Build Your Interval Timer: Precise Timing Control

Description

This circuit provides an adjustable output timer that will re-trigger at regular intervals. The output period can range from a fraction of a second to half-an-hour or more – and it can recur at regular intervals of anything from seconds to days and beyond.

Schematic Diagram

The Output Section

The output section is a monostable circuit. When Pin 6 of the Cmos 4001 is taken high, the monostable triggers, and the relay energizes. It remains energized for a duration determined by C1 and R3.

With the specified values, R3 will deliver output periods of up to about 30 minutes. However, component values can be modified to suit your requirements. For example, reducing R3 to 1 meg and C1 to 4.7 uF results in a maximum output period between 3 and 5 seconds. Due to manufacturing tolerances, the precise length of the time period varies depending on the characteristics of the utilized components.

The Cmos 4060

The Cmos 4060 is a 14-bit binary counter with an integrated oscillator. The oscillator comprises two inverters connected to Pins 9, 10, and 11, and its frequency is controlled by R7. The output from the oscillator is internally connected to the binary counter. While the oscillator is operating, the IC counts the number of oscillations, and the state of the count is reflected in the output pins.

Adjusting R7 allows setting the length of time it takes for any given output pin to become high. Connect that output to Pin 6 of the Cmos 4001, and every time it goes high, it will trigger the monostable.

Ideally, C4 should be non-polarized, but a regular electrolytic will work provided it doesn’t leak excessively in the reverse direction. Alternatively, you can simulate a non-polarized 10uF capacitor by connecting two 22uF capacitors back to back, as shown.

IMPORTANT

Do not use the “on-board” relay to switch mains voltage. The board’s layout does not offer sufficient isolation between the relay contacts and the low-voltage components. If you need to switch mains voltage, mount a suitably rated relay somewhere safe – Away From The Board. I've used a SPCO/SPDT relay, but you can use a multi-pole relay if desired.

Veroboard Layout

Since the delays between outputs can last for hours, or even days, using “Trial and Error” to set up the timer would be highly tedious. A better solution is to utilize the Setup Table provided, and calculate the time required for Pin 7 of the Cmos 4060 to go high.

For instance, if you want the monostable to trigger every Six Hours, the Setup Table indicates using Pin 1 of the Cmos 4060. You need Pin 1 to go high every 6 x 60 x 60 seconds, which equates to 21,600 seconds. The Setup table tells you that for Pin 1, you should divide this figure by 512, giving approximately 42 seconds. Adjust R7 so that the Yellow LED illuminates 42 seconds after power is applied. This will cause Pin 1 to go high after about 3 Hours.

The Setup Tables

When Pin 1 goes high, it remains high for three hours. It then goes low for three hours before going high once again. Thus, Pin 1 goes high once every six hours. The act of going high triggers the monostable. Consequently, after an initial delay of three hours, the relay will energize. It will then re-energize every six hours thereafter.

The reset button should NOT be used during setup. The time it takes for Pin 7 to go high, and the Yellow LED to light, MUST be measured from the moment power is applied.

Although R4, R5 and the two LEDs help with the setup, they are not necessary to the operation of the timer. If you want to reduce the power consumption, disconnect them once you’ve completed the setup. If you want the LEDs to glow brighter, use brighter LEDs. Don’t be tempted to reduce the values of the series resistors, especially R5. If you reduce its value too far, the oscillator output will not operate the counter.

The timer is designed for a 12-volt supply. However, provided a suitable relay is used, it will work at anything from 5 to 15 volts. Applying power starts the timer. It can be reset at any time by a brief interruption of the power supply – so a reset button is not strictly necessary. If you need delays in excess of 32 hours, increase the value of C4.

The Support Material for this circuit includes a step-by-step guide to the construction of the circuit-board, a parts list, a detailed circuit description, and more.