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12 volt switch from 5 volt supply

  1. Nov 2, 2011 #1
    Please my apologies for mistakes as I am a new member, and I would like to ask a how to question,I am trying to make a system up that I can use on my racing cars ( classic hence no onboard computer )
    What I want to do is to have a 12 volt pulse(from 12 volts to o volts) that I can use on my recording equipment to record my engine rpm. I have done it mechanically with a magnet and reed switch and it is succesful up to 7000rpm ( taking it off a shaft ) but now I am trying to make one using a photo transistor with a reflected light as cannot use an interupted beam. I seem to have a system working of sorts but I am using a 5 volt supply with the phototransistor and my recording hardware will not recognise the +5volt to 0 volt range so what I would like is a system that will utilize the on off of the 5 volt to give me a 12 volt wave at up to 7000rpm, seems insurmountable to me but then I am old and decrepit !!!!
    any help gratfully recieved and if I can get it working correctly will send you a link to you tube to see it working
    cheers Twm Sion
  2. jcsd
  3. Nov 2, 2011 #2
    http://www.national.com/pf/LM/LM2621.html#Overview [Broken]

    is this what you are looking for?

    have to check your propgation delays but 2MHZ should be sufficient for 7000 rpm (116 revs per second times the number of teeth in your crank sensor)
    Last edited by a moderator: May 5, 2017
  4. Nov 2, 2011 #3
    thanks hxtasy will take some looking into or is there a way just to use the 5volt supply to switch an external 12 volt supply ( from the car ) on and off to give me the 12volt spike
  5. Nov 2, 2011 #4
    Oooh for some reason it sounded to me like you did not have any 12 volts available. Yes you can just use a simple op amp circuit to do that. if you can't find anything online let me know i can help you design it. i love automotive electronics!

    just remember the 12 volts will change and fluctuate depending on your alternator system. not sure if this is a problem or not. You could always use a 7812 voltage regulator or something similar. I would rather order a $1 chip then build an op amp circuit though...just easier and cleaner.

    Also, why cant you use an interrupted beam? you may be able to design a simpler circuit to get around this, depending on your telemetry/data logging system you are using and it's inputs. i cant imagine it not accepting a 5 volt signal as a high...
  6. Nov 2, 2011 #5
    Thanks hxtasy, the system is that is used is for the old cars with coil ignition and it uses the 12 volt pulse from the low tension side for that ( works fine on my old, 1964 lightweight e-type ) but on my 1969 porsche it has electronic ignition fitted with dual ignition system and I think that the only place to put an interupted beam would be on the cooling fan blades, but with I think about 20 blades the pulse rate is getting rather high but I am still dreaming up ideas
  7. Nov 2, 2011 #6
    7,000 rpm is 116 revolutions per second, with 20 teeth thats 2.33 KHz. what is the sampling rate of the RPM input of your telemetry system?

    My obd1 honda uses the cam sensor for the crank angle sensor and is less than a dozen teeth. I have some scope shots of it somewhere. i can count the ones on my other car that uses a sensor on the crank pulley, but its still not many compared to most cars. 20 should be a decent number for sampling.
  8. Nov 3, 2011 #7
    Hello hxtasy thank you for the information on the e-type I guess that I get at 6500 revs x 6 cylinders approx 650 pulses per sec which the race logic setup ( race logic is the recording equipment ) and it records the 12 volt spike and with some input from me ,just inputting a conversion number it gives the rpm
    Trouble with the Porsche is that the electronic ignition gives multiple sparks from each system and also access to fit attachments are very few.I am not very good,very poor, at electronics and am self teaching myself.The RaceLogic system can be seen working in the clip of me driving my 1961 jag Mk11( ) where the rev counter is the blue gauge.
    Last edited by a moderator: Sep 25, 2014
  9. Nov 3, 2011 #8
    Hello hxtasy, just come accross something that looks as if it might work Opto-isolator,4N25 5300Vac/60mA DIP6
    If I read it correctly I can input from the phototransister and pass a 12 volt current through it, I hope ! t ogive you a laugh what I would do is order a few and test it practically if it works it does or it blows up !!
    If you would like link for porsche please ask
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