Instructables

Security System Power Saver

   This circuit is used for automating a day/night scheduled security system which, best of all, could only cost cost you $0.25 a YEAR to run. (All electricity prices based on a 2009 national average cost found here .) If you have a security system circuit you would like to run for a long time, then you probably don't want to pay more than you need to pay nor do you want to have to switch it on and off manually in order to save money and have it run at nighttime. This circuit takes care of both those problems. The circuit can be changed around to run on a daytime or nighttime schedule and is low cost to run along with the other circuit. (I'll explain how to calculate these prices of run time in the last step).
   Materials differ depending on which of the two circuits you plan to build, but here are the components used by both circuits:

1x 555 timer
3x resistors (1x 470 ohm; 2x 470K)
1x Photoresistor
1x LED
1x non-Zener diode
various jumper wires

   For the 5v relay circuit, you will need these in addition to the components listed above:

1x 5v relay
1x 12v regulator
at least 2 high rated capacitors with voltages ratings of 20 volts or above)

   For the 12v relay circuit, you will need these in addition to the components listed above:

1x LM741 op amp
1x NPN type transistor
1x 12v relay
2x 65K resistors
 
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Cunning use of a 555, but you could use an op-amp as a Schmitt trigger instead. Or a setup with a thyristor.

And you realise in one of your circuit diagrams, you've left the non-inverting input of your op-amp floating, right?
MacDynamo (author)  LegendofPedro3 years ago
I'm glad you caught that floating pin. I was actually unsure about using a pull-up/pull-down resistor on the inverting input because I wasn't sure if that would affect the output regardless of whether the non-inverting input receives power or not. If you happen to know, tell me and I'll fix it as soon as I can.
Your op-amp is running as an open loop inverting amplifier. The gain of the amplifier is unimportant as it is going to be very high; you're essentially just flipping the waveform (as you put a signal on the inverting input).

Since an op-amp is a differential amplifier and the output of the 555 will always be between GND and +Vcc I would assume you want to tie the non-inverting input to GND. It's bad form to have a floating input since it's supposed to be amplifying a 'difference'.
surfingump3 years ago
Well done and thank you for posting!
aslanidis3 years ago
very nice job!!!
jensenr303 years ago
amazing! i love this!