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!
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!