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#18 |
The CD4001 quad 2-input NOR gate is a very versatile IC that can be used in an almost infinite variation. Here we will use it in the application of a simple alarm system . The power supply suggested for the CD4001 is 5 to 15Vdc Max.The CD4001 has a typical transition voltage value of 50% of its supply voltage with a possible spread of 30% to 70% depending on its production.
The first application is for a Basic alarm system with a delayed trigger.
The delay is set by R1/C1) for exemple , R1=680k and C1=10uf give a time delay of approx 8 secs. Thus time can adjusted by the choice of R/C values.This delayed trigger allows time to set the module in the "Standby" and exit within the time delay period before the alarm is triggered. Again on re-entering the alarm must be reset within that delay period . Gate A forms the trigger circuit and gates B&C form a tone generator of 800hz to the speaker activated by the transistor.
![]() In the next circuit in addition to the delay ON circuit we added an extra gate to provide for an auto/delayed turn off function. C2 & R2 are used to automatically turn off the alarm . Like C1 & R1 , C2 & R2 can be selected for the duration the alarm will stay on then automatically turned off. It must be reset to be reactivated. Two outputs are available as shown .
![]() But , the delay period may not be long enought to manually reset the unit . We can eliminate the guess work by integrating an auto-exit /entry delay that will not require a manual reset as shown in the next circuit.
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R1/C1 and R2/C2 forms the exit /entry delay ciruits . When the power is first applied (standby) all sensor switches (S3,S4) are enabled except S1a/b which is disabled for a pre-set period via time-delay network C1/R1 . At the end of this period S1 is automatically enabled . When S1 is activated the output of gate-A goes high and is locked by the action of D1 diode(1N4148) and gate-D. This high output voltage is applied to the input of gate-B via C2/R2 delay network an after a pre-set delay the voltage reaching gate-B rises to such a value that the alarm is activated.
Notes
Add an auto turn-off option
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© Laurier Gendron, Burnaby, B.C., Canada. 1999/mg