555 timings

[scrawny git]

Originally posted this yesterday in the 555 timer tutorial thread below, but no response.

I have built the circuit shown in the attachment to automate the on/off for a 24V motor - works fine, but now I need to adjust the timings. I have been playing around with different value resistors at R1 and R2 to do this but whatever I do I can't manage to get a big discrepancy between the on time (t1) and off time (t2).

Ideally what I want is for t2 to be at least five times the length of t1, so if it's on for about 1 second, it's then off for say five or six, perhaps even ten. I have had a look at the timing calculator linked in the other thread and no matter what values I input I can't get such a wide discrepancy. Any ideas?

 

[ericgibbs]

Originally posted this yesterday in the 555 timer tutorial thread below, but no response.

I have built the circuit shown in the attachment to automate the on/off for a 24V motor - works fine, but now I need to adjust the timings. I have been playing around with different value resistors at R1 and R2 to do this but whatever I do I can't manage to get a big discrepancy between the on time (t1) and off time (t2).

Ideally what I want is for t2 to be at least five times the length of t1, so if it's on for about 1 second, it's then off for say five or six, perhaps even ten. I have had a look at the timing calculator linked in the other thread and no matter what values I input I can't get such a wide discrepancy. Any ideas?

hi,
Have you seen this 555 tutorial, if dosnt help let us know.
https://www.unitechelectronics.com/NE-555.htm

EDIT: one method of changing the mark/space ratio is to use a diode across one the timing resistors.

EDIT: image, example.

Or across the lower resistor.

 

[scrawny git]

Thanks.

Right, apologies for my ignorance (i never learnt lectronix) but

the only diode i've got to hand is the rectifier diode I used on the other side of this circuit (IN4003) which I'm assuming is a bit much for this and that I would be better off with a simpler signal diode, say IN4148, right? wrong?

and you've put in a variable resistor: what sort of value does this need?

 

[ericgibbs]

Thanks.

Right, apologies for my ignorance (i never learnt lectronix) but

the only diode i've got to hand is the rectifier diode I used on the other side of this circuit (IN4003) which I'm assuming is a bit much for this and that I would be better off with a simpler signal diode, say IN4148, right? wrong?

and you've put in a variable resistor: what sort of value does this need?

hi,
That rectifier diode will be OK.
If you have 1MΩ variables try one of those.

All the diode/resistor do is to make the cap charge and discharge times different.
This changes the mark/space ratio of the pulse, a little experimentation will be required...

 

[scrawny git]

a little experimentation will be required...

great. thanks again.

 

[scrawny git]

wow. works a treat. and with very little experimentation i have got a good idea about how to change the timings.

Just for the record, the way eric drew it increases the mark and putting the diode and variable the other way around over the same resistor increases the space. magic.

 

[neon]

Originally posted this yesterday in the 555 timer tutorial thread below, but no response.

I have built the circuit shown in the attachment to automate the on/off for a 24V motor - works fine, but now I need to adjust the timings. I have been playing around with different value resistors at R1 and R2 to do this but whatever I do I can't manage to get a big discrepancy between the on time (t1) and off time (t2).

Ideally what I want is for t2 to be at least five times the length of t1, so if it's on for about 1 second, it's then off for say five or six, perhaps even ten. I have had a look at the timing calculator linked in the other thread and no matter what values I input I can't get such a wide discrepancy. Any ideas?

the total period is =t1+t2=t1=.693[ra+rb]c t2=.693 [rb]c
the f=1/tau=1.44/[ra+2rb]c
duty cycle =rb/ra2rb
you should be able to figure it out this way.
50% =1/t1+t2 up 97% or so. for those timiming you might consider a current source. 10mega is big

 

[Hero999]

Increasing the voltage on pin 5 also helps to increase the delay, for example, set it to 86.5% of the supply and you'll get double the RC time constant.

 

[neon]

Increasing the voltage on pin 5 also helps to increase the delay, for example, set it to 86.5% of the supply and you'll get double the RC time constant.

that pin input is for modulation any noise fron vcc and your frqency will vary greatly

 

[Hero999]

Then add a 10nF capacitor to it like you should anyway.