Laser alarm

[LaserAlarm]

All good ideas guys.
But I would still like to make this my self. Half the fun is making something.

I could take apart a old smoke detector and use the alarm that's in side.
What type of relay could I use to keep the alarm on after the laser beam has been broken?
**broken link removed**

 

[ericgibbs]

hi Dan,
None of those bits you are trying to string together are compatible.

The night light is switching your local mains supply voltage.
You would need a mains relay, then a dc supply or battery to drive the piezo sounder.
The 3.2Vac adaptor is 'ac' the laser is most likely 'dc' powered

 

[Andy1845c]

All good ideas guys.
But I would still like to make this my self. Half the fun is making something.

I agree But seeing that your new to electronics, you might want to go back to the original battery powered circuit. Messing with ac mains can be dangerous.

As Eric said, the nightlight is switching the mains voltage. If your in the USA, thats 120v ac. Works fine to light a small night light bulb, but will damage a small, low volatage dc alarm. They do make 120v ac buzzers, but the ones I am familar with draw more current then the little night light can probably supply.
You could use a small relay with a 120v ac coil to switch a small low voltage dc alarm powered by a battery, but this could be a shock hazard if it isn't done right or you make a mistake. Plus you will still have to have a battery on the alarm end, it won't all be mains powered unless you want an addtional wallwart transformer, and that just dosn't make alot of sense to have two power supplys when one will do fine.

I suppose you could use an alarm out of an old smoke detector, but I've never tried to remove one so I can't offer any advice on how hard they are to get out. If you take one out of a battery power smoke alarm, it will likly be a 9v alarm, so it sould work well in the original circuit that John re-drew for you.

 

[LaserAlarm]

Andy1845c
Like this?
**broken link removed**
**broken link removed**
What about a relay to keep siren on after beam is broken?
Also could I use just the photocell from the nightlight?

 

[ericgibbs]

hi,
Something like circuit this would be OK.

EDIT: Measure the resistance of the light detector, when dark and light, let us know the value.

 

[LaserAlarm]

ericgibbs
How do you measure the resistance of the light detector, when dark and light?
I am a novice at this.

 

[ericgibbs]

ericgibbs
How do you measure the resistance of the light detector, when dark and light?
I am a novice at this.

hi,
If its an LDR [light dependent resistor] use a ohm meter across the LDR, shine a torch/bright light onto the LDR.
The resistance indicated by the meter will fall to a low value,,, note it.

Now cover the LDR so no light, the resistance will rise, note it.
These are the light/dark limits.

If its a PD [photo diode] connect the cathode to say +5V via a 10K resistor and the anode of the PD to 0V.
Measure the voltage limits across the PD as you did the LDR.

If its a PT [photo tansistor] , connect the collector to +5V via a 10K and the emitter to 0V, measure the voltage across the PT as you did for the PD.

The 2N7000 circuit I posted is working just fine on the bench, using an LDR.

The 2N7000 costs about 50cUS.

 

[ericgibbs]

hi Dan,
Simpler version of the latching 'missing beam' detector.

Tested, works OK. Use a piezo acoustic sounder on the FET output.

 

[LaserAlarm]

**broken link removed**
Iam a novice I have no idea what parts are in your drawing. I would need a list or pictures of each part.

 

[SMUGangsta]

making the relay latch is really easy, just split the 'switched' output so it goes to the siren AND back into the coil, so once it is activated you need to cut the power to turn off the siren - although you could add a keyswitch or hidden switch or something to interupt the supply

 

[ericgibbs]

making the relay latch is really easy, just split the 'switched' output so it goes to the siren AND back into the coil, so once it is activated you need to cut the power to turn off the siren - although you could add a keyswitch or hidden switch or something to interupt the supply

hi,
Thats the circuit I posted on 17thOct.

 

[SMUGangsta]

So it is, i looked at most of the diagrams but i missed that one - a lot of diagrams in this thread.

Sorry about that

 

[ericgibbs]

So it is, i looked at most of the diagrams but i missed that one - a lot of diagrams in this thread.

Sorry about that

hi,
No problem, for the simple application that first circuit would be OK.

I think that the second circuit is better in that it dosn't require a relay and the power requirement is lower
in the event Dan used say a 9Vbty.