How do I install a monostable 555 timer IC on this circuit?

retched

Joined Dec 5, 2009
5,207
This only works as a SHADOW detector, not as a motion detector. So you just have to put your finger (or a card) on the photo-diode to see it works. As long as your finger is there, it will be active, when it sees enough light, inactive.
 

seanbein

Joined Mar 16, 2010
21
I would like to know whether it is possible to use a 555 timer because when there is difference in light intensity of the photodiodes (i wave my hand across horizontally) the buzzer sounds but only for a short period.What i am trying to do is to trigger the buzzer to sound longer.Do you think it is possible?I need help to connect the 555 to the given circuit
 

KMoffett

Joined Dec 19, 2007
2,918
You might try increasing the values of C3 and/or R4, since this is supposed to function somewhat as a monostable to prolong the buzzer.

Ken
 

Wendy

Joined Mar 24, 2008
23,415
I like KMoffets idea, but if you feel you have to add a 555 Monostable try this...

The 555 Projects

555 Monostable

I really like this little chippie, but sometimes I think it is a situation where if the only tool you have is a hammer everything starts looking like a nail. :D
 

seanbein

Joined Mar 16, 2010
21
Thank you all..Kmoffet do you have the calculation for the c3 and r4 to calculate the duration of time where the buzzer turns on?
I find this circuit interesting too=D
 

KMoffett

Joined Dec 19, 2007
2,918
Benching your beeper circuit: D3 and everything to the right, using a 2N7000, and triggering with a 50mS/9V pulse to D3, I get a ~1.5 second beep, with a 1/4 second tone trail-off. With a 2.2uF cap for C3 I get a ~3 second beep with a 1/2 second tone trail-off. So doubling C3 doubles the beep length. Doubling R4 would also double the beep length. Increasing R4 would also lengthen the trail-off. Doubling C3 and halving R4 would keep the same beep length but reduce the tone trail-off. If you don't want the trail-off, a 555 monostable would be a better solution. How long do you want the beep to last?

Ken
 

KMoffett

Joined Dec 19, 2007
2,918
Google RC Discharge caluclation

First you would need to know know the voltage that the capacitor charges to when pulsed. In that schematic it would be ~9V (Vdd).
Next you would need to know the exact gate voltage at which the MOSFET just stops conducting, Vgs(th)...0.8v to 3v for a 2N7000. Quite a wide range spec.
The time from turn-on to turn-off is the time for the capacitor to discharge from 9v to Vgs(th).

The formula to discharge the capacitor from Vo (9V) to Vgs(th) is:
Vgs(th) = Vo*e^[-t/RC]...where e is 2.718, t is in seconds, R is in Ohms and C is in Farads


http://www.welwyn-tt.com/products/resistors/calculation-tools.asp Click in Capacitor Discharge Calculator.

With all the wide tolerances in Vgs(th), capacitors, and resistors you will wind up experimenting with component values. With 1M, 10μF, and a 2N7000 I get ~15s

Ken
 

seanbein

Joined Mar 16, 2010
21
Thank you Ken,you have helped a lot. Since you have already assembled the circuit, I would like to ask if you know how to install a basic dark detector circuit whereby during night an led or a bulb turns on in the circuit?Iwant to add on that feature
 
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