This simple digital timer circuit can be used to obtain timing output through selectable ranges, which can be set from 0 to 99 second, with 1 second interval, 0 to 990 seconds with 10 second interval, and 0 to 99 minutes with 1 minute interval. All these timing outputs can be visualized and tracked through a 2 digit common anode LED display.
As shown in the diagram, the IC 555 is wired as an astable clock generator circuit. This circuit forms the basic time interval generator stage.
The clock pulses are fed to pin 14 of IC2 7490 which is a divide-by-10 decade counter, and it divides the clocks from the IC 555 by 10, and the output is generated at its pin11.
The addition of the IC2 enables the design to produce reasonably longer time delays through an ordinary IC like the IC 555, since it converts the single pulse time intervals from the IC 555 into 10 times longer time intervals.
Thus, 1 second time periods from the IC 555 gets converted into 99 seconds, 10 seconds gets converted to 90 seconds, and 1 minute is scaled up to 99 minutes.
IC2 also allows the timing capacitor C1 to be relatively smaller and compact.
However, if you want the output delays many times higher than this, then IC 555 could be replaced with a more accurate timer IC like the IC 4060, for enabling bigger selectable ranges which can be 10 times higher than the proposed ranges.
The circuit has a 3 way selector switch, which can be used for setting one of 3 timing ranges. Each timing range has its own separate variable resistor or potentiometer which can be calibrated to further to breakdown each range into smaller time interval divisions.
Counter and Display Module
The counter and the display stage is built using IC3, IC4, IC5, IC6 and is used for showing the lapsed time intervals over a 2 digit 7 segment LED display.
The divide by 10 pulses from IC2 is applied to pin#14 of IC3, which is a binary decimal divider IC. IC3 converts the divide-by-10 pulses from IC2 into binary coded output across its pinout numbers 11, 8, 9, 1, and 12.
These binary signals are fed to the pins 6, 2, 1, 7 of IC4 which is a decoder-divider IC. The function of IC4 is to convert these binary signals into an appropriate sequence which can be interpreted as digital numbers over the attached 7 segment common anode display.
The pair IC3, and IC4 are able to process pulses upto 9 counts, after which it is carry forwards the signal to the next counter display stage consisting of IC5 and IC6.
IC5 and IC6 works exactly the same way as IC3, and IC4, but its job is to process the pulse counts higher than 9 pulses, so that the timing count above 9 can be correctly displayed over the two displays, upto the figure 99.
The integrated circuits from IC2 to IC6 all being TTL ICs require a regulated 5 V supply and therefore the circuit must be strictly operated through a 7805 IC.
How to Operate
Operating the proposed simple digital timer circuit is very simple:
Switch S4 is the ON/OFF switch which is shown on the negative line with no specific reason, it can be put on the positive line as well.
When power is switched ON through S4, the two displays may exhibit random irrelevant digits, which can be set to zero by opening the switch S3 momentarily and closing it.
Now, if the switch S2 is in the the switched ON position the digital timer will now start counting and displaying the ongoing counting process through the 2 common anode LED displays, and as per the selected time range.
If the switch S2 is in the OFF position, the timer will stay in the standby mode, and start the counting as soon as S2 is switched ON.