The Two-door Buzzer with Display is applicable and well suited to be used in a house having two gates or doors that are distant from each other. It helps the person inside the house by providing an alarm and a number display determining which door where the visitor is. It also saves time and effort as the owner does not need to check both doors.
The circuit is basically made up of a monostable and astable multivibrators (a.k.a free-running multivibrator). I suggest you read more about these fascinating timers. Okay, let’s cut right to the chase! The monostable multivibrator provides the timing on how long the buzzer would go while the astable multivibrator provides the nice alarm tone. You can change the frequency of the alarm tone produced by the astable multivibrator by rotating the potentiometer (R18 in the circuit).
I designed the circuit in a way that there would be an alarm tone when a switch either SW1 or SW2 is pressed. The alarm tone activates only for 1 second while the display turns “ON” for 1 minute and eventually turns “OFF” to save energy.
The tone intensity can also be varried by tweaking the trimmer (R19). In addition, the circuit can also have a 9 volts supply if a transformer is not present. Just simply remove the transformer together with the diodes and the filter capacitors and replace it with a fresh 9-volt battery.
What’s good about this circuit? Well, this homebrew circuit is useful and has low power consumption, and I’m actually using it at home. There are times that a guest is being missed because I failed to notice them that is why I constructed this and indeed It is reliable and cool. Hope this would also be a great use to you. Salamat po!
Here are the formulas that I worked on to obtain the values for the RC network employed in the multivibrator circuit.
For the monostable multivibrator circuit:
T = 1.1R6C5
F= 1/(1.1 R6C5)
For the astable multivibrator circuit:
t1 = 0.693(R7 + R18)C7
t2 = 0.693(R18)C7
T = t1 + t2
T = 0.693(R7 + 2R18)C7
P = VI = 4.7*56.98mA = 0.2W