EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: N TYPE on March 29, 2019, 06:06:01 am
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Ive got this speedo corrector to convert my cars tacho signal.
the input is a 12v square wave pulse to gnd.
Theres no trouble with the input sensing circuitry, that all works fine.
My problem is that the maximum output voltage swing is only 8.2V, its not enough to trigger the edge detection in my tachometer. I need this output to be at 12V
Ive got the circuit diagram attached.
If you look at the output circuit in the lower right hand corner, Im thinking I just need to cut the trace at "A", and connect it to the 12V input. Would this be correct?
Another option would be to use the standard 8.2v output to drive an external transistor connected between 12V and Gnd (with some current limiting resistors in series of course) however Id like to keep everything neat and tidy.
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Yes, just connect A to 12V, at the input node of the LM7805.
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I recognise that! It was a Silicon Chip magazine project (republished in EPE for the UK market) from about a decade ago.
Found it: http://archive.siliconchip.com.au/cms/A_108157/article.html (http://archive.siliconchip.com.au/cms/A_108157/article.html)
No track cuts required- just fit a fly-wire instead of LK1/LK2
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You're absolutely right, It is from silicon chip and has had several revisions over the years.
One question I have is that If i run a fly wire from the lk1/lk2 common to +12v . What effect will it have on the input / sense circuitry?
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Not much. If S2 is in position B the input pullup will be to 12V instead of 8.2V/5V selected by LK1/2, otherwise absolutely no effect.
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You could also just remove ZD3, no wire needed. Tidy.
Note that there is basic protection here. A 16V zener that will limit the voltage, but there can be voltage spikes on the +12V in some cars (especially older ones), so you could maybe replace ZD3 with a 12V zener instead. That would give added protection for the circuit you're feeding the output with (not sure what max voltage it can tolerate).
Note: check the voltage rating of the 10µF capacitor if you're going to do that. If they have used a 10V cap for instance, that wouldn't be good.
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That won't work. You cant get enough current through the 220R resistor too get more than 8V output swing due to the current requirements of the 470R load on Q6 and 1K load on Q5. The only practical option is to jumper to the regulator input. It doesn't matter that its not a particularly clean 12V (probably more like 13V), as any automotive circuit designed for 12V input signals will accept the supply voltage as a high level.