain voltage (indicating higher RPM. i'm thinking > 500RPM) the primary circuit will be activated, which is... "starter cutoff circuit" i've been asking this long time ago (How to Detect our Car's Engine is Running or Not?) and now i'm thinking of doing it since i'm in the process of "grooming" my car (did a teardown few days ago changed all aircon parts and repairing dvd player and etc etc), now it put back together again, but i'm still at it. having said this... linearity accuracy is not crucial since i can adjust the comparator ref volt (-ve input) using trimpot to get the > ~500rpm threshold. my "starter cutoff circuit" is nothing but just a relay activated (open circuit) by the said comparator.Quote from: Aurorai guess i've put wrong keyword, i entered "starter cutoff circuit" and "frequency to voltage circuit" then i got directed to "car immobilizing system", the said chips etc etc, nothing hit what i have in mind. i thought "car tachometer circuit" is building the "tachometer sensor" which i'm not up to it sighthanks.i also saw that LM331 its V-F, circuit for F-V seems to need many auxiliary components, but i just looked at it quickly with little interest since i dont have the chip as well. using PLL maybe a little complicated for me duh. the 555 circuit posted by Skimask looks tempting, it maybe what i'm looking for and 555 is ready in my draweranyway thanks all for the links and suggestions, all of them are appreciated. i'll look into F-V chip based or PLL if i cannot get around with the 555 chip.ps: maybe i can add an opamp as buffer to feed the 555 circuit, so extra protection for the tacho line from the ECM that i want to tap.

author: PA0PBZ
author_rank: Super Contributor
timestamp: December 23, 2012, 08:34:09 pm
content: Is the duty cycle fixed or is the positive pulse always the same width? In the latter case you could just see it as a PWM signal and a resistor and capacitor should do the trick.

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 23, 2012, 09:36:15 pm
content: Quote from: PA0PBZ on December 23, 2012, 08:34:09 pmas i said i havent probe the signal but i anticipate the worst, ie fixed 50% PWM with varying frequency. i already noticed some thread in here on how to do PWM-V conversion. this F-V is a bit trickier i cant just do with RC trick.