Maximum input voltage

Series8217

I want to connect the turn signal indicators and high beams indicator to my digital dash project (runs on a BS2p40) so I can see their status on my LCD. Rather than the usual setup where the switches ground the indicator lights, the turn signal and high beam lamps work by the appropriate switches providing unregulated positive voltage to the lamps. This can be anywhere from 12v+ to 18v+.
Is 18v within the maximum voltage range for the BS2p inputs? If so, how do I figure out how big the current limiting resistors should be for the inputs?
If not, how should I go about hooking these up?

-Steven

allanlane5

You could check out the PIC 16C54 data sheet, for the max current the 'protection diodes' can take.

Chris Savage

18V is definately too much for the pin...I would use an Opto-Isolator for this connection.· Not only will it isolate the BASIC Stamp I/O from the voltage, but depending on the package you use it could simply multiple connections.

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Chris Savage
Parallax Tech Support
csavage@parallax.com

Mike Green

You can also use a simple voltage divider with perhaps a 100K resistor connected to the switches and a 33K resistor between the other end and ground. That would divide down the 12-18V to 3-4.5V and the 100K resistor would certainly protect the Stamp pin. Actually, anything from 10K/3.3K on up would be more than adequate.