Post by thread:Detecting a Pulse

<x-flowed>I am playing around with my cars turn signal. When the turn signal is on I want to be able to just read an input and get a 1 on one of the pics input. Is there a chip that can do this? It is safe to assume that the pulse width can be as large as 3 seconds. If there is no such chip I plan to put some code in an timmer/interupt handler which checks for the pulse. It will just set a variable depending on if it detects a pulse. If it hasnt seen the pulse in a while it will just clear a variable. Has anyone done this? Schematics and web links would be a big help thanks. Greg ________________________________________________________________________ Get Your Private, Free E-mail from MSN Hotmail at http://www.hotmail.com </x-flowed>

You could do something fairly simple: Hook a diode from the signal into a capacitor and resistor in parallel (other ends connected to ground). Pick a large cap, and a large resister such that when the 12v from the turn signal charges the cap, it takes 3 seconds to discharge through the resister. Take that and hook up another diode which leads into a resister which then goes into the PIC pin (protected by a 5.1v zener, ideally). When the cap discharges to around 3-4v, the pin will go low. Use a pin with the schmidt trigger (porta.4) for better readings. Behold the ascii art: Turn Signal -----|>|---+---+---|>|---/\/\/---+--- PIC Input | \ | +--)|--+ / +---|>|---+ | \ | Zener | / | Ground ---------+----------+-------+ Otherwise you could make a routine to check the input a few times a second. It depends on how cost-sensitive your application is. This makes programming much easier, but you also have to include 4 extra parts (you'd need the zener and resister anyway) and that might cost you as much as $0.05 per unit in quantity (oh the horror!). Or you could use a 12c508... ;-) Anyway, I hope this gives you some ideas. -Adam Greg Hastings wrote: {Quote hidden}> > I am playing around with my cars turn signal. When the turn signal is on I > want to be able to just read an input and get a 1 on one of the pics input. > > Is there a chip that can do this? It is safe to assume that the pulse width > can be as large as 3 seconds. > > If there is no such chip I plan to put some code in an timmer/interupt > handler which checks for the pulse. It will just set a variable depending > on if it detects a pulse. If it hasnt seen the pulse in a while it will > just clear a variable. > > Has anyone done this? Schematics and web links would be a big help thanks. > > Greg > > ________________________________________________________________________ > Get Your Private, Free E-mail from MSN Hotmail at http://www.hotmail.com

On Mon, 12 Jun 2000, Greg Hastings wrote: > I am playing around with my cars turn signal. When the turn signal is on I > want to be able to just read an input and get a 1 on one of the pics input. This shouldn't be a problem. Just run the lamp signal into a PIC input pin thru a 100K or larger resistor to limit the current. With a 100K resistor, the current will be limited to 100uA (did I get that decimal in the right place?), which is well below the 250-400uA rated capacity of the PORTB weak pull-up resistors even if you assume a 15V input. This is what I (and many others) do with RS232 inputs to a PIC. > Is there a chip that can do this? It is safe to assume that the pulse width > can be as large as 3 seconds. Most, if not all of them. > Has anyone done this? Schematics and web links would be a big help thanks. Not (yet) specifically with a turn signal line, but with RS232 voltages, yes. 50K Ohm resistor works well with +/- 10V swings. Dale --- The most exciting phrase to hear in science, the one that heralds new discoveries, is not "Eureka!" (I found it!) but "That's funny ..." -- Isaac Asimov

From the left and right outputs (wires going to lights) take a tap through a diode - combine these isolated inputs and pass them through a small filter circuit then in to the PIC via a level shifting transistor (inverts signal). The filter reduces current demand and does hardware de-bouncing for you (may have to try a few values to suit your indicators). Use a fixed font (like courier) to view this diagram - 1K /--/\/\/\--+5V 1N4004 | Left ------|\------\ |------ PIC |/ | / | 1K5 10K |/ Right -----|\------\-----/\/\/\/\/\----\---/\/\/\/\---| BC337 |/ | |\ 4.7uF === _\| --- | | | | | 0V -----------------------------------/ \------ 0V Bye. {Original Message removed}

What you plan to do sounds reasonable to me. Don't have the signal interrupt on an edge but instead set a timer to interrupt at a rate much higher that the pusle wide expected and then sample the status of the pulse input. You could average the sample to smooth your data; that is, say you must receive at least three HIGHS in a row so you don't false. Phil On Monday, June 12, 2000 5:03 PM, Greg Hastings [SMTP:spam_OUTghpicTakeThisOuTHOTMAIL.COM] wrote: > I am playing around with my cars turn signal. When the turn signal is on I > want to be able to just read an input and get a 1 on one of the pics input. > > Is there a chip that can do this? It is safe to assume that the pulse width > can be as large as 3 seconds. > > If there is no such chip I plan to put some code in an timmer/interupt > handler which checks for the pulse. It will just set a variable depending > on if it detects a pulse. If it hasnt seen the pulse in a while it will > just clear a variable. > > Has anyone done this? Schematics and web links would be a big help thanks. > > Greg > > ________________________________________________________________________ > Get Your Private, Free E-mail from MSN Hotmail at http://www.hotmail.com