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555 timer operation

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smanches

New Member
You need a trace from Vcc to the positive side of C2 to charge the cap. There is no power to it.
 

Thunderchild

New Member
well with a discharged capacitor it should set the output high alowing for the cap to charge, when 2/3 vcc is obtained the output will go low which should mean connected to ground and the cap can discharge again. I'm sure I've done it before (in real life infact) it should give a roughly constant frequency and variable duty cycle.

at the moment the output just goes high
 

MikeMl

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Most Helpful Member
Here is a simple astable
 

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Thunderchild

New Member
yes thats it but you don't have pulse width control, now all I'm doing is adding diodes and hoes that going to cause an issue ?
 

k7elp60

Active Member
well I can't understand why the atached circuit will not work....
It should work. Perhaps you have the pot to close to one end and there is no resistance in series with the capacitor. I would try a small value say 220 ohms in series with the wiper of the pot.
 

Thunderchild

New Member
well pin 7 is discharge, you wont get any power from it. if your comtrolling a mosfet its quite ok to use pin 3 as output, my circuit is the same as the one your showing me.....
 

ericgibbs

Well-Known Member
Most Helpful Member
well pin 7 is discharge, you wont get any power from it. if your comtrolling a mosfet its quite ok to use pin 3 as output, my circuit is the same as the one your showing me.....
hi TC,
Is your problem solved.?:)
 

Thunderchild

New Member
I've done it in real life si I be damned if I know why it won'y work, its the same as the above linked circuit too, I think I may try and make it in real life later
 

ericgibbs

Well-Known Member
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I've done it in real life si I be damned if I know why it won'y work, its the same as the above linked circuit too, I think I may try and make it in real life later
hi,
Look at your Sim Scope connections!!!!!

It works OK in LTS.

EDIT: is that XSC1 a dual scope, I maybe misreading that scope connection..
 

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Thunderchild

New Member
pcbheaven: your using a pullup resistor on pin 7 to create the output signal to drive the mosfet, this is a good way of doing it however it has nothing to do with the actual operation of the circuit, the actual circuit drive is the same as what i have you are using pin 7 coupled with a resistor to drive the mosfet I am not driving anything at all just sampling pin 3 with a scope, if I were to drive a mosfet weather or not I use pin 3 or 7 (with the extra resister) it would make no difference to the circuit actually working.

Eric: yes its a dual channel scope, I think my simulator is playing me up
 

ericgibbs

Well-Known Member
Most Helpful Member
pcbheaven: your using a pullup resistor on pin 7 to create the output signal to drive the mosfet, this is a good way of doing it however it has nothing to do with the actual operation of the circuit, the actual circuit drive is the same as what i have you are using pin 7 coupled with a resistor to drive the mosfet I am not driving anything at all just sampling pin 3 with a scope, if I were to drive a mosfet weather or not I use pin 3 or 7 (with the extra resister) it would make no difference to the circuit actually working.

Eric: yes its a dual channel scope, I think my simulator is playing me up
hi,
What sim are you using.?

BTW: tried with a TTL and CMOS 555 works OK.
You dont need those power diodes, 1N4148 etc work OK.
 

Thunderchild

New Member
its multisim 9 so should do a good job, I think though sometimes it tried to take so much reality into theoretical consideration that it runs you round in circles doing things you wouldn't expect. But I can't see what the problem would be with this circuit, last time I had a problem it was because it refused to turn on a led unless the resistor in series was able to supply at least 10 mA in the real world the led would have just lit dimly
 
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