The post explains an innovative solution which could be used for pausing the counting process of a timer IC during power failures, and also restart the process when mains is restored, ensuring an uninterrupted functioning of the timer. The idea was requested by Mr. Arun Dev.
The Circuit Request
hello sir... this time i am gonna discuss about a different subject.... The intention behind making of this one is given below :
I need a timer circuit which should activate a relay for the determined time interval and thereafter deactivate it until a manual operation is detected...
The main purpose of doing this circuit is to charge my laptop or mobile phone over night keeping it charging for a few hours ( say max. of 4 hrs ) only......
thereafter charging cut off immediately.... even though the main intention is for charging, i also want to integrate this to operate a particular electrical equipment as per user's time decisions....
This can easily be carried out using a relay to switch the AC voltage as seen in the attached figure.....
but the only one problem regarding this is that :
whenever a power failure occurs during its ( timer ) working period, the CD4060 IC automatically gets reset and the timer starts from beginning when power returns.....
SO ANY IDEA TO PAUSE THE WORKING ( COUNTING ) OF THIS IC DURING POWER FAILURE AND RESUME FROM THAT POINT WHEN POWER RETURNS TO MAKE SURE THAT THE CONNECTED EQUIPMENTS ARE WORKING ONLY IN THE USER DEFINED TIME ??
The Circuit Design
The modified version of the above 4060 timer circuit can be witnessed in the following schematic. The circuit includes an automatic pause and restart feature of the IC's counting process during power failures and restorations respectively.
The sections which are colored in blue are the inserted modifications, we can see a battery backup being added at pin16 of the IC via diodes, and a relay at pin9 of the IC.
Since the capacitor C3 is responsible for initiating the counting process of the timer while it gets fully charged, this component could be targeted for the intended pausing/resuming of the timer.
As may be seen in the diagram, this is simply implemented by connecting C3 to the "hot" pin9 of the IC via a pair of relay contacts (N/O to be precise).
However for making the above implementation work, the IC needs to be supplied with its fundamental operating current and voltage while the mains is not available.
This is done by adding a battery back up to the IC via isolating diodes at pin16 of the IC.
The associated 10K resistor makes sure that the battery keeps getting the required trickle charge as long as the mains continues to be present.
When power is first switched ON, the relay at pin9 activates and connects C3 in the line so that the IC is able to initiate normally and begin its counting process.
During Mains Failure
In an event of a mains failure,the battery takes over and keeps the IC powered in an uninterruptible manner, while also simultaneously the relay at pin9 of the IC disconnects C3 from the line in order to stop the capacitor from losing the stored instantaneous charge via pin9, this ensures that the elapsed time period gets locked inside the capacitor for that particular moment until the mains is restored.
The moment mains power returns C3 is connected back with the circuit by the relay, enabling it to resume the counting process exactly from where it had stopped and not from zero as it would otherwise do if the above mods weren't included.
The above could be also identically implemented in other timer ICs such as in IC 555 monostable circuit or IC 4047, IC 556 IC 4022 etc.
As discussed in the comments the above designs may have some limitations and flaws, a reasonable approach may be witnessed in the below given diagram which would hopefully allow minimum discrepancy, not more than 1% +/-. See the relay connection in blue across R4 and the inclusion of the high value 10M hold resistor.