[SOLVED] Burglar Alarm Circuit

I'm trying to make a simple burglar alarm circuit. My basic idea is to construct a transmitter circuit using led/infra-red/laser diode connected to a cell, and the receiver circuit consisting of a photodiode/phototransistor and a relay.

When light falls on the receiving element (photodiode/phototransistor), it drives the relay to it's ON position... When the light is interrupted (by the burglar), the relay switches to it's off position and the alarm is sounded.

Is my idea okay ? Or am I making some mistakes ? Also, can anyone suggest a circuit simulator that'll help me to simulate my design ? I have multisim but it is poor for relays and photo elements.

Your design should be fine, and also using multisim for simulation.
You can try also LTspice or CircuitMaker SV for simulation.

https://www.linear.com/designtools/software/
**broken link removed**

Okay vfone many many thanks for your help. However I'm having a little difficulty in connecting the relay with the photodiode and circuitmaker is displaying an error. Any idea how to solve this ?

I'd recommend one small change to the basic plan - modulate the LED / laser at a few KHz. This can be as simple as driving it from the output of a 555 oscillator. Then at the receiver, detect that frequency, rather than the average light level.

Two advantages:
1) At the receiver, the amplifier can be AC-coupled so you don't have to worry about DC offset.
2) The receiver won't be fooled by the ambient light level.

Pick a frequency of at least a few KHz and use a high-pass filter in the receiver so it doesn't get fooled e.g. by flickering fluorescent lights.

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I wouldn't recommend Circuitmaker. I used the student version a few years ago until I realized it had bugs. Endless hours wasted trying to figure out what was wrong with my calculations before I realised Circuitmaker was giving wrong answers.

Anyway, I use the free version of SIMetrix SIMPLIS now. LTSpice is good too, but I found SIMetrix easier to learn and use.

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IMHO it's not worth trying to model the optical stuff in a simulator. I don't know if any of them even cater for that.

Okay that modulating the LED part, I can handle that... I just need to set a timer 555 in astable mode to oscillate at few KHz at any duty cycle. Right ?

Two advantages:
1) At the receiver, the amplifier can be AC-coupled so you don't have to worry about DC offset.
2) The receiver won't be fooled by the ambient light level.

Click to expand...

What amplifier are you talking about ? I didn't get that one. And how can the receiver not get fooled by the ambient light ? Sorry I'm a little new to this field, so it'll really be very kind of you if you can explain.

The alarm will sound only for the moment the IR beam is cut as the burglar walks past it, then then alarm turns OFF.
You need to latch the relay ON until somebody turns it off.

Thank you Audioguru, for your reply... Yes, it struck me right after I posted my previous post. I'm currently is at ZERO knowledge in digital electronics... So I thought of setting up a timer 555 in bistable mode. The SET pulse will be provided from the receiver circuit and the RESET pulse will be provided by the user, manually. Am I going right ?

But currently, I'm having a hard time setting up the timer as bistable in Multisim 13.0, it just won't work !! Can you give some suggestions ?

You probably DO NOT WANT the burglar alarm to sound for hours if you are away. You probably want it to trigger a 555 monostable timer that resets itself (or manually) after a time duration.
I don't think that Multisim learned anything about electronics.

Okay So monostable it is... Done !! Which circuit simulator do you recommend ? And can a monostable multivibrator be reset manually ?

I don't use a SIM program for simple circuits. I (and many of us) use LTspice IV program (free from Linear Technology) for more complicated circuits.
A 555 has a reset pin. It might also be needed the moment it is powered up.

Okay, will surely look into that. Thanks a lot. I'm running the timer in monostable mode with a output pulse width of 30 seconds, the output goes high when someone breaks the path of light. Now, to make this design more robust, I'm trying to implement what godfreyl told me.

I'd recommend one small change to the basic plan - modulate the LED / laser at a few KHz. This can be as simple as driving it from the output of a 555 oscillator. Then at the receiver, detect that frequency, rather than the average light level. Pick a frequency of at least a few KHz and use a high-pass filter in the receiver so it doesn't get fooled e.g. by flickering fluorescent lights.

Click to expand...

Now, for the high pass filter, should a normal R-C circuit work ?

Also, there are many models of 555 timer out there, for simulation purposes it is okay to use any of them, but for practical, I would like to use the one which works at low voltages and doesn't drain the battery out quickly. Can you help me in this matter ?

A highpass filter is a series capacitor feeding a resistor to ground. The high frequencies appear across the resistor.

The first 555 was an NE555 made with normal transistors. Then a different manufacturer made the LM555 which works exactly the same. They work from a supply of 4.5V to 16V at fairly high current.

The second kind of 555 is the LMC555, TLC555 and ICM7555 and are made with complementary Mosfets called Cmos. They are almost the same as each other. They work from a supply of 2V to 15V at very low current. Some SIM programs do not know about these ones.

Okay now I got it running !! I'll probably use a CMOS due to their low power capabilities... Now I'm having some difficulty in building up the receiver... I want to use a photodiode/phototransistor to drive a relay. Can you give some idea ? I've searched the net and got confused with so many complicated circuits, A simple circuit example will really help.

A photo-diode or photo-transistor needs an amplifier.
They are both activated by ordinary light or heat.
They need a circuit to detect the modulation frequency of the transmitter.
They need the timer circuit.
That is why the circuit is complicated.

Hmmmm.... Maybe "Simple" isn't so simple anymore ! I'm uploading the circuit diagram and multisim file along with this post. Can you please take a look and give your suggestions ?

1. The Circuit Diagram : View attachment Door Alarm.pdf
2. The Multisim File : View attachment Door Alarm.zip

No new replies !! Have I done something wrong ?

I don't understand some of your door alarm cct. What is "k" ? That whole input cct looks a bit bizarre. 1mH / 1ohm would refer to some inductor , that I don't see.
What is switch S1 connected to / trying to indicate?
Maybe you are getting no replies , because people don't understand your diagram. Of course it may just be me :0)
Are you working from another design or are you making this up as you go along.
Maybe you should start with a working circuit until you get the hang of it.
Quick search turned up this. I have not built this , but it looks ok from a quick glance.

**broken link removed**

neddie said:

That whole input cct looks a bit bizarre.
Of course it may just be me :0)

Click to expand...

Not just you, it really is bizarre.

From what the OP's previously writtrn, I suspect "K1" is supposed to represent a relay, and I suspect he hopes the circuit will work as follows:
a) When a beam of light falls on the photodiode, it produces current which is used to activate the relay.
b) When the beam of light is broken, the photodiode switches off and the relay contact opens, triggering the 555, which sounds an alarm.

There's a lot wrong with that plan. Mainly:
a) The output current from the photodiode is tiny, not nearly enough to activate a relay.
b) You don't need a relay. Using a relay is actually a step backwards as it requires much less current to trigger a 555 directly than it does to switch a relay.
c) The output from a photodiode isn't even enough to trigger a 555. The output from the photodiode must be amplified before it can do anything useful.

Even if the output of the photodiode is amplified enough to trigger the 555, the system probably won't work anyway unless it uses a very powerful LED/laser and operates in a dark room. Otherwise the ambient light falling on the photodiode will be brighter than the LED/laser light. That's why I suggested modulating the LED/laser light, and detecting the modulation of the photodiode current.

From what the OP's previously writtrn, I suspect "K1" is supposed to represent a relay, and I suspect he hopes the circuit will work as follows:
a) When a beam of light falls on the photodiode, it produces current which is used to activate the relay.
b) When the beam of light is broken, the photodiode switches off and the relay contact opens, triggering the 555, which sounds an alarm.

Click to expand...

Yeah you got that correct, that's how I intend it to work. But maybe I made some mistakes... Okay, so I do away with the relay, and replace it with what ? And about detection of the modulating part, can I use a transistor to amplify the power and then drive the trigger input ?

You need to modulate the transmitter.
The TX section of the link below will do the job.
**broken link removed**
On the receiver side the signal is going to be very small. A few mV probably , so it needs to be amplified quite a lot.
Do a Google search on "Transimpedance Amplifiers". You will find hundreds of circuits. Find one that you can get the parts for and is simple enough
for your application. In your case even a basic circuit (opamp , resistor and photodiode) will probably work. You will probably need another gain stage and the signal will
be big enough for you to work with.
Do you have an oscilloscope?
If not I'd recommend getting one. If you are serious about doing electronics , it's a must have. For beginners and even pro's , depending on the work you are doing
a 50MHz scope will be fine.I'm still using an old Tektronix TDS210.It has it's limitations obviously ,but it's fine for most applications.
As a beginner you will not run into it's limitations in all likelihood.
cheers
Neddie