555 timer circuit help

Hi guys

i need help with a 555 timer circuit that i am trying to design , the 555 timer is used as a LED flasher , it has three user controlled settings Low ,Medium and High.

Low denotes a time interval of 60 Min,Medium of 24 Min,High of 10 Min , the problem is i want the circuit also to compensate for the time when the user changes the setting , will try to explain what i mean by compensate .

Suppose the circuit is in Low which means 60 Min and a time period of 20 Min has elapsed and suppose the user changes the setting to Medium which means 24 Min ,since already 20 Min has elapsed , the circuit should take only 4 additional minutes to turn on.

how can the circuit remember the previous time elapsed without a lot of additonal components , just with a basic knowledge of it i thought since the capacitor already has charged/discharged some amount of charge when the resistance is changed the remainder charge would set the time accordingly .

please correct me if i am wrong and all suggestions are welcome.

risingcurrent

You can use this circuit or modify it to match your circuit.
C1 & R1 will give you about 60Min.
C1 & R2 will give you about 24Min.

When you switch between the 2 resistors you'll switch between the 2 periods without reseting the period.

Note: To be honest with you, I haven't tried this ckt, I only simulate it.

Be carfull when planing long timming periods from a 555 timer . The circuit shown relies on a 1000uf capacitor. This is a component with a high leakage current and I would recomend a tantalum capacitor or a none electrolitic equivelent. Even so it may appear to work when first constructed but may well change duo to ageing and enviromentl factors

Barrybear

Thank you seadolphine2000 for the circuit , will let you know if it works actually after i build it ,thank you barrybear for your suggestion , after i build it will post whether it works and if not the problems.

thank you

risingcurrent

Basically, the capacitor is what can "remember" the elapsed time.

However, for your requirement, I would recommend you change the period by adjusting the voltage on the control pin, rather than changing the resistor in the timing circuit (you cannot change the capacitor, since you need it to retain the voltage or "memory" of the previous time).

Take a look at the picture. The circuit times out when the cap voltages reaches the top dashed line.
In the first case, assume you are using the long time constant (blue line). Then you change the resistor and hence the time constant, when 10 minutes have elapsed. The new time constant follows the red curve. But look what happens. For the red line, the voltage on the capacitor corresponds to only 4 minutes. In other words, the circuit will now behave as if only 4 minutes have elapsed, timing out after another 20 more minutes, not after 14, as you want.
So changing time constants will not provide the desired result. In a similar way you can start with the red curve and see what happens as you switch to the blue one.

In the second picture, you only use the longest time period that you need, but adjust the threshold (by altering the voltage at the control pin, so as to change the timeout period. So the circuit times out when the cap voltage crosses one of the horizontal dashed lines, representing the voltage on the control pin.
Now, look what happens. Almost 10 minutes have elapsed, and if you are using the lowest threshold, the circuit will soon time out. But, if before it does so you raise the bar and change the threshold, it will time out at 24 or 60 minutes, measured from the moment it started. You are not doing anything to the time constant, you are merely changing the trigger point, which means you decided to wait longer. If 20 minutes have elapsed and you change the threshold to 10 minutes, the circuit times out instantly, which is what you want.

So just use a switchable divider on the CONT (control) pin.
The voltage on the control pin can be made both higher and lower, so you may be able to use a smaller capacitor for your longest time period.
Pay attention to the datasheet and make sure you keep the control voltage within those limits.
The voltage on the TRIGGER input should always go lower than half the voltage you have at the control pin. If you pull TRIGGER all the way to GND, it will work.

VVV,...

you are great, this looks much much better than my picture.

I think you'll need potential divider to set the desired control threshold voltage.??
Right.????

There is a divider inside the chip, that establishes the control voltage to about 2/3 of Vcc. You could just use two external resistors to alter the voltage.
If you connect one resistor to GND you get a lower threshold voltage, if you connect a resistor to Vcc, the threshold will be higher, and if you leave the pin open (with a cap to GND), the threshold will be the nominal 2/3 Vcc.

The only problem is that the internal resistors may not be well controlled as an absolute value (their ratio is). So, the external resistors will be different from device to device.

I could not find it in the latest datasheet, but I remember there were some limits for this control voltage. I think the highest is 0.9Vcc, and the lowest 0.4Vcc, to make sure the chip still operates. Obviously, connecting the control pin to Vcc would prevent the operation, since the cap charges from Vcc, so the voltage may never reach Vcc, due to leakage in the cap. Too low a value my prevent the internal comparators from operating, because I think they have current sources, that need a minimum voltage, so you cannot make the control voltage zero, either.
I hope I remeber those 40%-90% of Vcc number correctly.
It's worth checking out the datasheet for the CMOS version, too.

The voltage at control pin(5) is:
min=9.0V, typ=10.0V, max=11.0V at VCC=15V and
min=2.6V, typ=3.333V, max=4.0V at VCC=5V.

You'll need to change the voltage at pin5, right.???

The voltages you mention are open-circuit, thus due to the internal divider. You can measure that voltage at pin 5 with a voltmeter.
Yes, to control the duration you need to force this voltage higher and lower, using an external divider.

I found an old NE555 Signetics datasheet that recommends that the control voltage be kept between 45% and 90% of Vcc. That does not allow a 6:1 period adjustment.
However, the same datasheet said that those values were just a guide and that in some applications both higher and lower voltage had been used.
To cover the required range (6:1), a 30% to 90% variation would suffice. I think that is perfectly achievable.

For the CMOS version (TLC555 from TI), the limits are 10% to 80%. That allows a much larger adjustment range of 15:1. The external divider for this CMOS part should have at least 0.5mA bias through it, which is quite reasonable.

One suggestion: when you first test the circuit, choose a reasonable time constant for testing, such as 2min instead of 60. Thus, the short interval becomes 20sec, which will allow you to see clearly how the circuit behaves, without wasting a lot of time.

vvv

the explanation you provided was really good , thanks a ton for taking the time to explain , seadolphine thank you too.

thank you guys

rising current

did you design a new IC ?