[SOLVED] Burglar Alarm Circuit

[Audioguru]

You need to modulate the transmitter.
The TX section of the link below will do the job.

It is WRONG like most circuits on that website.
Simple arithmetic shows that the transistor will soon burn out:
1) With a 9V supply and the IR LED has a forward voltage of 1.5V then the current is (9V - 1.5V)/18 ohms= 417mA.
But the maximum allowed current for a BC547 transistor is only 100mA. A 2N4401 transistor has a maximum current of 600mA and should be used.

2) The base resistor is much too high at 10k. With a 9V supply, the output high of the 555 could be only 7V. To properly saturate, the transistor needs a base current 1/10th its collector current. For 42mA, the base resistor value should be (7V - 0.8V)/42mA= 148 ohms, use 150 ohms.

EDIT: Will the IR LED survive 417mA?

 

[DasPreetam]

My Question is : Why use the transistor at all ?

 

[godfreyl]

My Question is : Why use the transistor at all ?

Good question. I would leave it out and drive the IR LED directly from pin 3 or pin 7 of the 555, with a suitable resistor in series.

edit: Just check the maximum output current of the chip you're using. I suspect some "low power" versions can't give as much as a "standard" NE555.

 

[DasPreetam]

Okay, so that goes the transmitter part... I'm having some problems with the receiver section. As far as my concept goes, the photodiode detects the light and converts it into current/voltage. Then an amplifier amplifies the voltage and feeds into a high pass filter which rejects out lower frequencies (such as some smartass burglar flashing his torch ), then pass it through a rectifier-and-filter stage (like those found in simple DC power supplies), and feed the output into the trigger terminal. Right ?

 

[Audioguru]

One of these attachments uses a TSOP IR receiver IC that reduces its gain a lot when it receives continuous 38kHz modulated IR to reduce interference from modern compact fluorescent light bulbs that continuously modulate their IR and light at 38kHz. The TSOP is designed to have full gain when the 38kHz is in bursts of data.
Therefore the IR LED in those circuits that is continuously modulated at 38kHz must have its current pulses VERY high to be received. Much higher than the max output current of an ordinary 555.

 

[DasPreetam]

No, I'm not talking about the TSOP IR Receiver, I'm just asking that my concept is right or not.

I'm uploading a file. I've made some modifications based on the suggestions I got from here. I've emulated the photodiode with a 0.2V 1Khz clock voltage supply. The op-amp amplifies the voltage and the output is filtered and fed into the trigger input of the timer 555. Please do look into the design and give your suggestions.

I'm still working on "transimpedance amplifier", can anyone suggest a site where I may learn about them ?

 

[Audioguru]

Why do you think a photo-diode produces 0.2V? Maybe during a very close by lightning bolt! You need A LOT more gain.
Your opamp has no bias voltage (or a dual-polarity supply) so it WILL NOT WORK anyway.
If the opamp worked then you are rectifying its output to produce a positive voltage but a 555 is triggered only when its trigger input goes low, not high.

A lousy old (46 years old design) 741 opamp is too noisy to be a preamplifier. Like many opamps, its inputs do nothing when they are closer than about 3V from the negative supply voltage which is 0V in your circuit.

A photo-diode can have zero bias voltage then produce an output when receiving light like a tiny photo-cell. Because it is so small then its output current is extremely low.
If it is reverse-biased then its capacitance is reduced for higher speed then light causes it to leak a tiny current.
A transimpedance amplifier is good to amplify a tiny current.

 

[DasPreetam]

So you suggest I replace the op-amp with a transimpedance amplifier ? Any good site from where I can learn about them ?

 

[Audioguru]

There are many good articles about transimpedance amplifiers and photo-diodes in Google.

 

[DasPreetam]

Okay, I've made some modifications in my design. Now I'm using a 10ua 1KHz current source to emulate the photodiode which drives the inverting input of the Transimpedance Amplifier. The output of the amplifier holds the trigger input above 1/3 Vcc. Rest part of the circuit remains same. The circuit is working fine in multisim. So, any suggestions on improvements ? Or should I proceed with buying the parts and testing on a breadboard ?

Interestingly, I found LEDs can also function as photodiodes in this video !

So can I use LED in place of the photodiode ? Which one will be better ?
I've uploaded the modified circuit. Please do look into it.

 

[Audioguru]

You are not using a transimpedance amplifier. Instead you have a 46 years old lousy 741 opamp that DOES NOT WORK when its inputs are anywhere near the pin 4 voltage and yours are the same voltage. Also many 741 opamps DO NOT WORK when their supply voltage is only 6V since it was designed to use a 30V supply.

You have R3 triggering the 555 all the time if the IR beam is blocked (it will sound the buzzer for half a minute when the circuit is first powered). Then when the opamp (if it was biased to work properly) receives a signal the rectifier produces a positive voltage then the 555 turns off after it times out.

So when a burglar blocks the IR beam the 555 is triggered and the buzzer keeps on buzzing until the burglar moves away from the beam.

I think you should use an IR photo-diode not an IR LED as a sensor. I would probably use an IR photo-transistor feeding an ordinary opamp.

 

[DasPreetam]

You are not using a transimpedance amplifier

What do you mean ? That's the circuit I've found all transimpedance amplifiers use ! Which op-amp should I use ? I've used the 741 in the lab using 6V and it works fine. Don't know if it will work now..

You have R3 triggering the 555 all the time if the IR beam is blocked (it will sound the buzzer for half a minute when the circuit is first powered). Then when the opamp (if it was biased to work properly) receives a signal the rectifier produces a positive voltage then the 555 turns off after it times out.

That's why the switch (key=B) near the 100uF capacitor is provided. It is used the RESET the timer when it is switched on initially.

So when a burglar blocks the IR beam the 555 is triggered and the buzzer keeps on buzzing until the burglar moves away from the beam.

I don't think so. The 555 is designed as a monostable multivibrator, so once someone blocks the light path, it will ring the buzzer for 30 seconds unless the key=B is pressed to reset it.

 

[Audioguru]

What do you mean ? That's the circuit I've found all transimpedance amplifiers use ! Which op-amp should I use ? I've used the 741 in the lab using 6V and it works fine.

Look in Google. A transimpedance amplifier usually has Fet inputs and very low input offset voltage. (NOT an old 741 opamp.)
Your opamp is not biased correctly to work.

The 555 is designed as a monostable multivibrator, so once someone blocks the light path, it will ring the buzzer for 30 seconds unless the key=B is pressed to reset it.

The datasheets for the 555 say that the trigger pin #2 must go high before the timeout or it will not timeout. Signetics invented the 555 and their datasheet shows how to capacitor-couple the trigger input to fix this problem.

 

[DasPreetam]

Which op-amp should I use in place of 741 ?

 

[Audioguru]

Which op-amp should I use in place of 741 ?

I do not know which country you are in and I do not know which manufacturer's parts are available there.
Some opamps are made with a very high input resistance and work from a single positive supply voltage unlike a 741 that has a fairly low input resistance and usually needs a dual-polarity supply.
I found in Google that Linear Technology has some good transimpedance amplifier ICs: https://www.linear.com/solutions/Transimpedance_Amplifiers

 

[DasPreetam]

Okay many many thanks, will definitely look into it. I live in India.

 

[Audioguru]

I am in Canada. I do not go to a forum in India and ask what opamp should I buy there.
But you are in India and you come to this website and ... never mind.
There are many people in India on this forum. I wonder why they do not reply.

 

[DasPreetam]

Hmmm okay... Another question. I need to raise the output from the Transimpedance amplifier to a level above 1/3rd Vcc in order to hold the trigger input high. So my question is... Can I use a transformer instead of an amplifier ? Because I don't need much gain in current or power, I only need voltage gain.

 

[Audioguru]

I need to raise the output from the Transimpedance amplifier to a level above 1/3rd Vcc in order to hold the trigger input high. So my question is... Can I use a transformer instead of an amplifier ? Because I don't need much gain in current or power, I only need voltage gain.

Increasing the value of one resistor of the amplifier increases its voltage gain.
But your circuit will not work with a lousy old 741 opamp so which opamp will you use as a transimpedance amplifier that will work with a positive 6V supply and no negative supply?

 

[DasPreetam]

After much considerations, I've decided to drop the idea of using a Transimpedance Amplifier. Way too complex for my present level of knowledge. Instead I've found out about an IR sensor called TSOP1738. So, I've emulated it using a 5V Battery with a PBNO switch.. and it works !! So, is this the final solution ? Or do I have areas to improve ?

I'm uploading the circuit design alongwith.