I'm looking for help in safely connecting my BS2 to a 12volt auto electrical sy
I'm new at this and looking for some guidance. I want to connect my BS2 to a 12 volt automotive system. From forum searches, Google searches, etc., I understand that I need to protect my BS2 from inductance voltage spikes by using something called a flyback diode. Correct? I read that it should go as close to the load as possible. The load in this case is my BS2, not a transformer as most diagrams show?
I have a positive and negative wire coming off of the accessory terminal of the key switch to a plug on my project box. Is it correct that the diode would go across the BS2 pins Vin and Vss, but inverted with the anode(+) to Vss and the cathode(-) to Vin?
And...because an alternator puts out higher voltages for awhile after starting (to recharge the battery), would it be safer for the BS2 if I were to use a couple of resistors as a voltage divider before I connect to the Vin? Something to drop the voltage down around 9 volts, but still high enough to allow the BS2's on board regulator to do it's job?
Or, if I going about this all wrong please set me straight.
Thank You in advance,
Steve
I have a positive and negative wire coming off of the accessory terminal of the key switch to a plug on my project box. Is it correct that the diode would go across the BS2 pins Vin and Vss, but inverted with the anode(+) to Vss and the cathode(-) to Vin?
And...because an alternator puts out higher voltages for awhile after starting (to recharge the battery), would it be safer for the BS2 if I were to use a couple of resistors as a voltage divider before I connect to the Vin? Something to drop the voltage down around 9 volts, but still high enough to allow the BS2's on board regulator to do it's job?
Or, if I going about this all wrong please set me straight.
Thank You in advance,
Steve
Comments
I am not clear on what 12V signals you want to connect and how, however I do have experience at this using both the BASIC Stamp and the Z80. Modern car electrical systems are significantly less noisy than older cars used to be. That said, I’m sure you’re aware that the actual voltage varies significantly depending on engine RPM and charging cycle. For this reason, diodes and resistor voltage dividers are not the best choice for interfacing. My suggestion would be optical isolation for any inputs you want to read and MOSFET or relay outputs depending on what you want to control. Because cars are negative ground to the chassis it is often desirable to switch devices on the high-side which can make otherwise simple interfaces slightly more complex.
If you could list details on what you’re trying to do exactly, I might be able to provide you with some examples of how to go about this. Others may have input as well. But it is necessary to first define your needs in detail. Take care.
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Chris Savage
Parallax Engineering
Thank you for your speedy reply.
I'm just trying to learn about the stamp by playing around with it. I want to use the BS2 to interface with some of the sensor modules Parallax has available like the temperature/humidity, compass, acceleration/tilt and Ping. The interface is to an old 4 cylinder diesel tractor and is purely to keep from eating up nine volt batteries with the Board of Education.
Steve
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Chris Savage
Parallax Engineering
for input from your 12 Volt switches [noparse][[/noparse]i.e. connected to the brake lights] I'd optically isolate it.
Output I'd use transistor driver or maybe a ULN2803 Driver
Obviously devices not 12V need to be connected as needed, your modules will all be TTL (5V) level
[noparse][[/noparse]oh, and connect pin 10 on the 2803 to +12V to prevent surges backfeeding into the circuit]
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Smile ... It increases your face value!
Post Edited (MrBi11) : 2/11/2009 10:43:41 PM GMT
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Chris Savage
Parallax Engineering
Optoisolators are often recommended here in applications for which they are not the best choice (because they seem gee-whiz to many, I guess), but in an automotive sensing application they are the perfect choice.· Applications like yours are what optoisolators were invented for.
You do have to wire'em up correctly, though, which is easy.· The way shown by MrBi11 cannot possibly work.
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· -- Carl, nn5i@arrl.net
I'll get on that
Okay, changes made.. it's amazing what you can come up with if you drink enough NyQuil
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Smile ... It increases your face value!
Post Edited (MrBi11) : 2/11/2009 10:52:04 PM GMT
You should add some inductance and a large capacitor at the input of the 7805 to filter unwanted spikes from the electrical system. I have attached a schematic for a·design that we use that includes a 220 uh inductor and a
47uF capacitor at the input to the 7805.
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Craig Eid
www.TriadRD.com
Again, thank you for your time and help,
Steve