Help!!
want to make a simple GCSE circuit that triggers both an monostable and an astable by the use of an LDR (and simple infrared phototransistor).
The idea is for an alarm system that is triggered by light then does not go off for 30 seconds, (will use counter!!)
i dont know how to do it?????
cheers, jd

 

We can show you how to trigger a 555 from an LDR or a phototransistor. Why do you need both?

 

because i help with gcse projects, and i will use the circuit as a generic base, one pupil wants to have LDR for a mobile phone display alarm (alarms when picked up for >30secs, and another pupil wants infrared as an intruder alarm!
jd

 

Do you want the 555 to run as the clock for the counter, or do you simply want to use it as a Schmitt trigger to give you a clean logic level from your light sensor?

 

as a clock to delay the buzzer, the time up to the pupil!!http://www.doctronics.co.uk/555.htm#triggering
the above url seems to be the type of thing, just required to coonect the ldr???? more importantly will it work?

 

LDR = light-dependant resistor?

 

yep, both light dependant resistor, and/or infrared photo transistor

 

When light hits the LDR, this triggers the 555 timer, pulses the counter, and after the determined time length ( to calculate later) the thyristor alarm goes off.
HOWEVER if the object is replaced, the pulse remains at its last output position!!!! As can be seen by the blip on pin 2 above.
HOW can I have a reset that will get it back to 0 when the LDR goes dark again????

Attached Images

Untitled-1.GIF (16.0 KB, 466 views)

 

If you connect the SCR cathode to 0V and the alarm load to the anode, then you can temporarily short the SCR with a transistor which will turn it off. You will no doubt need a second 555 one shot to drive the transistor.

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Try this one:

Attached Images

ldralarm.jpg (14.2 KB, 439 views)

 

This solution is a good start, but an SCR is the wrong device, use an FET instead.

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oops, did anyone spot my mistake??? ops: missing resistor between lower diode and 10k resistor - output 4 is accidentally tied to ground....

 

can u labek the parts????????
or make a list of needed parts

 

This thread has been dead for four and a half years. If you look at the schematics, the parts you need are labelled (although they are almost too small to be read).

Please start a new thread to explain what you are trying to do and what you have done so far. This will help you get a useful answer to your question.

Also, it's impolite to use txtspeak (i.e. 'u' instead of 'you') on forums where you don't know the culture. On this forum, txtspead is discouraged. People will be more likely to help if you use English instead of txtspeak.


Torben

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What I need is the buzzer will activated when a suddenly shadow hit the LDR.
Is this circuit working?

I also building this circuit,
but i use 2N3904 NPN transistor since it is cheaper.

My question is,
since my buzzer minimum voltage consumption is 5V.
I made a modification with insert another extra LED parrellel with buzzer.
Is it will work?

I also confuse with those variable resistor connection.
There is 3 pin of it.
How should I connect it?

Please refer to my attachment.

Are somebody suceed to make it through stripboard?
Have any photo?

Thx for advise.

Attached Images

Circuit 2.gif (11.9 KB, 16 views)