Burningmace
New Member
The following works in Multisim, but I'm dubious as to if it would work in real life.
**broken link removed**
To set the alarm, one would open the door and turn the alarm key to ON (J1 closed). When the door closes, J2 closes and ALARM_SET goes high. After this point, J2 can open and close as much as it wants, but ALARM_SET won't go low again until J1 is opened again.
In theory, U1A and U2A keep each other's outputs high using feedback. R1 works as a weak pull-up to help keep the feedback stable. However, in Multisim you can remove R1 and it continues to work. R2 is probably completely unnecessary, but I have it there because Multisim bitches at me if I don't include a ground.
The reason I'm dubious is that somebody once told me that when you use feedback designs such as this in TTL logic, it doesn't work. I seem to remember them mentioning the switching speed of the transistors as the cause of the problem, but I don't remember the details.
I'm sure there's a better way to do this using a PIC, but I'd rather stick to standard logic for this project.
**broken link removed**
To set the alarm, one would open the door and turn the alarm key to ON (J1 closed). When the door closes, J2 closes and ALARM_SET goes high. After this point, J2 can open and close as much as it wants, but ALARM_SET won't go low again until J1 is opened again.
In theory, U1A and U2A keep each other's outputs high using feedback. R1 works as a weak pull-up to help keep the feedback stable. However, in Multisim you can remove R1 and it continues to work. R2 is probably completely unnecessary, but I have it there because Multisim bitches at me if I don't include a ground.
The reason I'm dubious is that somebody once told me that when you use feedback designs such as this in TTL logic, it doesn't work. I seem to remember them mentioning the switching speed of the transistors as the cause of the problem, but I don't remember the details.
I'm sure there's a better way to do this using a PIC, but I'd rather stick to standard logic for this project.
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