Digital Time Switch

Yu might have noticed the street lights suddenly switching on in the evening and switching off early in the morning. Such operations for a complete row of lights are achieved automatically with the help of a ‘time switch’ which is usually mounted on an electric pole. It is generally an analogue type of ‘time switch*. Digital time switches are not yet commonly available in our country.


Using the principle of ‘ Master Slave Clock1 (published in EFY Nov.’91), 1 have developed a digital time switch. The block schematic diagram of the same is given in Fig. 1. Basically, there arc two independent clocks running synchronously. The ‘on’ clock gives a criterion for switching on the device. 1110 desired on time can be adjusted up to the last minute. Similarly, the ‘off* clock gives a criterion for switching off the device. The desired off time can also be adjusted (up to the Iasi minute) for any time up to 24 hours. The ‘on clock’ criterion operates the relay and the ‘off clock’ criterion releases the relay through a relay control circuit, This way, any device am be com rolled for a desired duration.

Circuit description

The complete circuit diagram of the project is shown in Fig. 2. IC1 uses 5369 as a master oscillator/divider and supplies 601 Hz drive at pin 1. IC2 and 1C3 use 8361, a 40-pin clock chip. The 60Hz input at pin 35 receives the drive through 100-kilohm resistors R26 and R27 respectively. To economise the design, a common display is used for both the identical clocks. Segment outputs of 1C2 and IC3 are paralleled through diodes D1- D24 and D25-D48. Common points are connected to limiting resistors R1-R24, which are connected to common-cathode display chips FND500 or their equivalent. The display will normally show the real clock time.

Set switches

S. F and R are the slow set, fast set and reset controls respectively. Applying VoC(R)at pins 32,33 and 34 simultaneously, reset the clock to 0000 hour (as the clocks are operating in 24-hour mode). As it is desired to run both the clocks synchronously, reset operation is made common through diodes D49-D54.

To set ‘on time’ bring switch SI towards ‘on CIV. Pressing the switch I) will display the on time for the device. By keeping D pressed, set the desired time by operating respective switches S and l

Similarly, by throwing the switch S1 towards ‘off CTI off time of the time switch am be set to desired time.

Relay control

In fact, the adjustment of ‘on time/ off time’ is nothing but setting of two alarms for two synchronous docks. When the alarm of ‘on clock’ is active, pin 25 of IC2 will be high and hence SR latch will set. Please note that four gates of IC 4011 have be ;n used as SR flip-flop. The output `Y` will be high. Hence, saturating transistor T1 activates relay RL1 to operate the device under control. Indication LEI)2 will light whenever the relay is operated.

Whenever the alarm of ‘off clock’ is active, SR latch will reset and the output Y will become 0. This in turn will release the relay RL1. LED2 will be no more lit.

Please note that the ‘device under control’ will be controlled by the time switch only when the automanual switch S7 is kept in ‘Auto’. Whenever the time switch circuit poses problems or the device has to be operated in addition to the automatic set time, the device can be controlled manually by operating switch S7 to ‘Manual’ mode.

Power supply

The power supply is a conventional one. Diodes D55 and D56 arc used as full-wave rectifier with transformer XI. Vcc(A) is the supply with battery backup whereas Vcc(B) is without back-up. Two Ni-Cd batteries of 3.6V each can be used, Varta make Ni-Cd rechargeable batteries are now easily available in the market. However, A 9V ordinary battery can also be used.

Vcc(B) is supplied through SI to , pins 23 of IC2 and IC3 where as Vcc( A) is extended to pins 28 of IC2 and IC3 as well as pin 8 of I Cl and pin 14 of IC4. j This arrangement will keep the oscillator IC1, memory of clocks and SR flip-flop energised even during power interruptions or failures. Only the display will be inactive during power failures. Under no-load conditions, more than 12V appearing on Vcc(A) bus may cease oscillation of IC 1 — Introduction of LED3 and D60-D62 keep Vcc(A)/ Vcc(B) within 12V and result in belter stability.


The project can be simplified if the components arc arranged as per two recommended PCBs. A motherboard (155mm x 135mm) will house all the components except the transformer and the relay whereas a display board (75mm x 35mm) will accommodate only four display chips. Both the PC’Bs arc interconnected through 26 flexible wires. The completed time switch can be given the shape of a wall-hanging and kept near the device to be controlled.

Before starting the assembly check all resistors, capacitors, transformers, transistors etc with the help of a multimeter or otherwise. First, assemble display PCB and check all the segments of all the four display chips by extending lest bench supply to + 12V or 9 V battery to anodes of every LED segment, one by one, in series with a 1k resistor. After the display PCB is tested, start assembling the motherboard. Mount all the diodes and resistors and solder them. 1

After soldering cheek all the diodes/resistors on the PC13 with the help of a multimeter once again.

Complete the assembly of master oscillator IC1+ quartz crystal and associated resistors and capacitors. Extend 9V or 12V through test bench power supply and test the functioning of the master oscillator. If oscillating, 60 Hz output should be available at pin 1 of IC1. This can be easily seen through a i DMM (digital multimeter) having provision for frequency measurements.

Now complete the soldering job of both the clock chips. Interconnect both 1 the PCBs through 26 flexible leads of 1 approximately 150mm length. Connect the various leads on the motherboard which go to various switches. Through these leads test the functioning of both the clock chips.

Now fit rest of the components, e.g. IC4, X1. switches, power supply circuit and two Ni-Cd batteries. These batteries will automatically gel charged and supply back-up current during power interruptions. Mount the motherboard on the back cover of the box and mount display PCB, switches and indicators on the front panel.


After the assembly job is over, test the project as follows, keeping switch S2 in‘on position:

(a) When the time switch is switched on. the display may be blinking. If so.

note the position of SI and with the help of respective S or F switches make the display steady.

(b) By pressing R reset both the clocks.

(c) By keeping SI in off CTL position adjust the real clock time. Similarly, in on CTL position adjust the real clock time.

(d) Keep SI in on CTL position, and by keeping I) pressed, adjust on time with the help of respective S and F switches.

(e) Keep SI in off CTL position and similarly set off time.

Now the time switch is ready for use. Extend the power socket to device under control through auto/manual switches S7 and S8. Switch S2 is an optional switch, which when operated to disable position will prevent clock time from getting disturbed by accidental pressing of S or Fswitches. Keep switch S2 in enable position while setting the time etc.

Sometimes it may happen that, depending upon the setting of SR latch.

the device may remain switched off when it should be on. and vice-versa. Under these conditions, keep the switch SI to on CTL or off CTL position and make the real clock time ahead by 24 hours with the respective F and S switches. It may set to correct position after one or two trials.

The digital times witch will be more useful when the device has to be controlled precisely up to the last minute. The analogue time switch has a least count of about 15 minutes.

Tie most popular application of th is time switch can be in controlling the daily pump-motor operation, the duration of which is normally fixed, depending upon the water consumption. This time switch can be designed for 5 A or 15 A of the load current, The relay contacts will have to be of the appropriate rating, the complete design remaining the same.

It has typical industrial applications such as controlling street lights, compound lights and in oil heating installations, water processing plants, electric heaters etc. In general, it can be used to control any electrical device or appliance.

Readers’ Comments:

I could not make the Digital Time Switch project published in EFY March’92 issue because of certain discrepancies in figures.

Capacitor C2 and crystal XL are shown connected between pins 5 and 6 in Fig, 2 but they arc shown between pins 5 and 8 in the PCB pattern! Please clarify and give some more information about pins 24, 26, 27 etc ofIC 8361.



I am thankful to the author for his superb Digital Time Switch project published in EFY Mal:92 issue. As IC LM8361 is not available in the market can I use IC LM8362 instead?



The author, Mr S.Batra, replies: Capacitor C2 and crystal XL are shown correctly in Fig. 2 whereas they have been shown wrongly connected in the PCB layout Inconvenience caused is regretted.

IC836l’s pin 24 is used when alarm facility of the clock is in use. When the alarm rings, but you want to sleep some more (snooze), a high potential (snooze input) is extended to this pin through a pushbutton. This enables snooze and the alarm rings automatically after nine minutes. One can have this facility for an hour. Pin 26 is used to stop the alarm. When the alarm is ringing, if the high potential is extended to pin 26 (alarm off input), the alarm stops and rings the next day at the same time.

Pin 30 comes into action when IC 8361 is used for a clock-cum-radio. This pin for ‘sleep display’ feature enables one to listen to a radio up to 59 minutes. It is found useful if you wish the radio to switch off automatically after a desired period when you are going to bed. With the pushbutton pressed, if the desired time is set through the slow set switch, a maximum 59-minute down counter is activated and the radio is switched on.

Pin 27 is a ‘sleep out’ point which is used to activate the radio while using the sleep down counter.

I have gone through the technical data of both the ICs and have found them pin-to-pin compatible to MM5397 except for a minor difference which is not relevant to this project. Therefore, 8362 may be used in place of the 8361 without any hesitation.

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