Coupling PIR to 555 questions

Discussion in 'General Electronics Chat' started by Mike33, May 31, 2012.

1. Mike33 Thread Starter AAC Fanatic!

Feb 4, 2005
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I'm trying to "hack" a PIR I have, which outputs a hex (?) code to a command unit. I only have the PIR. I know, getting the right one is cheap, but this is more of an experiment to learn more about coupling digital circuits etc.

I've found a good location on its board to take an output, and I can drive an LED with this output. I'm getting an AVERAGE of about 4V when tripped, and it lies very near ground when in resting state.

So, I'd like to couple this output to a 555 in monostable mode, and maybe use that to trigger a 2nd 555 to sound an alarm, which is easy. Getting the input to the first 555 is my question.

I have a varying voltage here, a digital sequence going hi/lo all the time when tripped. Should I capacitively couple this to the trigger of a 555, which will then dump the 'average' DC when the input stops?
OR
Should I rig a transistor to be normally H, which will go L when the signal is received (invert the signal)?

I've read the Marsden tutorial on 555's (very good!), and am thinking of using something like this circuit for my 'front end':

http://forum.allaboutcircuits.com/attachment.php?attachmentid=4895&stc=1&d=1259354506

Mainly I'm asking if a digital signal like this is "clean" enough to reliably trigger a 555, and how the mechanics of connecting them up would work.
Thanks!

2. Brownout Well-Known Member

Jan 10, 2012
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First, I think you need to invert the signal, as the timer will trigger with a "low" input. So, you want the quiescent level to be 'hi'. Then you should want to capacitively couple the signal. Now, the pulse should be significantly long enough to trigger the 555. I don't know that number off the top of my head, but a couple mS should do it. The RC time constant of the input network should be long enough to produce a low of a couple mS.

On second thought, you can invert the signal and connect it directly to the 555's input. I don't think it is necessary to capacitively couple it.

3. Mike33 Thread Starter AAC Fanatic!

Feb 4, 2005
349
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Thanks. I think I got it...I just used the basic monostable configuration, and tied the reset pin to the trigger. Direct coupled. Seems to work; I have a 3v buzzer on the output and it goes off when I trip the PIR....
Next step is just to add another 555 astable for a 'siren', and I can use it to alarm my shed )

I was more worried about the digital signal affecting how it would work, but true to what they say, it seems to ignore any input after the first negative-going pulse...

4. Brownout Well-Known Member

Jan 10, 2012
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I don't see how that could possibly work.

5. Wendy Moderator

Mar 24, 2008
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Which is why I like schematics. We can assume, or be shown.

A 555 monostable without the signal conditioner will ignore additional inputs, but if the input sticks high then the monostable will also stick high. It will stay high until in input goes low, in which case the monostable recycles. In your case this is probably no big deal, but it is something to be aware of.

6. Bernard AAC Fanatic!

Aug 7, 2008
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If pin 2 goes lo then sticks hi, 555 will time out & reset; if pin 2 sticks lo, 555 willl time out, then re trigger??

7. Wendy Moderator

Mar 24, 2008
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The 555 will retrigger when a high is on pin 2, assuming a standard monostable schematic.

8. Mike33 Thread Starter AAC Fanatic!

Feb 4, 2005
349
25
Yup, you're right. If I tie the reset to my input signal, it's really just following along with the input (retriggering).
If I leave it floating, I get very odd results (not working, intermittent trash).

Tied to Vcc, it is just "always on". Perhaps self-triggering and I have a large timing cap, so it looks 'always on'...

I'll try using a BJT as an inverter and see what we get. The main thing throwing me is the nature of the input signal, which is a series of pulses rather than one square wave or something. I sense some sort of filter is in order?

I'd like to have my first 555 trigger on receipt of the input, and ignore any further inputs for its timing cycle; say 10 secs. Then, when it goes low at the end of its cycle, to trigger a second 555 which will be the alarm.

9. Mike33 Thread Starter AAC Fanatic!

Feb 4, 2005
349
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Thanks - inverting the signal is now giving normal operation.

Next thing is how to use the H output of the 555 to trigger one that's set up astable...off to the data sheet

10. Bernard AAC Fanatic!

Aug 7, 2008
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Quote by Bill Marsden," A 555 timer starts when pin 2 goes under 1/3 Vcc. If pin 2 stays there[low] the timer will time out, and the 555 will stay forced on[ high].

11. Mike33 Thread Starter AAC Fanatic!

Feb 4, 2005
349
25
I see that (now) - been a while since I've played with these since I usually do audio work.

What I'm trying to do now is solve problem two...how to trigger an astable 555 after a DELAY (to allow you to step up and shut off the system if you're NOT the guy after the gas can, lol). A simple on/off power switch is fine for this, I don't need fancy tamper-proofing on this.

SO...it's a logic thing. I have PIR going high, 1st 555 input goes low and so triggers the 555, which is H for about 10s. Then goes low again, unless the PIR is tripped again. This block is all set.

I need to use this logic state to turn on the astable 555, BUT allow that 10s to lapse first. I'm thinking of some sort of RC job that will charge up as the 1st 555 times out, and when it goes low again, it can 'latch' something as it discharges. An SCR, perhaps?

Does anyone have any suggestions on how this can be done? I feel 2/3 of the way there but still a little bungled; I've never used SCRs. I know it's simple, and this is re-inventing the wheel, but it's the best way for me, at least, to understand these systems and make them do useful work...
Thanks!

12. Mike33 Thread Starter AAC Fanatic!

Feb 4, 2005
349
25
Using the KISS rule: I probably should just cascade a 556, and use the 2nd section to power a piezobuzzer or similar alarm....no frills needed.

13. Bernard AAC Fanatic!

Aug 7, 2008
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Here's my attempt: Using piezo buzzer [ built in osc ] would connect to U2-3; U2 is used as a latch & will stay hi untill reset by SW.

File size:
153.5 KB
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14. absf Senior Member

Dec 29, 2010
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Is R4 a 10K pot with wiper connected to pin 7 of U2?

Allen

Bernard likes this.
15. Bernard AAC Fanatic!

Aug 7, 2008
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Whoops, R4 suposed to connect to U2-2, pull-up, & not to ground; thanks for catch.