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Old 8th October 2006, 06:52 PM   #11
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I will be taking 12v signals from the car (either 12v or 0) and feeding that into a microcontroller (run off a 5v supply), so the currents will be very small).

Poobah, I did google those phrases, but most of the results were 5v to 3.3v converters. Is there a specific name for the 12v devices?
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Old 8th October 2006, 08:46 PM   #12
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1. Voltage divider 12 => 5 V 10k + 10k

2. 4.7 V zener in parallel with the lower resistor, transient protection

3. Lowpass filter into the MCU. 100k + 100n Noise filter
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Old 8th October 2006, 09:22 PM   #13
poobah is offline poobah  United States
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Xplod,

http://www.onsemi.com/PowerSolutions...?id=MC14504BCP

Don't be fooled by the fact the this says CMOS output only... It can be used for exactly this purpose. You just cinnect the output supply pin to your 5V. It really depends on the type of TTL you are using... regular TTL no... LS TTL yes.

Really, the circuits posted here are just as easy. In fact, when you consider you still need input protection... these circuits are probably simple to imlement.


Now... when you say 0 volts... do mean 0 Volts... or NO Volts, as in an open circuit? There is a diiference and TTL inuts will need a sink to ground to register a "low" state.

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Old 9th October 2006, 01:11 PM   #14
clem_o is offline clem_o  Philippines
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Do you want to just get the microcontroller to know if the car amp is on or off? If this is so, follow perander's suggestion... Check the microcontroller data sheet - most will switch logic states at the half voltage, i.e. 2.5V or greater is considered logic 1, anything below 2.5 is logic 0. If it's really "TTL" then anything from 2.0V to 5V is a logic 1, and 0.8V to 0V is logic 0.

Cheers
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Old 1st June 2007, 03:13 PM   #15
eckertd is offline eckertd  United States
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Would the resistor/zener setup work if I wanted to monitor a +12V square wave (20% duty cycle) on a laptop serial/parallel/USB port?
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Old 7th June 2007, 10:09 AM   #16
clem_o is offline clem_o  Philippines
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Howdy,

Depends on the frequency of the signal. Using the parallel port, you might be able to monitor the presence of a signal, if the frequency is low enough... (or, with a high frequency, you'd probably miss a transition or two), but it should work. Serial would be more iffy, and USB - well, writing a driver to talk to the USB device is probably not worth the time - well, unless someone else already has... :-)


Cheers
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Old 8th June 2007, 03:38 PM   #17
eckertd is offline eckertd  United States
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Thanks for the reply.

My current thinking is to put the signal (130Hz max) to pin-10 of the parallel port (ACK) and monitor with an interrupt (IRQ 7).

With TTL "high" being 2.4v-5v, a 100-ohm resistor and 7.5V Zener should do the trick, no?

It's an OLD laptop, but I'd still rather not fry parallel port.

Doug
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Old 8th June 2007, 11:23 PM   #18
clem_o is offline clem_o  Philippines
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It "should" but I'd opine that it isn't the safest solution. If ever the signal goes above 12V, (i.e a transient), you may still zap the parallel port. Also, if the zener were ever to fail (again, perhaps due to a transient), it can fail as a short, and there again goes the port.

I think a resistive divider is safer. Use values to get the 12V down to about 3.5V then add a zener to ground rated at 5.1V to catch any wayward signal that may come around.


Cheers!
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