We are in a digital era! Now we can add some intelligence to our doors and gates without too much investments. Just assemble this little door timer circuit built around a few cheap and simple components and link it to any door/gate as per your requirement and taste. If the door/gate remains in open condition for a prefixed time an acoustic sounder starts beeping to raise an alert. Enough?
At the heart of the circuit is CD4060, which is a 14-stage ripple-carry binary counter/divider plus oscillator. Here CD4060 is configured as an independent timer. In standy by state, ie when switch S1 is in closed condition, the reset terminal ( pin12) of CD4060 is at a high level and the timer is in inactive mode. When the door is opened, S1 also opened as per the mechanical arrangement and CD4060 gets activated.
After a short period,transistor T1 conducts to power the active piezo-sounder. Red LED in parallel with the piezo-buzzer also lights up instantly. When the door(and hence S1) is closed, circuit returns to standby state automatically.
The open door alarm circuit works off 9V (PP3/6F22) battery supply. Assemble the circuit on a small PCB and enclose in a plastic box of appropriate size. Mount the switch S1 in the door/gate frame and connect it to the circuit through external cable. If possible use a key-lock type switch for power on/off function (S2).
Note: Preset delay time can be changed by changing the value of timing resistor R3. SK1 is an optimised connection terminal for attaching a standard 6-9Volt electromagnetic relay.