I have been making my way through Charles Platt’s Make Electronics book and this is one of the circuits involving the use of 555 timer ICs. This will a brief discussion of what the circuit does and how it works. If you want more information on how to build the circuit, I recommend buying Mr. Platt’s book as it has been a great learning resource for myself.
Overall Design:
This alarm circuit, when implemented could use reed switch sensors to trigger a siren if a connection was broken. The circuit is designed with a grace period upon activation so that the user could exit the area without triggering the alarm by opening a particular door. It also has a grace period upon entering so that the alarm can be switched off by the user upon entering the area, else it will sound after a certain period of time.
How it works:
This is more so a learning exercise for me, so please forgive any errors or unclear explanations.
Full Schematic
I recreated the schematic representation of Mr. Platt’s design to better understand the functions.
Delay upon entry:
The lower 555 timer is set up in a semi-bistable mode with pin 2 (trigger pin) connected between a 68mF capacitor and a 1M ohm resistor that is connected to ground. When this part of the circuit receives power, the capacitor will charge and keep pin two high. The charge on the capacitor however will leak out through the 1M ohm to ground. Once the capacitor reaches 1/3 of the supply voltage pin two will trigger the output at pin 3 where the siren is connected. In the picture there is simply a red LED connected for testing purposes which signifies the alarm sounding.
Delay upon activation:
The upper 555 IC controls how long the user has to exit the area after activating the circuit. The circuit is armed by pushing the button connected to pin 2 of the IC. This allows the transistor Q2 connected to the output pin on the IC to conduct and cut off the supply to the transistor Q1 which controls the relay and therefore the power to the alarm. The 68uF capacitor connected to the threshold pin will keep the output high for sufficient enough time for the user to leave the area. There is a yellow LED that illuminates while the timer output is high. Once the capacitor reaches 2/3 of the supply voltage the output will go low Q2 will stop conducting and Q1 will turn the relay on.
Sensor Check:
The toggle switch at the top of the schematic allows the user to check if all the sensors are closed. If they move the switch to the upper position, and all the sensors are closed, a closed path through the sensors will allow the top led to conduct. If a sensor is open of course there will be no closed path and the LED will not illuminate.
Those are the main features to this design. This was just learning exercise, so I don’t have any plans to create a finished product of this design.
Overall, this was a nice project displaying a practical use of the 555 IC functionalities.