The timing of events and processes is becoming an ever increasing necessity in virtually every automation task, not only in the industrial sphere but even in the domestic sphere. No wonder timers arc often referred to as the first step towards automation.
This universal timer offers an ultrawide timing range which can be set with pin point precision anywhere from one-tenth of a second to hundred hours! Dispensing with the ever-problematic analogue dial, this timer is built around common CMOS chips, and features autolatch facility plus such a low current consumption (hat most multimeters would fail to detect it. All these features make it all the more versatile.
Readers are quite familiar with timers and must be well acquainted with the ubiquitous 555 timer which probably stands as the most widely used (rather overused) timer chip. It is indeed a versatile device if the delays required are less than about quarter of an hour. Hut for greater delays the 555 fails miserably.
Then, obviously, there are two options left. One, to use a microprocessor based timer which is not only costlier, but is much cumbersome to use as well, since most microprocessors arc ‘specialised*. The second option is to resort to digital electronics and keep on dividing the basic timing again and again! And that seems to be the most sound idea as it enables implementation of fairly large delays, which sometimes are not quite possible even with microprocessors. That’s how this design works as well.
The universal range of applications this timer can be put to are evident from the specifications (see box). The timer dispenses with the usual linear knob setting control that can hardly ever be calibrated correctly. This timer instead uses a combination of I wo thumbwheel/ rotary switches along with a range selector with 600 possible settings, which enables it to be adjusted directly with pin-point precision.
The PCBs used for the timer have been designed on an IBM compatible PC-XT. using powerful PCB CAD software to ensure maximum miniaturisation. For readers’ convenience the PCB can be had readymade from EFY associates (Kits’n’Spares. New Delhi).
Even with one of the widest ranges ever provided on a timer, this timer still keeps the cast factor the narrowest at about Rs 140.
The working of the circuit can be understood from the block schematic in Fig. 1. The actual circuit is shown in Fig. 2.
The 4060 oscillator generates a high-frequency signal of the order of several kilohertz, which is precisely adjusted to remove device imprecision errors, using preset VR 1. The oscillator inside the 4060chip oscillates to give a timing period equal to2.2 x Timing Resistor x Timing Capacitor (in farads). This basic frequency is divided by all the internal dividers inside the chip, i.e. by a factor of 16384 or 213 times. ITiis in turn provides a 10 Hz frequency with a very high degree of precision.
The same frequency is simultaneously divided by 10,6,10.6,10 by the set of five 4017 counters wired in maximum mode and divide-by-six decimal mode respectively. The output is tapped by the range selector at each stage.
This provides time periods of 0.1 sec, 1 sec., 0.1 minute. 1 minute. 0.1 hour, 1 hour respectively. These, in turn, provide a precise master clock frequency, which is fed to the two ‘serially’ connected counters, again two 4017 chips. When ICS counts ten mas-ter pulses. ICT7 counts one. Switches S2 and S3 provide control over the timer setting, the range for which is selected via switch St. S2 provides ‘tens’ digit control and S3 controls the unit digit settings.
The output of both switches are taken and fed to a NAND gate the much popular 4011 quad NAND chip. This gate changes state when both of its inputs assume high state, i.e. when the preset time is reached. This, in turn, latches the next gate with the help of ‘feedback’ diode 1N4148. The following transistor buffer drives a relay whose outputs control the load. The relay contacts can be selected to switch the load ‘on’ or ‘off’ after the preset time. The audio alarm is also driven in a similar way, and the same may be disabled by the corresponding switch S5.
The entire circuit/cycle may be reset by pressing the reset control or by interrupting power supply to the circuit.
The circuit, owing to its low current intake, may be run on batteries, or on mains—using the circuit shown in Fig.8. The power supply need not be regulated but may include a 7812 regulator, if desired. It is strongly suggested to use the 18-volt version of the CMOS ICs, and not the 15-volt type, and then run the whole circuit on 15 volts rather than 12 volts, to improve accuracy of the oscillator.
The batteries included for back-up during power failure arc a must. It should be wise to opt for rechargable batteries. In that case, the Varta make 9-volt flat battery would be a better and a lower-cost choice as compared to a set of few Ni-Cd cells.
The circuit may be constructed on a double-sided PCB as shown in Figs 3.4 and 5. The PCB, being very compact, allows the entire timer to be a compact unit. Hut the track spacing has been kept liberal to enable even a normal constructor to make the PCB himself.
The PCB must be cleaned to remove any dirt which may. otherwise, lead to shoddy joints. First solder the resistors, then capacitors, and last semi-conductors. It is strongly advisable to use the sockets for all the ICs. Or else, the PCB may get damaged in case even one chip turns out to be defective.
After the soldering is complete, check the positions of all polarised components, especially the ICs. Then recheck alt joints which should appear shiny. If all seems fine, turn the PCR upside down and clean it with some petrol/ spirit and, if feasible, give it a coat of varnish after the construction is complete.
Next, select a suitable cabinet, and fix all controls on the panel then the relay towards the rear end. Better use a socket as found in most stabilisers.
Make all ‘light’ connections with the indicators using ribbon cable. It will be a good idea to insert cable ties or tape wiresets. 16-way ribbon cable connect ions are preferable. After all connections are made, proceed with the high-current connections by using the well insulated wires used for mains current. After these connections are complete, test for mains leakage with the help of a continuity tester.
If all is well, you may close the cabinet and turn on the mains/power supply. The power indicator will light up. Now open the enclosure again and put a load, i.e. connect a device to the timer and set it to a short period, say, a few seconds, and adjust VR1 till exactly that lime delay is obtained. The accuracy on rest of the ranges will follow automatically. That’s all the adjustment needed.
After the timer is complete, it’ll be a good idea to see how simple it is to operate. Suppose you have to record a movie after five hours and also need an alarm. The following steps are needed:
1. Connect the VCR to the timer.
2. Select the second last range using SI (up to 9.9 hours).
3. Set S2 to 5 hours marking and S3 to zero setting.
4. Set delayed on using S4 and turn the alarm on using S5.
5. Turn the power on and press reset..
And that’s it.