greg123 said:I'm struggling help please!
Looked on RS and Maplin and can find little 120uF which is needed for my 555 timer circuit other than low spec alloy, which I read isn't recomended for timer circuits as this circuit needs to be stable over time.
Can anyone suggest a suitable supplier for a good and stable 120uF cap for attached circuit?
Also, while on the subject I just got a basic 10mm trimmer for the Pot, it's vital that once the pot is set the circuit doesn't 'drift' much over time do I need any special kind of pot or is a normal trimmer, used once to set the time value going to be good at sitting there and not playing up/altering resistance?
Greg.
ericgibbs said:hi,
Get a 100uF and a 22uF 16v tantalum from Maplins and connect them in parallel.
Use a 20/22 turn Cermet potentiometer.
What accuracy in timing over time are you expecting from a 555 timer.
ericgibbs said:hi Greg,
Thats the type of trimmer I often use, it will be OK in your circuit.
As you are looking for longish term stability in the time period, use 1% metal film resistors in the timing section.
If its working in a car, you should consider the 'harsh' environment in a car, eg:
temperature range, humidity and vibration and electrical noise.
A low cost 9V battery will give a poor performance at very low temperatures, also use a CMOS version of the 555, it has a wider operating temperature than a standard TTL version.
Eric
PS: is Rediffusion still in Castle Boulvard.???
EDIT: if no one else posts a diagram, I'll draw and post it.
greg123 said:I'm struggling help please!
Looked on RS and Maplin and can find little 120uF which is needed for my 555 timer circuit other than low spec alloy, which I read isn't recomended for timer circuits as this circuit needs to be stable over time.
Can anyone suggest a suitable supplier for a good and stable 120uF cap for attached circuit?
Also, while on the subject I just got a basic 10mm trimmer for the Pot, it's vital that once the pot is set the circuit doesn't 'drift' much over time do I need any special kind of pot or is a normal trimmer, used once to set the time value going to be good at sitting there and not playing up/altering resistance?
Greg.
Rolf said:I have seen 555 circuits with over 10 meg for R. I would increase R to 10Meg var. + one Meg fixed (for starters). This should require just about 5uf for the timing capacitor and overall you should have a much more stable circuit.
ericgibbs said:hi Greg,
The easiest way would be to get a low power 12Vdc relay [a high resistance coil] with change over contacts.
Connect the 12V relay across the car battery via a fuse.
Use the contacts on the 12V relay to connect the junction of the 10K and 1n0 to 0V when the relay becomes denergised.
This will trigger the monostable.
It could also be done using a couple of transistors or if you have on the shelf any CMOS gates, invertors you could use an ic.
What 9V battery life are you expecting.?
Remember also that on a cold day the car battery could drop to around 9V or so when operating the starter motor.
Does this help.?
Hero999 said:555 circuits or 7555 circuits?
Don't forget that as you increase the resistor value the leakage current becomes more important, past a certain point even the input current of the 555 needs to be taken into account.
Another way of increasing the delay is to add a resistor from pin 5 to +V. I can't remember the formula but it's on one of the application notes.
greg123 said:Hi Rolf, thanks for that. Is the circuit unstable as it is? Are you saying reduce the 120uf cap to 5uf and replace the 500k pot with a 10m pot?
Why would this make it better?
{snip}
Greg.
greg123 said:Won't the above circuit be triggered by loss of 12v feed into the trigger side? I think the way Hero did it that it will breifly drop to 0v across the trigger and thus when I cut the 12v supply to the trigger side (12v will be kept to the power side, naturally) it will activate the output.
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