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How do you design a circuit so that a capacitor will never go below a certain voltage?
Ok I'm trying to make a traffic light out of 555 timers, however the charging time towards 1/3 Vcc is throwing my caculations off.... so how do you make it so that the capacitor will never go below that value so i dont have to worry about those things...
I am using the 555 timer as a astable multivibrator in conjunction with other 555 timers. The output of the main 555 timer is used as the source for the next 555 timers. Because it is a design of a traffic light, the up times are very long so charging towards 1/3 Vcc is very disruptive...
its for a school project and 555 timers are the requirement without other logic gates (actually, our class has already done similar projects on logic gates) or such so microcontrollers are definitely not allowed... its very disruptive because i can calculate the resistors for charging time up to 2/3Vcc, which is the uptime at the first cycle, however the uptime for the succeeding cycles are only from Vcc/3 to 2*Vcc/3.
In other square wave oscillators i can just ignore the first cycle; however, for this one i used the output of the main 555 as the source for the other 555 timers... so that their charging and discharging timings are off.
I'm thinking if i can vary the frequency for the next cycles however i do not know how to do so as well.. Again, Thanks for all the feedback
2 Answers
- veeyesveeLv 77 years ago
Try LM2240 . For traffic light when times are of the order of 100's of secs, you will have to use a large capacitor. large capacitor is fine but its leakage is not.I guess you will use electrolytic and tantalum is better. nevertheless the leakage cannot be less than about 10 times the sum of the reistsors r1 and r2 connected from pin 8 to 7 and 7 to cap. the 2240 reduces the value required to almost 1/128 of this. perhaps 1/256 ??
Thus instead of using a 1000uF, you can use a 8uF with much higher leakage resistance.
What is it about not charging the capacitance to negative? The capacitor starts from 0 and oscillates between 4V and 8V if supply is 12V. A 12V rated capacitor should be fine. Another point is that leakage of a 12V capacitor is about 5 times that of a 50V capacitor. So 50 V rating gets you a lower leakage (desired). Low leakage means high leakage resistance.
- billrussell42Lv 77 years ago
The 555 is setup internally to switch at Vcc/3, so you would need a different timer circuit, which you can do with opamps and other components.
But I don't see your problem. What calculations?
why is charging towards 1/3 Vcc very disruptive ?
Is your problem large electrolytic caps that have leakage problems?
555 timers are not well suited to this application. You should do this digitally with a microcomputer of some sort.