laser alarm problem

[jepoy12]

hi, Im currently making an Laser alarm using the 741 op-amp as a comparator.
heres the circuit.

but then I came up with a problem. If I put the laser beam directly to LDR and a moving object(for example like man) moves in across the laser it will only buzz if ever he/she will stay and block the beam hitting the LDR. What I wanted to do is when an object PASS unto it, it will then trigger the alarm and start buzzing and it will only stop when someone hit the reset button. so I start searching on how to make that possible then I saw this video showing the one that I wanted.

www.youtube.com/watch?v=1k9bZklac6Q

I tried to connect my 741 dark detector to his 4017 decade counter and follow his schematic.
and the result was the buzzing wont stop even if I hit the reset button. but all works fine like what I want only the reset is not working.
help me please anyone.
thanks.

 

[Laplace]

The 4017 counter will not meet the stated design specifications. What you need is a CMOS RS latch or SR flip flop (Google 'RS Latch'). Connect the comparator output to the S input. Use the switch for the R input. The 741 on a 9V power supply may not reliably drive a CMOS logic input due to insufficient voltage swing. Try the LM358 instead. Then use one of the quad gates to buffer the logic output and drive the transistor.

 

[jepoy12]

were not allowed to use other than 741.
umm...how did the guy at the YT make it work? all I did was follow his schem.

 

[duke37]

I could not make out much from the video, it would be better to sketch out the circuit and scan it and post it.
The circuit you show does not have a reset.
This circuit can be made to latch by feeding some positive feedback from p2 to p6 (try 100k). Look up Schmitt trigger.
The problem here is that it may latch on switch on. To reset, short R1.

 

[jepoy12]

heres what I made.
it keeps buzzing but if you hold the reset it will stop.
what I wanted to make is when I hit the reset it will stop the buzzing then device goes back to normal.
need help people. : (

 

[duke37]

That is an excellent drawing, I wish I could do as well.

I have not studied the circuit around the Johnson counter but the 741 has the + input connected to ground with R1 just wasting power. This cannot be right.
What does R6 do?

 

[engineering king]

can you show me how to draw this circuit in the board

 

[duke37]

Engineering King

I am not happy with this circuit, I think it needs quite a bit of correction. I have not analysed the circuit but have pointed out obvious errors.

Are you asking how to construct the circuit?
For a simple circuit such as this I would use strip board (Vero board) and work from one end of the diagram to the other.
Make sure that you get the pin numbers right, they are normally given in the top view whereas you will be soldering on the under view.