# triggering a 555 timer from an LDR????

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#### jsdriscky

##### New Member
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

#### Roff

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

#### jsdriscky

##### New Member
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

#### Roff

##### Well-Known Member
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?

#### fat-tony

##### Member
LDR = light-dependant resistor?

#### jsdriscky

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

#### jsdriscky

##### New Member
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????

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#### Russlk

##### New Member
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.

#### Phasor

##### Member
HOW can I have a reset that will get it back to 0 when the LDR goes dark again????
Try this one:

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#### Russlk

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

#### Phasor

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

#### tejas612

##### New Member
yep, both light dependant resistor, and/or infrared photo transistor
can u labek the parts????????
or make a list of needed parts

#### Torben

##### Well-Known Member
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).

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

#### ysgoay

##### New Member
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?

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

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#### ericgibbs

##### Well-Known Member
What I need is the buzzer will activated when a suddenly shadow hit the LDR.
Is this circuit working?
The circuit looks OK.

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

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?
You need a resistor in series with the LED

I also confuse with those variable resistor connection.
There is 3 pin of it.
How should I connect it?
Link the centre pin to one of the end pins.
then connect into the circuit, one wire to the link and the other wire to the pin with no wire..
Code:
Connection>  ------0+++0      0------------

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