5 Different Timer Circuits

The Timer circuits are used to produce time delay intervals for triggering a load. This time delay is set by the user.

Below are few examples of timer circuits used in different applications

1.     Long Duration Timer

This timer circuit is designed to switch on a 12 V load in a solar-powered installation for a preset period at the press of a button. When the period has expired a latching relay disconnects both the load and the controller circuit from the 12 V supply. The length of the period can be configured by making suitable changes to the microcontroller’s source code.

Video on Long Duration Timer Circuit Diagram

Working

The IC4060 is a 14- stage binary ripple counter which generates the basic time delay pulses. Variable resistor R1 can be adjusted to get different time delays. The delay pulse is obtained at IC 4060.  The counter output is set by a jumper.  The output from the 4060 goes to a transistor switch arrangement. A jumper sets the option. – the relay can turn ON when power and counting start then turn OFF after the count period, or – it can do the opposite. The relay will turn ON after the end of the count period and stay on so long as power is supplied to the circuit. When the supply is ON then transistors T1 and T2 are activated, then supply voltage is slowly goes to low. The supply voltage start at 12V when supply is ON then slowly goes down. This is working of long duration timer.

2.     Fridge Timer

Generally the power consumption by the domestic refrigerator is quite large during peak hours from 6 pm to 9 pm and is much more on low voltage lines. Hence it is most appropriate to switch off the fridge during these peak hours.

 Here a circuit is demonstrated which automatically switches off the fridge during this peak period and switches it on after two and half hours, thus enabling the saving of energy.

Circuit Working

An LDR is used as the light sensor to detect the darkness around 6 pm. During day light, LDR has less resistance and it conducts. This keeps the reset pin 12 of IC1 high and the IC remains off without oscillating. VR1 adjust the resetting of IC at the particular light level in the room, say around 6 pm. When the light level in the room drops below the preset level, IC1 starts oscillating. After 20 seconds, its pin 5 turns high and triggers the relay driver transistor T1. Normally the power supply to the fridge is provided through the Comm and NC contacts of the relay. So when the relay triggers, the contacts break and the power to the fridge will be cut off.

The other outputs of the IC1 turns high one by one as the binary counter advances. But since the outputs are taken to the base of T1 through the diodes D2 through D9, T1 remains on during the entire period until the output pin 3 turns high after 2.5 hours. When the output pin 3 turns high, diode D1 forward biases and inhibits the oscillation of IC. At this time, all the outputs except pin 3 turns low and T1 switches off. Relay deenergizes and the Fridge again gets power through the NC contact. This condition remains as such till the LDR gets light again in the morning.IC1 then resets and pin3 again turns low. So during day time also, the Fridge works as usual. Only during the peak hours say between 6 pm and 8.30 pm, the Fridge remains off. By increasing the value of either C1 or R1, you can increase the time delay to 3 or 4 hours.

How to Set?

Assemble the circuit on a common PCB and enclose in a Box. You can use the case of a stabilizer so that the output plug can be fixed easily. Use a 9 volt 500 mA transformer power supply for the circuit. Take the phase line from the Transformer primary and connect it to the Common contact of the relay. Connect another wire to the NC contact of the relay and connect its other end to the Live pin of the socket. Take a wire from the Neutral of the transformer primary and connect it to the Neutral pin of the Socket. So now the socket can be used to plug in the Fridge. Fix the LDR outside the box where day light is available (note that the room light during night should not fall on the LDR). If the room light is not sufficient during day time, keep the LDR outside the room and connect it to the circuit using thin wires. Adjust the preset VR1 to set the sensitivity of LDR at the particular light level.

3.     Programmable Industrial Timer

Industries often require programmable timer for certain repetitive nature of load on and off. In this circuit design we used an AT80C52 microcontroller which is programmed to set the time by using set input switches. A LCD display helps in setting the time period while a relay duly interfaced from the microcontroller operates the load as per the entry time for on period and off period is made.

Video on Programmable Industrial Timer

Circuit Description

On pressing the start button, the display interfaced to the Microcontroller starts showing the relevant instructions. The ON time of the load is then entered by the user. This is done by pressing the INC button. Pressing the button more than once increases the ON time. Pressing the DEC button decreases the ON time. This time is then stored in the microcontroller by pressing the enter button. Initially the transistor is connected to the 5V signal and starts conducting and as a result the relay is energized and the lamp is glowing. On pressing the relevant button, the time for which the lamp glows can be increased or decreased. This is done by the Microcontroller sending high logic pulses accordingly to the transistor based on the time stored. On pressing the emergency off button, the Microcontroller receives a interrupt signal and accordingly generates a low logic signal to the transistor to switch off the relay and in turn the load.

4.     RF based Programmable Industrial Timer

This is an improved version of the programmable Industrial Timer where the time of switching of loads in controlled remotely using RF communication.

At the transmitter side, 4 push buttons are interfaced to the Encoder-the start button, the INC button, the DEC button and the Enter button. On pressing the relevant buttons, the Encoder accordingly generates a digital code for the input, i.e. converts the parallel data into serial form. This serial data is then transmitted using RF module.

At the receiver side, the Decoder converts the received serial data into parallel form, which is the original data. The Microcontroller pins are connected to the output of the Decoder and accordingly, based on the input received, the Microcontroller controls the conduction of the transistor, so as to control the switching of the relay and thus the load remains switched on for the time set at the transmitter side.

5.      Auto Dimming Aquarium Light

We all are familiar with Aquariums which we often use at homes for decorative purpose for some one having a desire to keep fish at home (not for eating of course!).Here a basic system is demonstrated through it is possible to lighten up the aquarium during the day and night and switch it off or dim it around midnight.

Basic principle involves controlling the triggering of the relay using an oscillating IC.

The circuit uses the Binary counter IC CD4060 to get the time delay of 6 hours after the sunset. An LDR is used as light sensor to control the working of IC. During day time, LDR offers less resistance and it conducts. This keeps the reset pin 12 of IC high and it remains off. When the intensity of day light decreases, the resistance of LDR increases and the IC starts oscillating. This happens around 6 pm (as set by VR1). The oscillating components of IC1 are C1 and R1 which gives a time delay of 6 hours to turns the output pin 3 to high state. When the output pin3 goes high (after 6 hours), transistor T1 turns on and the Relay triggers. At the same time, diode D1 forward biases and inhibits the oscillation of IC.IC then latches and keeps the relay energized till the resetting of the IC in the morning.

Normally the power supply to the LED panel is through the Common and NC (Normally Connected) contacts of the relay. But when the relay energize, the power supply to the LED panel will be bypassed through the NO (Normally Open) contact of the relay. Before entering the LED panel, power passes through R4 and VR2 so that the LEDs turn dim. VR2 is used to adjust the brightness of LEDs. The light from the LED panel can be adjusted from dim state to fully off state using VR2.

The LED panel consists of 45 LEDs of single color or two colors. The LEDs should be high bright transparent type to give sufficient brightness. Arrange the LEDs in 15 row each consisting of 3 LEDs in series with a 100 ohms current limiting resistor. Only two rows are shown in the diagram. Arrange all the 15 rows as shown in the diagram. It is better to fix the LEDs in a long sheet of common PCB and connect the panel to the relay using thin wires. The LDR should be placed in a position to get day light. Connect the LDR using thin plastic wires and place it near the window or outside so as to get day light.

IC4060

Let us now have a brief about IC 4060

IC CD 4060 is an excellent IC for designing timer for different applications. By selecting suitable values of the timing components, it is possible to adjust the timing from a few seconds to several hours. CD 4060 is the Oscillator cum Binary counter cum Frequency divider integrated circuit that has a built in oscillator based on three inverters. The basic frequency of the internal oscillator can be set using the external capacitor- resistor combination. IC CD4060 works between 5 and 15 volts DC while the CMOS version HEF 4060 works down to three volts.

Pin 16 of the IC is the Vcc pin. If a 100 uF capacitor is connected to this pin, the IC gets more stability even if the input voltage fluctuates slightly. Pin 8 is the ground pin.

Timing Circuit

IC CD4060 requires external timing components to feed oscillations to the Clock in pin 11. The timing capacitor is connected to pin 9 and the timing resistor to pin 10. Clock in pin is 11 which also require a high value resistor around 1M. Instead of the external timing components, clock pulses from an oscillator can be fed to clock in pin 11. With the external timing components, the IC will start oscillating and the time delay for the outputs depends on the values of the timing resistor and timing capacitor.

Resetting

Pin 12 of the IC is the reset pin. IC oscillates only if the reset pin is at ground potential. So a 0.1 capacitor and a 100K resistor are connected to reset the IC at power on. Then it will start oscillating.

Outputs and Binary Counting

The IC has 10 outputs each can source around 10 mA current and voltage slightly less than that of Vcc. The outputs are numbered as Q3 through Q13. Output Q10 is missing so that double time can be obtained from Q11. This enhances more flexibility to get more timing. Each output from Q3 to Q13 goes high after completing one timing cycle. Inside the IC there is an oscillator and 14 serially connected Bistables. This arrangement is called Ripple Cascade arrangement. Initially, the oscillation is applied to the first bistable which then drives the second bistable and so on. The signal input is divided by two in each bistable so a total 15 signals are available each of half the frequency of the previous one. Out of these 15 signals, 10 signals are available from Q3 to Q13. So the second output gets double time than that of first output. The Third output gets double time than that of the Second one. This continues and maximum time will be available at the last output Q13. But during that time, other outputs will also give high output based on their timing.

Latching the IC

CD 4060 based timer can be latched to block the oscillation and to keep the output high until resetting. For this IN4148 diode can be used. When the high output is connected to Pin11 through the diode, the clocking will be inhibited when that output becomes high. The IC will stars oscillation again only if it is reset by switching off the power.

Formulae for the Timing Cycle

Time t = 2 n / f osc = Seconds

n is the selected Q output number

2 n = Q output number = 2 x Q no times Eg. Q3 output = 2x2x2 = 8

f osc = 1 / 2.5 (R1XC1) = in Hertz

R1 is the resistance at pin 10 in Ohms and C1, the capacitor at pin 9 in Farads.

For example if R1 is 1M and C1 0.22 the basic frequency f osc is

1 / 2.5(1,000,000 x 0.000,000 22) = 1.8 Hz

If the selected output is Q3 then 2 n is 2 x 2 x 2 = 8

Therefore time period (in seconds) is t = 2 n / 1.8 Hz = 8 / 1.8 = 4.4 seconds

Now you have got an idea about the five different types of timer circuit if any queries on this topic or on the electrical and electronic projects leave the comments section below.