Among the many anti-theft devices that are available, presence simulators have a special role to play. In fact, while an alarm system generally reacts the instant the intrusion is detected, or sometimes a little afterwards, in all cases the damage has already occurred. The purpose of the presence simulator is to stop intrusions beforehand by making crooks think that someone is at home. Working from the principle that the majority of home burglaries, with break-in, happen particularly at night, a properly designed presence simulator turns on the lights as evening falls, then turns them off a few hours later, causing an observer with bad intentions to believe that the premises are occupied.
Creating such a function with a microcontroller is certainly very easy and has already been done many times in the past, but the project we are proposing now is intended for those among you who do not want to, or who cannot program this type of circuit. As a result, our diagram only includes very common logic circuits from the CMOS 4000 family, with quite respectable results.
Ambient light is measured using the LDR R3 and, when it goes below a threshold determined by the adjustable potentiometer (P1) setting, like when night falls, it drives the IC1.A gate output to a low level. This has the effect of triggering triac T3 through gates IC1.C, IC1.D and transistors T1 and T2. At the same time, this clears the reset input from IC2 which is none other than the classic 4060 in CMOS technology.
Considering the values of C2, R4 and P2, the internal continuous oscillator in IC2 functions at a frequency on the order of 5 Hz. Consequently, its output Q12 (pin 2) changes state at the end of approximately one to two hours (depending on the P2 setting) while Q13 (pin 3) does the same, but in two to four hours. Depending on whether a link has been installed on S1 or on S2, gate IC1.B output thus changes state after one to four hours, having the effect of blocking triac TRI1 through IC1.D, T1 and T2. Simultaneously, diode D1 blocks the oscillator contained in IC2 and, therefore, the assembly stops in this state. It is dark, the light was lit for one to four hours, according to the setting of P2 and the wiring of S1 or S2, and it just went out. A return to the initial state can only happen after IC2 is reset to zero, which occurs when
its input from reset to zero (pin 12) goes to high level, in other words at dawn and LDR R3 detects lights again.
Thanks to its low consumption, this circuit can be directly powered by the mains using capacitor C4. The latter must be a class X or X2 model, rates for 230 VAC. Such a model, called a self-healing capacitor, is actually the only type of capacitor we should use for power supplies that are directly connected to the mains supply.
To ensure proper operation, we should pay careful attention to the placement of the LDR, to prevent the device being influenced not only by light from the house to be protected, but also by potential street lights, or even headlights of passing cars. Finally, since it is directly connected to the mains, the assembly must be mounted in an insulating housing, for obvious
safety reasons.
Author: Christian Tavernier
(Elektor Electronics Magazine – 2006)
 | * Download this article. |
0 comments: