Time to ask the big guns... 555-556 issues...

Discussion in 'General Electronics Chat' started by mbohuntr, Oct 27, 2013.

  1. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
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    I have built a latching alarm circuit using a 556 for two different timing purposes... the left side holding the discharge and threshhold pins high and using the trigger pin to turn on the output high by a 10uF cap and a 1meg resistor in parallel as a voltage decay delay. Works great. (A contact opening removes the supply voltage from the pin.)

    Now the right side output was intended to shut off the power to the circuit after about an hr by latching a NC relay open and disabling the rest of the circuit. Using a 470uF cap and a 1meg resister and a PNP transistor that clamps off when the alarm sounds starting the decay. On a 555 works great.

    Now when I connect the cap, resistor and transistor to the right side of circuit, it triggers the left side of the circuit and latches it into alarm?? I removed all the right side components one at a time and discovered when I connect tha 1meg resistor to pin 8 (trigger pin) it pulls the opposite side pin 6 (trigger)low, triggering the circuit??? I thought they were electrically isolated???

    Now I checked the resistance from 6 to gnd and get 4k? There is a 1Meg and 1uF in parallel to gnd there? Same with the right side??? (470uF and 1Meg resister.) Before I reconnected the right side, I checked the pin 6 to 8 continuity and it was open like I would expect...??? I then rechecked the 6 to 8 continuity and it starts at 4k and climbs about 1k per second??? Anobody seen this before? Everything works great before I connect up the big cap and resister....

    Thanks in advance!
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Can we have the circuit to look at please???
     
  3. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
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    Oh god, I KNEW you were going to ask that... I have been on a schematic free binge for several weeks and it isn't pretty. I would draw up a schematic and then have to modify it as things weren't playing well together and needed to be modified... :p It will take quite a while because my laptop holding my multisim program fried... This laptop is a temporary (old and worn out) one and pretty unstable at times.... I should and will try to draw one by hand...
     
  4. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
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    I just read in another post that 555's using decay RC's can start up high unless you delay the power on using another RC delay??? Possibly my glitch.......Hmmmmm... I will get to work om a schematic as soon as possible.
     
  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    It is unlikely you will manage to get a reliable 1 hr time delay using a 555/556, because of capacitor leakage current when using a high value electrolytic. A CD4060-based circuit would be better.
     
  6. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
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    Ummm... uhhh.. I sorta forgot to reconnect the power to the right side circuit after I lifted it to add a resistor to change the voltage. (another undocumented modification...:rolleyes:) Problem seems resolved for now... DUH!!

    I only need the longer delay to shut off the alarm anywhere from 40 minutes to several hours. I didn't want to annoy the neighbors any more than necessary with the siren blaring... actual time not really important.

    I will still try and draw up the schematic, it is becoming quite a little Rube Goldberg design...:cool:

    Thanks!
     
  7. MrChips

    Moderator

    Oct 2, 2009
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    Put a 0.1μF capacitor across the power supply rails as close to the chip as possible AND add a 10μF electrolytic capacitor across the power supply rails.
     
  8. Wendy

    Moderator

    Mar 24, 2008
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    You could always hand draw it, then scan it in, or use my PaintCAD to draw it. I've been using M/S Paint with good results for decades.
     
  9. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
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    Here it is... PFEW!!! I got this old laptop to work w/ multisim and I think I got it all.

    Q1 and Q2 function as a latch when triggered instantly by J1, or delayed by J2 from the output of the left side of the 556. (about 20 seconds)
    Q3 is then pulled low, triggering Q4. (actually a tip120, but multisim didn't offer it.) Q4 then activates U1 (actually a piezo siren...)

    I want to use the right side of the 556 output to activate a similar latch on another board which will open a NC relay disabling the siren after a long delay and giving the neighbors ears a break. Everything works right now except I can't seem to clamp the base of Q5 high when the siren sounds to shut off Q5 and start the decay at the right side of the 556. (I will be adding the relay on the siren when I get the long decay to work...)
     
  10. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
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    Suggestions anyone???
     
  11. Wendy

    Moderator

    Mar 24, 2008
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    I will have to study it a while. I may end up redrawing the schematic, I like my 555 blocks modular. I see the SCR analog in the circuit, and the reset function.

    If it were a 555 I have the pinouts on those memorized.
     
  12. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
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    Thanks Bill, I cannot get it working on my simulator...,but you get the idea. I cannot seem to get more than 6 volts out of the tip120 emitter to bias the Q5 3906 and shut it off.

    Update... I finally got it working. Here is the latch and tip120. I wanted to pick up an output from the Tip 120, or siren input and use it to clamp off Q5.
     
    Last edited: Nov 3, 2013
  13. Wendy

    Moderator

    Mar 24, 2008
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    OK, in redrawing I found one 555 "sin". Pin 4 must be connected to Vcc, as should pin 10. These are the inverted reset pins.

    Drawing continues.
     
  14. Wendy

    Moderator

    Mar 24, 2008
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    OK, here is the original schematic with Reset connected redrawn:

    [​IMG]

    The schematic you just showed will burn up the LED.

    [​IMG]

    So what is the function of U1a again? It is not a timer.

    C2 looks like it may be an error too.

    The circuit on U2b can be eliminated from this print.

    The 555/556 Cookbook sheets are pretty handy references in keeping the pins straight.
     
    Last edited: Nov 3, 2013
  15. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
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    Thanks Bill,

    I drew the led that way (without the 1k resistor) because multisim makes the led on full time with the resistor in there?? I just tried it again and it only works correctly without it??? Dunno!:( I redrew the latch from the web page I borrowed the idea from, the actual circuit is correct and functions properly. The LED is working correctly and runs on maybe 12 mA. I drew the 2nd latch more for concept than the as built. Sorry for the confusion. Now to the actual circuit...

    U1(S2) is a door shunt timer so you can use the man door giving you 20 seconds to de-activate the system. I got that idea from another website as well.Try and put one together on a board, If S2 is kept open for 20 seconds or more the voltage on R5 slowly decays until the output goes high... It really works!. Pin 4.... I thought the same thing and tied it to pins 1&2. If pin 4 is pulled up, the circuit doesn't work.:confused: I tested both timers on breadboard as well as the left side timer on the soldered board and they both work as designed. Perhaps I drew something wrong???
    U2 is simply a 2nd timer designed to be started by the activation of the siren, decaying for about an hour and using the output on that side to activate another latch circuit that doesn't appear yet. It's going to be on a separate board because of lack of room. I'm trying to get alarm power onto the base of Q5, but it won't clamp it off...
     
  16. Wendy

    Moderator

    Mar 24, 2008
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    The configuration of U1a is not that of a timer, it resembles a Schmitt trigger inverter.

    There is also no preconditioning for the input of the timer, as long as the input is held low the input will be held at high. The timer will time out, then once the input goes high the output will immediately go low (assuming the timer has timed out).

    The one time I build a practical SCR analog I had to put a high resistance on both collectors to quieten its response to noise.
     
  17. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
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    I just reviewed the breadboard timer I built previously to gather some more info. Pin 2 on a 555 is held high and connected to ground via a 1M resistor and a cap in parallel. 4, 6 and 7 are pinned high thru a 10k.

    I measured the voltage on 2 as 12Volts. Then I pulled the wire out of the supply power from 2 and watched the meter. When the voltage hits 4 Volts,(1/3 vcc) the output goes high and lights the LED. On the circuits I built I had to put a diode on the output as it was causing all kind of glitches. I think it was sinking current and messing with my PNP's. After the diode, no more glitches so your inverter comment is probably accurate!

    I'm not sure I understand your preconditioning comment other than a floating input could swing the output either way on startup? Are my output diodes protecting me from this? (if so, it was stupid luck, not smarts...:p) I also don't understand why when I pinned the reset high on the soldered board, it latched the circuit every time until I clipped it open. then works normally...

    Thanks for taking time to help me bill, I appreciate it!
     
  18. Wendy

    Moderator

    Mar 24, 2008
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    555 Monostable

    I explain it in depth on this article.

    If the input is held at a low the timer is fundamentally disable, though internally the timer still uses the RC circuit. As long as the input is a low the output will be a high.

    Check the schematic out on the above link.
     
  19. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
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    If I'm understanding this correctly, at the initial power-up of the circuit, since the trigger is low, and there is a large cap that takes time to get to 2/3 vcc, that the output will be briefly high and possibly long enough to trip the alarm latch?
     
  20. Wendy

    Moderator

    Mar 24, 2008
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    A monostable is wired a bit different so this is not a problem.

    A monostable configuration does not have Trig and Threshold wired together. Anytime these two pins are connected the result is an inverting Schmitt trigger.

    It the input pulse exceeds the length of the monostable output other problems pop up, hence the signal conditioner C1 and R4.

    [​IMG]
     
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