555 mono (restriction on Resistance value)

[vinodquilon]

Find the attachment (555 mono schematic in Cadsoft Eagle.) . The purpose of the circuit is to turn on reed relay for 60sec, when
first ring_signal occurs. For that I have to use C= 100 uF & R= 560 K. But somewhere I have
read, in the design of 555 mono take R= 10 K to limit current through the internal transistor
to 1 mA.
Is it correct ? Can I use 560 K ?

It is my first time experience with Eagle to draw schematic. If someone notice any drawing faults
in my schematic, pls inform me to improve my drawing skill with Eagle.....

 

[Hero999]

560k is higher than 10k which means a lower current according to Ohm's law so why would exceed the maximum rating?

Incidentally I thought the minimum resistance was more like 470R but it depends on the power supply voltage.

 

[k7elp60]

For that I have to use C= 100 uF & R= 560 K. But somewhere I have
read, in the design of 555 mono take R= 10 K to limit current through the internal transistor
to 1 mA.....

My concern is the 100uF capacitor. That value probably will not work as the leakage resistance will not allow the capacitor to fully chargeso the timing will be off.

 

[Hero999]

It depends on the capacitor.

If it's a tantalum then there should be no problem.

With large capacitor and resistance values, it's a good idea to factor the leakage current into the equation, the trouble is it's unpredictable and varies depending on the voltage across the capacitor so it can be a bit of a headache.

 

[vinodquilon]

PICAXE-08 program

Simple PICAXE-08 program to create accurate 60 sec output time delay when first ring trigger occurs.

start:
if pin3=1 then holdoff
goto start

holdoff:
high 2
wait 60
low 2
goto start


See the attachment. Opto-couplers' output goes to IP 3 (pin no.4) of PICAXE-08.
I will connect IP/OP 2 (pin no.5) of PICAXE-08 to base of relay driving transistor BC547.
Thus I can accurately turn on relay for 60 sec when first ring signal comes.

I am newbie in the field of PIC microcontroller. Thanks for all help by IanP of edaboard.

I couldn't find any eagle libraries for PICAXE-08 on cadsoft. Does anyone have ?